The <img> tag is used to embed an
image in an HTML page. The src attribute
specifies the path to the image to be displayed:
Example
<img src="img_girl.jpg">
There are two ways to specify the URL in the src
attribute:
1. Absolute URL - Links to an external image that is hosted
on another website. Example: src="https://www.w3schools.com/images/img_girl.jpg".
Notes: External images might be under copyright. If you do
not get permission to use it, you may be in violation of copyright laws. In
addition, you cannot control external images; it can suddenly be removed or
changed.
2. Relative URL - Links to an image that is hosted within
the website. Here, the URL does not include the domain name. If the URL begins
without a slash, it will be relative to the current page. Example: src="img_girl.jpg".
If the URL begins with a slash, it will be relative to the domain. Example: src="/images/img_girl.jpg".
Tip: It is almost always best to use relative URLs. They
will not break if you change domain.
The width and height Attributes
The <img> tag should also contain the
width and
height attributes, which specify the width and
height of the image (in pixels):
Example
<img src="img_girl.jpg" width="500" height="600">
The alt Attribute
The required alt attribute for the <img>
tag specifies an
alternate text for an image, if the image for some reason cannot be displayed.
This can be due to
a slow connection, or an error in the src attribute, or if the user uses a screen
reader.
Example
<img src="img_girl.jpg" alt="Girl
with a jacket">
Example
See what happens if we try to display an image that does not exist:
<img src="img_typo.jpg" alt="Girl
with a jacket">
You will learn more about images in our .
The style Attribute
The style attribute is used to add styles to
an element, such as color, font, size, and more.
Example
<p style="color:red;">This is a red paragraph.</p>
You will learn more about styles in our .
The lang Attribute
You should always include the lang attribute
inside the <html> tag, to declare the
language of the Web page. This is meant to assist search engines and browsers.
The following example specifies English as the language:
Country codes can also be added to the language code in the lang
attribute. So, the first two characters define the language of the HTML page,
and the last two characters define the country.
The following example specifies English as the language and United States as
the country:
The title attribute defines some extra
information about an
element.
The value of the title attribute will be displayed as a tooltip when
you mouse over the element:
Example
<p title="I'm a tooltip">This is a paragraph.</p>
We Suggest: Always Use Lowercase Attributes
The HTML standard does not require lowercase attribute names.
The title attribute (and all other attributes) can be written with uppercase or lowercase
like title or TITLE.
However, W3C recommends lowercase attributes in HTML, and demands
lowercase attributes for stricter document types like XHTML.
At W3Schools we always use lowercase attribute names.
We Suggest: Always Quote Attribute Values
The HTML standard does not require quotes around attribute values.
However, W3C recommends quotes in HTML, and demands quotes for
stricter document types like XHTML.
Good:
<a href="https://www.w3schools.com/html/">Visit our HTML tutorial</a>
Bad:
<a href=https://www.w3schools.com/html/>Visit our HTML tutorial</a>
Sometimes you have to use quotes. This example will not display
the title attribute correctly, because it contains a space:
Example
<p
title=About W3Schools>
 At W3Schools we always use quotes around attribute values.
Single or Double Quotes?
Double quotes around attribute values are the most common in HTML, but single
quotes can also be used.
In some situations, when the attribute value itself contains double quotes, it is necessary to use single quotes:
<p title='John "ShotGun" Nelson'>
Or vice versa:
<p title="John 'ShotGun' Nelson">
Chapter Summary
All HTML elements can have attributes
The href attribute of
<a> specifies the URL of the page the link goes to
The src attribute of
<img> specifies the path to the image to be displayed
The width and height attributes
of <img> provide size information for images
The alt attribute of
<img> provides an alternate text for an image
The style attribute is used to add styles
to an element, such as color, font, size, and more
The lang attribute
of the <html> tag declares the
language of the Web page
The title attribute defines some extra
information about an element
Learn Java
Java is a popular programming language.
Java is used to develop mobile apps, web apps, desktop apps, games and much more.
Examples in Each Chapter
Our "Try it Yourself" editor makes it easy to learn Java. You can edit Java code and view the result in your browser.
Example
publicclassMain{
publicstaticvoidmain(String[] args){
System.out.println("Hello World");
}
}
Click on the "Run example" button to see how it works.
We recommend reading this tutorial, in the sequence listed in the left menu.
Java is an object oriented language and some concepts may be new. Take breaks when needed, and go over the examples as many times as needed.
Java Introduction
What is Java?
Java is a popular programming language, created in 1995.
It is owned by Oracle, and more than 3 billion devices run Java.
It is used for:
Mobile applications (specially Android apps)
Desktop applications
Web applications
Web servers and application servers
Games
Database connection
And much, much more!
Why Use Java?
Java works on different platforms (Windows, Mac, Linux, Raspberry Pi, etc.)
It is one of the most popular programming language in the world
It has a large demand in the current job market
It is easy to learn and simple to use
It is open-source and free
It is secure, fast and powerful
It has a huge community support (tens of millions of developers)
Java is an object oriented language which gives a clear structure to programs and allows code to be reused, lowering development costs
As Java is close to and , it makes it easy for programmers to switch to Java or vice versa
Get Started
It is not necessary to have any prior programming experience.
Java Install
Some PCs might have Java already installed.
To check if you have Java installed on a Windows PC, search in the start bar for Java or type the following in Command Prompt (cmd.exe):
C:UsersYour Name>java -version
If Java is installed, you will see something like this (depending on version):
java version "11.0.1" 2018-10-16 LTS Java(TM) SE Runtime Environment 18.9 (build 11.0.1+13-LTS) Java HotSpot(TM) 64-Bit Server VM 18.9 (build 11.0.1+13-LTS, mixed mode)
If you do not have Java installed on your computer, you can download it for free at oracle.com.
Note: In this tutorial, we will write Java code in a text editor. However, it is possible to write Java in an Integrated Development Environment, such as IntelliJ IDEA, Netbeans or Eclipse, which are particularly useful when managing larger collections of Java files.
Setup for Windows
To install Java on Windows:
Go to "System Properties" (Can be found on Control Panel > System and Security > System > Advanced System Settings)
Click on the "Environment variables" button under the "Advanced" tab
Then, select the "Path" variable in System variables and click on the "Edit" button
Click on the "New" button and add the path where Java is installed, followed by bin. By default, Java is installed in C:Program FilesJavajdk-11.0.1 (If nothing else was specified when you installed it). In that case, You will have to add a new path with: C:Program FilesJavajdk-11.0.1bin Then, click "OK", and save the settings
At last, open Command Prompt (cmd.exe) and type java -version to see if Java is running on your machine
Show how to install Java step-by-step with images »
Step 2 »
Step 3 »
Step 4 »
Step 5 »
Write the following in the command line (cmd.exe):
C:UsersYour Name>java -version
If Java was successfully installed, you will see something like this (depending on version):
java version "11.0.1" 2018-10-16 LTS Java(TM) SE Runtime Environment 18.9 (build 11.0.1+13-LTS) Java HotSpot(TM) 64-Bit Server VM 18.9 (build 11.0.1+13-LTS, mixed mode)
Java Quickstart
In Java, every application begins with a class name, and that class must match the filename.
Let's create our first Java file, called Main.java, which can be done in any text editor (like Notepad).
The file should contain a "Hello World" message, which is written with the following code:
Main.java
publicclassMain{
publicstaticvoidmain(String[] args){
System.out.println("Hello World");
}
}
Don't worry if you don't understand the code above - we will discuss it in detail in later chapters. For now, focus on how to run the code above.
Save the code in Notepad as "Main.java". Open Command Prompt (cmd.exe), navigate to the directory where you saved your file, and type "javac Main.java":
C:UsersYour Name>javac Main.java
This will compile your code. If there are no errors in the code, the command prompt will take you to the next line. Now, type "java Main" to run the file:
C:UsersYour Name>java Main
The output should read:
Hello World
Congratulations! You have written and executed your first Java program.
Java Syntax
In the previous chapter, we created a Java file called Main.java, and we used the following code to print "Hello World" to the screen:
Main.java
publicclassMain{
publicstaticvoidmain(String[] args){
System.out.println("Hello World");
}
}
Example explained
Every line of code that runs in Java must be inside a class. In our example, we named the class Main. A class should always start with an uppercase first letter.
Note: Java is case-sensitive: "MyClass" and "myclass" has different meaning.
The name of the java file must match the class name. When saving the file, save it using the class name and add ".java" to the end of the filename. To run the example above on your computer, make sure that Java is properly installed: Go to the Get Started Chapter for how to install Java. The output should be:
Hello World
The main Method
The main() method is required and you will see it in every Java program:
publicstaticvoidmain(String[] args)
Any code inside the main() method will be executed. Don't worry about the keywords before and after main. You will get to know them bit by bit while reading this tutorial.
For now, just remember that every Java program has a class name which must match the filename, and that every program must contain the main() method.
System.out.println()
Inside the main() method, we can use the println() method to print a line of text to the screen:
publicstaticvoidmain(String[] args){
System.out.println("Hello World");
}
Note: The curly braces {} marks the beginning and the end of a block of code.
System is a built-in Java class that contains useful members, such as out, which is short for "output". The println() method, short for "print line", is used to print a value to the screen (or a file).
Don't worry too much about System, out and println(). Just know that you need them together to print stuff to the screen.
You should also note that each code statement must end with a semicolon (;).
Print Text
You learned from the previous chapter that you can use the println() method to output values or print text in Java:
Example
System.out.println("Hello World!");
You can add as many println() methods as you want. Note that it will add a new line for each method:
Example
System.out.println("Hello World!");
System.out.println("I am learning Java.");
System.out.println("It is awesome!");
Double Quotes
When you are working with text, it must be wrapped inside double quotations marks "".
If you forget the double quotes, an error occurs:
Example
System.out.println("This sentence will work!");
System.out.println(This sentence will produce an error);
The Print() Method
There is also a print() method, which is similar to println().
The only difference is that it does not insert a new line at the end of the output:
Example
System.out.print("Hello World! ");
System.out.print("I will print on the same line.");
Note that we add an extra space (after "Hello World!" in the example above), for better readability.
In this tutorial, we will only use println() as it makes it easier to read the output of code.
Java Output / Print
Print Text
You learned from the previous chapter that you can use the println() method to output values or print text in Java:
Example
System.out.println("Hello World!");
You can add as many println() methods as you want. Note that it will add a new line for each method:
Example
System.out.println("Hello World!");
System.out.println("I am learning Java.");
System.out.println("It is awesome!");
Double Quotes
When you are working with text, it must be wrapped inside double quotations marks "".
If you forget the double quotes, an error occurs:
Example
System.out.println("This sentence will work!");
System.out.println(This sentence will produce an error);
The Print() Method
There is also a print() method, which is similar to println().
The only difference is that it does not insert a new line at the end of the output:
Example
System.out.print("Hello World! ");
System.out.print("I will print on the same line.");
Note that we add an extra space (after "Hello World!" in the example above),
for better readability.
In this tutorial, we will only use println() as it makes it easier to read the output of code.
Print Numbers
You can also use the println() method to print numbers.
However, unlike text, we don't put numbers inside double quotes:
Example
System.out.println(3);
System.out.println(358);
System.out.println(50000);
You can also perform mathematical calculations inside the println() method:
Example
System.out.println(3+3);
Example
System.out.println(2*5);
Java Comments
Comments can be used to explain Java code, and to make it more readable. It can also be used to prevent execution when testing alternative code.
Single-line Comments
Single-line comments start with two forward slashes (//).
Any text between // and the end of the line is ignored by Java (will not be executed).
This example uses a single-line comment before a line of code:
Example
// This is a comment
System.out.println("Hello World");
This example uses a single-line comment at the end of a line of code:
Example
System.out.println("Hello World");// This is a comment
Java Multi-line Comments
Multi-line comments start with /* and ends with */.
Any text between /* and */ will be ignored by Java.
This example uses a multi-line comment (a comment block) to explain the code:
Example
/* The code below will print the words Hello World
to the screen, and it is amazing */
System.out.println("Hello World");
Single or multi-line comments?
It is up to you which you want to use. Normally, we use // for short comments, and /* */ for longer.
Java Variables
Java Variables
Variables are containers for storing data values.
In Java, there are different types of variables, for example:
String - stores text, such as "Hello". String values are
surrounded by double quotes
int - stores integers (whole numbers), without decimals, such as 123 or -123
float - stores floating point numbers, with decimals, such as 19.99 or -19.99
char - stores single characters, such as
'a' or 'B'. Char values are surrounded by single quotes
boolean - stores values with two states:
true or false
Declaring (Creating) Variables
To create a variable, you must specify the type and assign it a value:
Syntax
type variableName = value;
Where type is one of Java's types (such as int or String), and
variableName is the name of the variable (such as x or
name). The equal sign is used to assign values to the variable.
To create a variable that should store text, look at the following example:
Example
Create a variable called name of type String and assign it the value "John":
String name = "John";
System.out.println(name);
To create a variable that should store a number, look at the following example:
Example
Create a variable called myNum of type int and assign it the value 15:
int myNum = 15;
System.out.println(myNum);
You can also declare a variable without assigning the value, and assign the value later:
Example
int myNum;
myNum = 15;
System.out.println(myNum);
Note that if you assign a new value to an existing variable, it will overwrite the previous value:
Example
Change the value of myNum from 15 to 20:
int myNum = 15;
myNum = 20; // myNum is now 20
System.out.println(myNum);
Final Variables
If you don't want others (or yourself) to overwrite existing values, use the final keyword (this will declare the variable as "final" or "constant", which means unchangeable and read-only):
Example
final int myNum = 15;
myNum = 20; // will generate an error: cannot assign a value to a final variable
Other Types
A demonstration of how to declare variables of other types:
In Java, there are different types of variables, for example:
String - stores text, such as "Hello". String values are surrounded by double quotes
int - stores integers (whole numbers), without decimals, such as 123 or -123
float - stores floating point numbers, with decimals, such as 19.99 or -19.99
char - stores single characters, such as 'a' or 'B'. Char values are surrounded by single quotes
boolean - stores values with two states: true or false
Declaring (Creating) Variables
To create a variable, you must specify the type and assign it a value:
Syntax
type variableName = value;
Where type is one of Java's types (such as int or String), and variableName is the name of the variable (such as x or name). The equal sign is used to assign values to the variable.
To create a variable that should store text, look at the following example:
Example
Create a variable called name of type String and assign it the value "John":
String name ="John";
System.out.println(name);
To create a variable that should store a number, look at the following example:
Example
Create a variable called myNum of type int and assign it the value 15:
int myNum =15;
System.out.println(myNum);
You can also declare a variable without assigning the value, and assign the value later:
Example
int myNum;
myNum =15;
System.out.println(myNum);
Note that if you assign a new value to an existing variable, it will overwrite the previous value:
Example
Change the value of myNum from 15 to 20:
int myNum =15;
myNum =20;// myNum is now 20
System.out.println(myNum);
Final Variables
If you don't want others (or yourself) to overwrite existing values, use the final keyword (this will declare the variable as "final" or "constant", which means unchangeable and read-only):
Example
finalint myNum =15;
myNum =20;// will generate an error: cannot assign a value to a final variable
Other Types
A demonstration of how to declare variables of other types:
Example
int myNum =5;
float myFloatNum =5.99f;
char myLetter ='D';
boolean myBool =true;
String myText ="Hello";
You will learn more about data types in the next section.
Display Variables
The println() method is often used to display variables.
To combine both text and a variable, use the + character:
Example
String name ="John";
System.out.println("Hello "+ name);
You can also use the + character to add a variable to another variable:
Example
String firstName ="John ";
String lastName ="Doe";
String fullName = firstName + lastName;
System.out.println(fullName);
For numeric values, the + character works as a mathematical operator (notice that we use int (integer) variables here):
Example
int x =5;
int y =6;
System.out.println(x + y);// Print the value of x + y
From the example above, you can expect:
x stores the value 5
y stores the value 6
Then we use the println() method to display the value of x + y, which is 11
Java Declare Multiple Variables
Declare Many Variables
To declare more than one variable of the same type, you can
use a comma-separated list:
Example
Instead of writing:
int x = 5;
int y = 6;
int z = 50;
System.out.println(x + y + z);
You can simply write:
int x = 5, y = 6, z = 50;
System.out.println(x + y + z);
One Value to Multiple Variables
You can also assign the same value to multiple variables in one line:
Example
int x, y, z;
x = y = z = 50;
System.out.println(x + y + z);
Java Identifiers
Identifiers
All Java variables must be
identified with unique names.
These unique names are called identifiers.
Identifiers can be short names (like x and y) or more descriptive names (age, sum, totalVolume).
Note: It is recommended to use descriptive names in order to
create understandable and maintainable code:
Example
// Good
int minutesPerHour = 60;
// OK, but not so easy to understand what m actually is
int m = 60;
The general rules for naming variables are:
Names can contain letters, digits, underscores, and dollar signs
Names must begin with a letter
Names should start with a lowercase letter and it cannot contain whitespace
Names can also begin with $ and _ (but we will not use it in this tutorial)
Names are case sensitive ("myVar" and "myvar" are different variables)
Reserved words (like Java keywords, such as int or
boolean) cannot be used as names
As explained in the previous chapter, a in Java must be a specified data type:
Example
int myNum = 5;Â Â Â Â Â Â Â Â Â Â Â Â Â Â // Integer (whole number)
float myFloatNum = 5.99f;Â Â Â // Floating point number
char myLetter = 'D';Â Â Â Â Â Â Â Â // Character
boolean myBool = true;Â Â Â Â Â Â // Boolean
String myText = "Hello";Â Â Â Â // String
Data types are divided into two groups:
Primitive data types - includes byte, short, int, long,
float, double, boolean and char
Non-primitive data types - such as , and (you will learn more about these in a later chapter)
Primitive Data Types
A primitive data type specifies the size and type of variable values, and it has no
additional methods.
There are eight primitive data types in Java:
Data Type
Size
Description
byte
1 byte
Stores whole numbers from -128 to 127
short
2 bytes
Stores whole numbers from -32,768 to 32,767
int
4 bytes
Stores whole numbers from -2,147,483,648 to 2,147,483,647
long
8 bytes
Stores whole numbers from -9,223,372,036,854,775,808 to
9,223,372,036,854,775,807
float
4 bytes
Stores fractional numbers. Sufficient for
storing 6 to 7 decimal digits
double
8 bytes
Stores fractional numbers. Sufficient for
storing 15 decimal digits
boolean
1 bit
Stores true or false values
char
2 bytes
Stores a single character/letter or ASCII values
Numbers
Primitive number types are divided into two groups:
Integer types stores whole numbers, positive or negative (such as 123 or -456), without decimals. Valid types are byte, short, int and long. Which type you should use, depends on the numeric value.
Floating point types represents numbers with a fractional part, containing one or more decimals. There are two types: float and double.
Even though there are many numeric types in Java, the most used for numbers are int (for whole numbers) and double (for floating point numbers). However, we will describe them all as you continue to read.
Integer Types
Byte
The byte data type can store whole numbers from -128 to 127. This can be used instead of int or other integer types to save memory when you are certain that the value will be within -128 and 127:
Example
byte myNum =100;
System.out.println(myNum);
Short
The short data type can store whole numbers from -32768 to 32767:
Example
short myNum =5000;
System.out.println(myNum);
Int
The int data type can store whole numbers from -2147483648 to 2147483647. In general, and in our tutorial, the int data type is the preferred data type when we create variables with a numeric value.
Example
int myNum =100000;
System.out.println(myNum);
Long
The long data type can store whole numbers from -9223372036854775808 to 9223372036854775807. This is used when int is not large enough to store the value. Note that you should end the value with an "L":
Example
long myNum =15000000000L;
System.out.println(myNum);
Floating Point Types
You should use a floating point type whenever you need a number with a decimal, such as 9.99 or 3.14515.
The float and double data types can store fractional numbers. Note that you should end the value with an "f" for floats and "d" for doubles:
Float Example
float myNum =5.75f;
System.out.println(myNum);
Double Example
double myNum =19.99d;
System.out.println(myNum);
Use float or double?
The precision of a floating point value indicates how many digits the value can have after the decimal point. The precision of float is only six or seven decimal digits, while double variables have a precision of about 15 digits. Therefore it is safer to use double for most calculations.
Scientific Numbers
A floating point number can also be a scientific number with an "e" to indicate the power of 10:
Example
float f1 =35e3f;
double d1 =12E4d;
System.out.println(f1);
System.out.println(d1);
Java Boolean Data Types
Boolean Types
Very often in programming, you will need a data type that can only have one of two values, like:
YES / NO
ON / OFF
TRUE / FALSE
For this, Java has a boolean data type, which can only take the values true or false:
Example
boolean isJavaFun = true;
boolean isFishTasty = false;
System.out.println(isJavaFun);Â Â Â Â // Outputs true
System.out.println(isFishTasty);Â Â // Outputs false
Boolean values are mostly used for conditional testing.
You will learn much more about and later in this tutorial.
Characters
The char data type is used to store a single character. The character must be surrounded by single quotes, like 'A' or 'c':
Example
char myGrade ='B';
System.out.println(myGrade);
Alternatively, if you are familiar with ASCII values, you can use those to display certain characters:
Example
char myVar1 =65, myVar2 =66, myVar3 =67;
System.out.println(myVar1);
System.out.println(myVar2);
System.out.println(myVar3);
Tip: A list of all ASCII values can be found in our ASCII Table Reference.
Strings
The String data type is used to store a sequence of characters (text). String values must be surrounded by double quotes:
Example
String greeting ="Hello World";
System.out.println(greeting);
The String type is so much used and integrated in Java, that some call it "the special ninth type".
A String in Java is actually a non-primitive data type, because it refers to an object. The String object has methods that are used to perform certain operations on strings. Don't worry if you don't understand the term "object" just yet. We will learn more about strings and objects in a later chapter.
Java Non-Primitive Data Types
Non-Primitive Data Types
Non-primitive data types are called reference types because
they refer to objects.
The main difference between primitive and non-primitive data types are:
Primitive types are predefined (already defined) in Java. Non-primitive types are created by the programmer and
is not defined by Java (except for String).
Non-primitive types can be used to call methods to perform certain operations, while primitive types cannot.
A primitive type has always a value, while non-primitive types can be null.
A primitive type starts with a lowercase letter, while non-primitive types starts with an uppercase letter.
Examples of non-primitive types are , , , etc. You will learn more about these in a later chapter.
Java Type Casting
Type casting is when you assign a value of one primitive data type to another type.
In Java, there are two types of casting:
Widening Casting (automatically) - converting a smaller type to a larger type size byte -> short -> char -> int -> long -> float -> double
Narrowing Casting (manually) - converting a larger type to a smaller size type double -> float -> long -> int -> char -> short -> byte
Widening Casting
Widening casting is done automatically when passing a smaller size type to a larger size type:
Example
publicclassMain{
publicstaticvoidmain(String[] args){
int myInt =9;
double myDouble = myInt;// Automatic casting: int to double
System.out.println(myInt);// Outputs 9
System.out.println(myDouble);// Outputs 9.0
}
}
Narrowing Casting
Narrowing casting must be done manually by placing the type in parentheses in front of the value:
Example
publicclassMain{
publicstaticvoidmain(String[] args){
double myDouble =9.78d;
int myInt =(int) myDouble;// Manual casting: double to int
System.out.println(myDouble);// Outputs 9.78
System.out.println(myInt);// Outputs 9
}
}
Java Operators
Operators are used to perform operations on variables and values.
In the example below, we use the +operator to add together two values:
Example
int x =100+50;
Although the + operator is often used to add together two values, like in the example above, it can also be used to add together a variable and a value, or a variable and another variable:
Example
int sum1 =100+50;// 150 (100 + 50)
int sum2 = sum1 +250;// 400 (150 + 250)
int sum3 = sum2 + sum2;// 800 (400 + 400)
Java divides the operators into the following groups:
Arithmetic operators
Assignment operators
Comparison operators
Logical operators
Bitwise operators
Arithmetic Operators
Arithmetic operators are used to perform common mathematical operations.
Operator
Name
Description
Example
+
Addition
Adds together two values
x + y
-
Subtraction
Subtracts one value from another
x - y
*
Multiplication
Multiplies two values
x * y
/
Division
Divides one value by another
x / y
%
Modulus
Returns the division remainder
x % y
++
Increment
Increases the value of a variable by 1
++x
--
Decrement
Decreases the value of a variable by 1
--x
Java Assignment Operators
Assignment operators are used to assign values to variables.
In the example below, we use the assignment operator (=) to assign the value 10 to a variable called x:
Example
int x =10;
The addition assignment operator (+=) adds a value to a variable:
Example
int x =10;
x +=5;
A list of all assignment operators:
Operator
Example
Same As
=
x = 5
x = 5
+=
x += 3
x = x + 3
-=
x -= 3
x = x - 3
*=
x *= 3
x = x * 3
/=
x /= 3
x = x / 3
%=
x %= 3
x = x % 3
&=
x &= 3
x = x & 3
|=
x |= 3
x = x | 3
^=
x ^= 3
x = x ^ 3
>>=
x >>= 3
x = x >> 3
<<=
Java Strings
Strings are used for storing text.
A String variable contains a collection of characters surrounded by double quotes:
Example
Create a variable of type String and assign it a value:
String greeting ="Hello";
String Length
A String in Java is actually an object, which contain methods that can perform certain operations on strings. For example, the length of a string can be found with the length() method:
Example
String txt ="ABCDEFGHIJKLMNOPQRSTUVWXYZ";
System.out.println("The length of the txt string is: "+ txt.length());
More String Methods
There are many string methods available, for example toUpperCase() and toLowerCase():
Java counts positions from zero. 0 is the first position in a string, 1 is the second, 2 is the third ...
Complete String Reference
For a complete reference of String methods, go to our Java String Methods Reference.
The reference contains descriptions and examples of all string methods.
Java Strings
Java Strings
Strings are used for storing text.
A String variable contains a collection of characters surrounded by double quotes:
Example
Create a variable of type String and assign it a value:
String greeting = "Hello";
String Length
A String in Java is actually an object, which contain methods that can perform certain operations on strings. For example, the length of a string can be found with the length() method:
Example
String txt = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
System.out.println("The length of the txt string is: " + txt.length());
More String Methods
There are many string methods available, for example toUpperCase() and toLowerCase():
Example
String txt = "Hello World";
System.out.println(txt.toUpperCase());Â Â // Outputs "HELLO WORLD"
System.out.println(txt.toLowerCase());Â Â // Outputs "hello world"
Finding a Character in a String
The indexOf() method returns the index (the position)
of the first occurrence of a specified text in a string
(including whitespace):
Java counts positions from zero. 0 is the first position in a
string, 1 is the second, 2 is the third ...
Complete String Reference
For a complete reference of String methods, go to our .
The reference contains descriptions and examples of all string methods.
String Concatenation
The + operator can be used between strings to combine them. This is called concatenation:
Example
String firstName ="John";
String lastName ="Doe";
System.out.println(firstName +" "+ lastName);
Note that we have added an empty text (" ") to create a space between firstName and lastName on print.
You can also use the concat() method to concatenate two strings:
Example
String firstName ="John ";
String lastName ="Doe";
System.out.println(firstName.concat(lastName));
Adding Numbers and Strings
WARNING!
Java uses the + operator for both addition and concatenation.
Numbers are added. Strings are concatenated.
If you add two numbers, the result will be a number:
Example
int x =10;
int y =20;
int z = x + y;// z will be 30 (an integer/number)
If you add two strings, the result will be a string concatenation:
Example
String x ="10";
String y ="20";
String z = x + y;// z will be 1020 (a String)
If you add a number and a string, the result will be a string concatenation:
Example
String x ="10";
int y =20;
String z = x + y;// z will be 1020 (a String)
Strings - Special Characters
Because strings must be written within quotes, Java will misunderstand this string, and generate an error:
String txt ="We are the so-called "Vikings" from the north.";
The solution to avoid this problem, is to use the backslash escape character.
The backslash () escape character turns special characters into string characters:
Escape character
Result
Description
'
'
Single quote
"
"
Double quote
\
Backslash
The sequence " inserts a double quote in a string:
Example
String txt ="We are the so-called "Vikings" from the north.";
The sequence ' inserts a single quote in a string:
Example
String txt ="It's alright.";
The sequence \ inserts a single backslash in a string:
Example
String txt ="The character \ is called backslash.";
Other common escape sequences that are valid in Java are:
Code
Result
n
New Line
r
Carriage Return
t
Tab
b
Backspace
f
Form Feed
The Java Math class has many methods that allows you to perform mathematical tasks on numbers.
Math.max(x,y)
The Math.max(x,y) method can be used to find the highest value of x and y:
Example
Math.max(5,10);
Math.min(x,y)
The Math.min(x,y) method can be used to find the lowest value of x and y:
Example
Math.min(5,10);
Math.sqrt(x)
The Math.sqrt(x) method returns the square root of x:
Example
Math.sqrt(64);
Math.abs(x)
The Math.abs(x) method returns the absolute (positive) value of x:
Example
Math.abs(-4.7);
Random Numbers
Math.random() returns a random number between 0.0 (inclusive), and 1.0 (exclusive):
Example
Math.random();
To get more control over the random number, for example, if you only want a random number between 0 and 100, you can use the following formula:
Example
int randomNum =(int)(Math.random()*101);// 0 to 100
Java Booleans
Java Booleans
Very often, in programming, you will need a data type that can only have one of two values, like:
YES / NO
ON / OFF
TRUE / FALSE
For this, Java has a boolean data type, which can store true or false values.
Boolean Values
A boolean type is declared with the boolean keyword and can only take the values true or false:
Example
boolean isJavaFun = true;
boolean isFishTasty = false;
System.out.println(isJavaFun);Â Â Â Â // Outputs true
System.out.println(isFishTasty);Â Â // Outputs false
However, it is more common to return boolean values from boolean expressions, for conditional testing
(see below).
Boolean Expression
A Boolean expression returns a boolean value: true or false.
This is useful to build logic, and find answers.
For example, you can use a , such as the greater than (>) operator, to find out if an expression (or a variable) is true or false:
Example
int x = 10;
int y = 9;
System.out.println(x > y); // returns true, because 10 is higher than 9
Or even easier:
Example
System.out.println(10 > 9); // returns true, because 10 is higher than 9
In the examples below, we use the equal to (==) operator to evaluate an expression:
Example
int x = 10;
System.out.println(x == 10); // returns true, because the value of x is equal to 10
Example
System.out.println(10 == 15); // returns false, because 10 is not equal to 15
Real Life Example
Let's think of a "real
life example" where we need to find out if a person is old enough to vote.
In the example below, we use the >= comparison operator to find out if the age (25) is greater than OR
equal to the voting age limit, which is set
to 18:
Example
int myAge = 25;
int votingAge = 18;
System.out.println(myAge >= votingAge);
Cool, right? An even better approach (since we are on a roll now), would be to wrap the code above in an
if...else statement, so we can perform
different actions depending on the result:
Example
Output "Old enough to vote!" if myAge is greater than or equal to18. Otherwise output "Not old enough to vote.":
int myAge = 25;
int votingAge = 18;
if (myAge >= votingAge) {
System.out.println("Old enough to vote!");
} else {
System.out.println("Not old enough to vote.");
}
Booleans are the basis for all Java comparisons and conditions.
You will learn more about in the next chapter.
Java Conditions and If Statements
You already know that Java supports the usual logical conditions from mathematics:
Less than: a < b
Less than or equal to: a <= b
Greater than: a > b
Greater than or equal to: a >= b
Equal to a == b
Not Equal to: a != b
You can use these conditions to perform different actions for different decisions.
Java has the following conditional statements:
Use if to specify a block of code to be executed, if a specified condition is true
Use else to specify a block of code to be executed, if the same condition is false
Use else if to specify a new condition to test, if the first condition is false
Use switch to specify many alternative blocks of code to be executed
The if Statement
Use the if statement to specify a block of Java code to be executed if a condition is true.
Syntax
if(condition){
// block of code to be executed if the condition is true
}
Note that if is in lowercase letters. Uppercase letters (If or IF) will generate an error.
In the example below, we test two values to find out if 20 is greater than 18. If the condition is true, print some text:
Example
if(20>18){
System.out.println("20 is greater than 18");
}
We can also test variables:
Example
int x =20;
int y =18;
if(x > y){
System.out.println("x is greater than y");
}
Example explained
In the example above we use two variables, x and y, to test whether x is greater than y (using the > operator). As x is 20, and y is 18, and we know that 20 is greater than 18, we print to the screen that "x is greater than y".
The else Statement
Use the else statement to specify a block of code to be executed if the condition is false.
Syntax
if(condition){
// block of code to be executed if the condition is true
}else{
// block of code to be executed if the condition is false
}
Example
int time =20;
if(time <18){
System.out.println("Good day.");
}else{
System.out.println("Good evening.");
}
// Outputs "Good evening."
Example explained
In the example above, time (20) is greater than 18, so the condition is false. Because of this, we move on to the else condition and print to the screen "Good evening". If the time was less than 18, the program would print "Good day".
The else if Statement
Use the else if statement to specify a new condition if the first condition is false.
Syntax
if(condition1){
// block of code to be executed if condition1 is true
}elseif(condition2){
// block of code to be executed if the condition1 is false and condition2 is true
}else{
// block of code to be executed if the condition1 is false and condition2 is false
}
Example
int time =22;
if(time <10){
System.out.println("Good morning.");
}elseif(time <18){
System.out.println("Good day.");
}else{
System.out.println("Good evening.");
}
// Outputs "Good evening."
Example explained
In the example above, time (22) is greater than 10, so the first condition is false. The next condition, in the else if statement, is also false, so we move on to the else condition since condition1 and condition2 is both false - and print to the screen "Good evening".
However, if the time was 14, our program would print "Good day."
Short Hand If...Else
There is also a short-hand if else, which is known as the ternary operator because it consists of three operands.
It can be used to replace multiple lines of code with a single line, and is most often used to replace simple if else statements:
String result =(time <18)?"Good day.":"Good evening.";
System.out.println(result);
Java Switch Statements
Instead of writing manyif..else statements, you can use the switch statement.
The switch statement selects one of many code blocks to be executed:
Syntax
switch(expression){
case x:
// code block
break;
case y:
// code block
break;
default:
// code block
}
This is how it works:
The switch expression is evaluated once.
The value of the expression is compared with the values of each case.
If there is a match, the associated block of code is executed.
The break and default keywords are optional, and will be described later in this chapter
The example below uses the weekday number to calculate the weekday name:
Example
int day =4;
switch(day){
case1:
System.out.println("Monday");
break;
case2:
System.out.println("Tuesday");
break;
case3:
System.out.println("Wednesday");
break;
case4:
System.out.println("Thursday");
break;
case5:
System.out.println("Friday");
break;
case6:
System.out.println("Saturday");
break;
case7:
System.out.println("Sunday");
break;
}
// Outputs "Thursday" (day 4)
The break Keyword
When Java reaches a break keyword, it breaks out of the switch block.
This will stop the execution of more code and case testing inside the block.
When a match is found, and the job is done, it's time for a break. There is no need for more testing.
A break can save a lot of execution time because it "ignores" the execution of all the rest of the code in the switch block.
The default Keyword
The default keyword specifies some code to run if there is no case match:
Example
Loops
Loops can execute a block of code as long as a specified condition is reached.
Loops are handy because they save time, reduce errors, and they make code more readable.
Java While Loop
The while loop loops through a block of code as long as a specified condition is true:
Syntax
while(condition){
// code block to be executed
}
In the example below, the code in the loop will run, over and over again, as long as a variable (i) is less than 5:
Example
int i =0;
while(i <5){
System.out.println(i);
i++;
}
Note: Do not forget to increase the variable used in the condition, otherwise the loop will never end!
The Do/While Loop
The do/while loop is a variant of the while loop. This loop will execute the code block once, before checking if the condition is true, then it will repeat the loop as long as the condition is true.
Syntax
do{
// code block to be executed
}
while(condition);
The example below uses a do/while loop. The loop will always be executed at least once, even if the condition is false, because the code block is executed before the condition is tested:
Java For Loop
When you know exactly how many times you want to loop through a block of code, use the for loop instead of a while loop:
Syntax
for(statement 1; statement 2; statement 3){
// code block to be executed
}
Statement 1 is executed (one time) before the execution of the code block.
Statement 2 defines the condition for executing the code block.
Statement 3 is executed (every time) after the code block has been executed.
The example below will print the numbers 0 to 4:
Example
for(int i =0; i <5; i++){
System.out.println(i);
}
Example explained
Statement 1 sets a variable before the loop starts (int i = 0).
Statement 2 defines the condition for the loop to run (i must be less than 5). If the condition is true, the loop will start over again, if it is false, the loop will end.
Statement 3 increases a value (i++) each time the code block in the loop has been executed.
Another Example
This example will only print even values between 0 and 10:
Example
for(int i =0; i <=10; i = i +2){
System.out.println(i);
}
Nested Loops
It is also possible to place a loop inside another loop. This is called a nested loop.
The "inner loop" will be executed one time for each iteration of the "outer loop":
Example
// Outer loop
for(int i =1; i <=2; i++){
System.out.println("Outer: "+ i);// Executes 2 times
// Inner loop
for(int j =1; j <=3; j++){
System.out.println(" Inner: "+ j);// Executes 6 times (2 * 3)
}
}
Java For Loop
Java For Loop
When you know exactly how many times you want to loop through a block of
code, use the for loop instead of a while loop:
Syntax
for (statement 1; statement 2; statement 3) {
 // code block to be executed
}
Statement 1 is executed (one time) before the execution of the code block.
Statement 2 defines the condition for executing the code block.
Statement 3 is executed (every time) after the code block has been executed.
The example below will print the numbers 0 to 4:
Example
for (int i = 0; i < 5; i++) {
  System.out.println(i);
}
Example explained
Statement 1 sets a variable before the loop starts (int i = 0).
Statement 2 defines the condition for the loop to run (i must be less than
5). If the condition is true, the loop will start over again, if it is false,
the loop will end.
Statement 3 increases a value (i++) each time the code block in the loop has
been executed.
Another Example
This example will only print even values between 0 and 10:
Example
for (int i = 0; i <= 10; i = i + 2) {
  System.out.println(i);
}
Nested Loops
It is also possible to place a loop inside another loop. This is called a
nested loop.
The "inner loop" will be executed one time for each iteration of the "outer loop":
Example
// Outer loop
for (int i = 1; i <= 2; i++) {
System.out.println("Outer: " + i); // Executes 2 times
// Inner loop
for (int j = 1; j <= 3; j++) {
System.out.println(" Inner: " + j); // Executes 6 times (2 * 3)
}
}
For-Each Loop
There is also a "for-each" loop, which is used exclusively to loop through elements in an array:
Syntax
for(typevariableName:arrayName){
// code block to be executed
}
The following example outputs all elements in the cars array, using a "for-each" loop:
Example
String[] cars ={"Volvo","BMW","Ford","Mazda"};
for(String i : cars){
System.out.println(i);
}
Note: Don't worry if you don't understand the example above. You will learn more about Arrays in the Java Arrays chapter.
Java Break and Continue
Java Break
You have already seen the break statement used in an earlier chapter of this tutorial. It was used to "jump out" of a switch statement.
The break statement can also be used to jump out of a
loop.
This example stops the loop when i is equal to 4:
Example
for (int i = 0; i < 10; i++) {
 if (i == 4) {
   break;
 }
 System.out.println(i);
}
Java Continue
The continue statement breaks one iteration (in the loop), if a specified condition occurs, and continues with the next iteration in the loop.
This example skips the value of 4:
Example
for (int i = 0; i < 10; i++) {
 if (i == 4) {
   continue;
 }
 System.out.println(i);
}
Break and Continue in While Loop
You can also use break and continue in while loops:
Break Example
int i = 0;
while (i < 10) {
  System.out.println(i);
  i++;
  if (i == 4) {
    break;
 }
}
Continue Example
int i = 0;
while (i < 10) {
  if (i == 4) {
   i++;
    continue;
  }
  System.out.println(i);
  i++;
}
Java Arrays
Arrays are used to store multiple values in a single variable, instead of declaring separate variables for each value.
To declare an array, define the variable type with square brackets:
String[] cars;
We have now declared a variable that holds an array of strings. To insert values to it, you can place the values in a comma-separated list, inside curly braces:
String[] cars ={"Volvo","BMW","Ford","Mazda"};
To create an array of integers, you could write:
int[] myNum ={10,20,30,40};
Access the Elements of an Array
You can access an array element by referring to the index number.
This statement accesses the value of the first element in cars:
Example
String[] cars ={"Volvo","BMW","Ford","Mazda"};
System.out.println(cars[0]);
// Outputs Volvo
Note: Array indexes start with 0: [0] is the first element. [1] is the second element, etc.
Change an Array Element
To change the value of a specific element, refer to the index number:
Example
cars[0]="Opel";
Example
String[] cars ={"Volvo","BMW","Ford","Mazda"};
cars[0]="Opel";
System.out.println(cars[0]);
// Now outputs Opel instead of Volvo
Array Length
To find out how many elements an array has, use the length property:
Example
String[] cars ={"Volvo","BMW","Ford","Mazda"};
System.out.println(cars.length);
// Outputs 4
Java Arrays
Java Arrays
Arrays are used to store multiple values in a single variable, instead of declaring separate variables for each
value.
To declare an array, define the variable type with square brackets:
String[] cars;
We have now declared a variable that holds an array of strings. To insert
values to it, you can place the values in a
comma-separated list, inside curly braces:
You can loop through the array elements with the for loop, and use the length property to specify how many times the loop should run.
The following example outputs all elements in the cars array:
Example
String[] cars ={"Volvo","BMW","Ford","Mazda"};
for(int i =0; i < cars.length; i++){
System.out.println(cars[i]);
}
Loop Through an Array with For-Each
There is also a "for-each" loop, which is used exclusively to loop through elements in arrays:
Syntax
for(typevariable:arrayname){
...
}
The following example outputs all elements in the cars array, using a "for-each" loop:
Example
String[] cars ={"Volvo","BMW","Ford","Mazda"};
for(String i : cars){
System.out.println(i);
}
The example above can be read like this: for eachString element (called i - as in index) in cars, print out the value of i.
If you compare the for loop and for-each loop, you will see that the for-each method is easier to write, it does not require a counter (using the length property), and it is more readable.
Multidimensional Arrays
A multidimensional array is an array of arrays.
Multidimensional arrays are useful when you want to store data as a tabular form, like a table with rows and columns.
To create a two-dimensional array, add each array within its own set of curly braces:
Example
int[][] myNumbers ={{1,2,3,4},{5,6,7}};
myNumbers is now an array with two arrays as its elements.
Access Elements
To access the elements of the myNumbers array, specify two indexes: one for the array, and one for the element inside that array. This example accesses the third element (2) in the second array (1) of myNumbers:
Example
int[][] myNumbers ={{1,2,3,4},{5,6,7}};
System.out.println(myNumbers[1][2]);// Outputs 7
Remember that: Array indexes start with 0: [0] is the first element. [1] is the second element, etc.
Change Element Values
You can also change the value of an element:
Example
int[][] myNumbers ={{1,2,3,4},{5,6,7}};
myNumbers[1][2]=9;
System.out.println(myNumbers[1][2]);// Outputs 9 instead of 7
Loop Through a Multi-Dimensional Array
We can also use a for loop inside another for loop to get the elements of a two-dimensional array (we still have to point to the two indexes):
Example
publicclassMain{
publicstaticvoidmain(String[] args){
int[][] myNumbers ={{1,2,3,4},{5,6,7}};
for(int i =0; i < myNumbers.length;++i){
for(int j =0; j < myNumbers[i].
A method is a block of code which only runs when it is called.
You can pass data, known as parameters, into a method.
Methods are used to perform certain actions, and they are also known as functions.
Why use methods? To reuse code: define the code once, and use it many times.
Create a Method
A method must be declared within a class. It is defined with the name of the method, followed by parentheses (). Java provides some pre-defined methods, such as System.out.println(), but you can also create your own methods to perform certain actions:
Example
Create a method inside Main:
publicclassMain{
staticvoidmyMethod(){
// code to be executed
}
}
Example Explained
myMethod() is the name of the method
static means that the method belongs to the Main class and not an object of the Main class. You will learn more about objects and how to access methods through objects later in this tutorial.
void means that this method does not have a return value. You will learn more about return values later in this chapter
Call a Method
To call a method in Java, write the method's name followed by two parentheses () and a semicolon;
In the following example, myMethod() is used to print a text (the action), when it is called:
Example
Inside main, call the myMethod() method:
publicclassMain{
staticvoidmyMethod(){
System.out.println("I just got executed!");
}
publicstaticvoidmain(String[] args){
myMethod();
}
}
// Outputs "I just got executed!"
A method can also be called multiple times:
Example
publicclassMain{
staticvoidmyMethod(){
System.out.println("I just got executed!");
}
publicstaticvoidmain(String[] args){
myMethod();
myMethod();
myMethod();
}
}
// I just got executed!
// I just got executed!
// I just got executed!
In the next chapter, Method Parameters, you will learn how to pass data (parameters) into a method.
Parameters and Arguments
Information can be passed to methods as parameter. Parameters act as variables inside the method.
Parameters are specified after the method name, inside the parentheses. You can add as many parameters as you want, just separate them with a comma.
The following example has a method that takes a String called fname as parameter. When the method is called, we pass along a first name, which is used inside the method to print the full name:
Example
publicclassMain{
staticvoidmyMethod(String fname){
System.out.println(fname +" Refsnes");
}
publicstaticvoidmain(String[] args){
myMethod("Liam");
myMethod("Jenny");
myMethod("Anja");
}
}
// Liam Refsnes
// Jenny Refsnes
// Anja Refsnes
When a parameter is passed to the method, it is called an argument. So, from the example above: fname is a parameter, while Liam, Jenny and Anja are arguments.
Multiple Parameters
You can have as many parameters as you like:
Example
publicclassMain{
staticvoidmyMethod(String fname,int age){
System.out.println(fname +" is "+ age);
}
publicstaticvoidmain(String[] args){
myMethod("Liam",5);
myMethod("Jenny",8);
myMethod("Anja",31);
}
}
// Liam is 5
// Jenny is 8
// Anja is 31
Note that when you are working with multiple parameters, the method call must have the same number of arguments as there are parameters, and the arguments must be passed in the same order.
Return Values
The void keyword, used in the examples above, indicates that the method should not return a value. If you want the method to return a value, you can use a primitive data type (such as int, char, etc.) instead of void, and use the return keyword inside the method:
Example
publicclassMain{
static
Method Overloading
With method overloading, multiple methods can have the same name with different parameters:
Example
intmyMethod(int x)
floatmyMethod(float x)
doublemyMethod(double x,double y)
Consider the following example, which has two methods that add numbers of different type:
Example
staticintplusMethodInt(int x,int y){
return x + y;
}
staticdoubleplusMethodDouble(double x,double y){
return x + y;
}
publicstaticvoidmain(String[] args){
int myNum1 =plusMethodInt(8,5);
double myNum2 =plusMethodDouble(4.3,6.26);
System.out.println("int: "+ myNum1);
System.out.println("double: "+ myNum2);
}
Instead of defining two methods that should do the same thing, it is better to overload one.
In the example below, we overload the plusMethod method to work for both int and double:
Example
staticintplusMethod(int x,int y){
return x + y;
}
staticdoubleplusMethod(double x,double y){
return x + y;
}
publicstaticvoidmain(String[] args){
int myNum1 =plusMethod(8,5);
double myNum2 =plusMethod(4.3,
Java Scope
In Java, variables are only accessible inside the region they are created. This is called scope.
Method Scope
Variables declared directly inside a method are available anywhere in the method following the line of code in which they were declared:
Example
publicclassMain{
publicstaticvoidmain(String[] args){
// Code here CANNOT use x
int x =100;
// Code here can use x
System.out.println(x);
}
}
Block Scope
A block of code refers to all of the code between curly braces {}.
Variables declared inside blocks of code are only accessible by the code between the curly braces, which follows the line in which the variable was declared:
Example
publicclassMain{
publicstaticvoidmain(String[] args){
// Code here CANNOT use x
{// This is a block
// Code here CANNOT use x
int x =100;
// Code here CAN use x
System.out.println(x);
}// The block ends here
// Code here CANNOT use x
}
}
A block of code may exist on its own or it can belong to an if, while or for statement. In the case of for statements, variables declared in the statement itself are also available inside the block's scope.
Java Recursion
Recursion is the technique of making a function call itself. This technique provides a way to break complicated problems down into simple problems which are easier to solve.
Recursion may be a bit difficult to understand. The best way to figure out how it works is to experiment with it.
Recursion Example
Adding two numbers together is easy to do, but adding a range of numbers is more complicated. In the following example, recursion is used to add a range of numbers together by breaking it down into the simple task of adding two numbers:
Example
Use recursion to add all of the numbers up to 10.
publicclassMain{
publicstaticvoidmain(String[] args){
int result =sum(10);
System.out.println(result);
}
publicstaticintsum(int k){
if(k >0){
return k +sum(k -1);
}else{
return0;
}
}
}
Example Explained
When the sum() function is called, it adds parameter k to the sum of all numbers smaller than k and returns the result. When k becomes 0, the function just returns 0. When running, the program follows these steps:
Since the function does not call itself when k is 0, the program stops there and returns the result.
Halting Condition
Just as loops can run into the problem of infinite looping, recursive functions can run into the problem of infinite recursion. Infinite recursion is when the function never stops calling itself. Every recursive function should have a halting condition, which is the condition where the function stops calling itself. In the previous example, the halting condition is when the parameter k becomes 0.
It is helpful to see a variety of different examples to better understand the concept. In this example, the function adds a range of numbers between a start and an end. The halting condition for this recursive function is when end is not greater than start:
Example
Use recursion to add all of the numbers between 5 to 10.
publicclassMain{
publicstaticvoidmain(String[] args){
int result =sum(5,10);
System.out.println(result);
}
publicstaticintsum(int start,int end){
if(end > start){
return end +sum(start, end -1);
}else{
return end;
}
}
Java - What is OOP?
OOP stands for Object-Oriented Programming.
Procedural programming is about writing procedures or methods that perform operations on the data, while object-oriented programming is about creating objects that contain both data and methods.
Object-oriented programming has several advantages over procedural programming:
OOP is faster and easier to execute
OOP provides a clear structure for the programs
OOP helps to keep the Java code DRY "Don't Repeat Yourself", and makes the code easier to maintain, modify and debug
OOP makes it possible to create full reusable applications with less code and shorter development time
Tip: The "Don't Repeat Yourself" (DRY) principle is about reducing the repetition of code. You should extract out the codes that are common for the application, and place them at a single place and reuse them instead of repeating it.
Java - What are Classes and Objects?
Classes and objects are the two main aspects of object-oriented programming.
Look at the following illustration to see the difference between class and objects:
class
Fruit
objects
Apple
Banana
Mango
Another example:
class
Car
objects
Volvo
Audi
Toyota
So, a class is a template for objects, and an object is an instance of a class.
When the individual objects are created, they inherit all the variables and methods from the class.
You will learn much more about classes and objects in the next chapter.
Java Classes and Objects
Java Classes/Objects
Java is an object-oriented programming language.
Everything in Java is associated with classes and objects, along with its attributes and methods. For example: in real life, a car is an object. The car has attributes, such as weight and color, and methods, such as drive and brake.
A Class is like an object constructor, or a "blueprint" for creating objects.
Create a Class
To create a class, use the keyword class:
Main.java
Create a class named "Main" with a variable x:
public class Main {
int x = 5;
}
Remember from the that a class should always start with an uppercase first letter, and that the name of the java file should match the class name.
Create an Object
In Java, an object is created from a class. We have already created the class named
Main, so now we can use this to create objects.
To create an object of Main, specify the class name, followed by the object name, and use the keyword new:
Example
Create an object called "myObj" and print the value of x:
public class Main {
int x = 5;
public static void main(String[] args) {
Main myObj = new Main();
System.out.println(myObj.x);
}
}
Multiple Objects
You can create multiple objects of one class:
Example
Create two objects of Main:
public class Main {
int x = 5;
public static void main(String[] args) {
Main myObj1 = new Main(); // Object 1
Main myObj2 = new Main(); // Object 2
System.out.println(myObj1.x);
System.out.println(myObj2.x);
}
}
Using Multiple Classes
You can also create an object of a class and access it in another class. This is often used for better organization of classes (one class has all the attributes and methods, while the other class holds the main() method (code to be executed)).
Remember that the name of the java file should match the class name. In this example, we have created two files in the same directory/folder:
Main.java
Second.java
Main.java
public class Main {
int x = 5;
}
Second.java
class Second {
public static void main(String[] args) {
Main myObj = new Main();
System.out.println(myObj.x);
}
}
You will learn much more about classes and objects in the next chapters.
Java Class Attributes
Java Class Attributes
In the previous chapter, we used the term "variable" for x in the example (as shown below). It is actually an attribute of the class. Or you could say that class attributes are variables within a class:
Example
Create a class called "Main" with two attributes: x and y:
public class Main {
int x = 5;
int y = 3;
}
Another term for class attributes is fields.
Accessing Attributes
You can access attributes by creating an object of the class, and by using the dot syntax (.):
The following example will create an object of the Main class, with the name myObj. We use the x attribute on the object to print its value:
Example
Create an object called "myObj" and print the value of x:
public class Main {
int x = 5;
public static void main(String[] args) {
Main myObj = new Main();
System.out.println(myObj.x);
}
}
Modify Attributes
You can also modify attribute values:
Example
Set the value of x to 40:
public class Main {
int x;
public static void main(String[] args) {
Main myObj = new Main();
myObj.x = 40;
System.out.println(myObj.x);
}
}
Or override existing values:
Example
Change the value of x to 25:
public class Main {
int x = 10;
public static void main(String[] args) {
Main myObj = new Main();
myObj.x = 25; // x is now 25
System.out.println(myObj.x);
}
}
If you don't want the ability to override existing values, declare the attribute as final:
Example
public class Main {
final int x = 10;
public static void main(String[] args) {
Main myObj = new Main();
myObj.x = 25; // will generate an error: cannot assign a value to a final variable
System.out.println(myObj.x);
}
}
The final keyword is useful when you want a variable to always store the same value, like PI (3.14159...).
The final keyword is called a "modifier". You will learn more about these in the .
Multiple Objects
If you create multiple objects of one class, you can change the attribute values in one object, without affecting the attribute values in the other:
Example
Change the value of x to 25 in myObj2, and leave x in myObj1 unchanged:
public class Main {
int x = 5;
public static void main(String[] args) {
Main myObj1 = new Main(); // Object 1
Main myObj2 = new Main(); // Object 2
myObj2.x = 25;
System.out.println(myObj1.x); // Outputs 5
System.out.println(myObj2.x); // Outputs 25
}
}
Multiple Attributes
You can specify as many attributes as you want:
Example
public class Main {
String fname = "John";
String lname = "Doe";
int age = 24;
public static void main(String[] args) {
Main myObj = new Main();
System.out.println("Name: " + myObj.fname + " " + myObj.lname);
System.out.println("Age: " + myObj.age);
}
}
The next chapter will teach you how to create class methods and how to access them with objects.
Java Class Methods
Java Class Methods
You learned from the chapter that methods are declared within a
class, and that they are used to perform certain actions:
Example
Create a
method named myMethod() in Main:
public class Main {
 static void myMethod() {
   System.out.println("Hello World!");
 }
}
myMethod() prints a text (the action), when it is
called. To
call a method, write the method's name followed by two parentheses () and a semicolon;
Example
Inside main, call
myMethod():
public class Main {
 static void myMethod() {
   System.out.println("Hello World!");
 }
 public static void main(String[] args) {
   myMethod();
 }
}
// Outputs "Hello World!"
Static vs. Public
You will often see Java programs that have either static or public
attributes and methods.
In the example above, we created a static
method, which means that it can be accessed without creating an object of the class,
unlike public, which can only be accessed by
objects:
Example
An example to demonstrate the differences between static and publicmethods:
public class Main {
 // Static method
 static void myStaticMethod() {
   System.out.println("Static methods can be called without creating objects");
 }
 // Public method
 public void myPublicMethod() {
   System.out.println("Public methods must be called by creating objects");
 }
 // Main method
 public static void main(String[] args) {
   myStaticMethod(); // Call the static method
   // myPublicMethod(); This would compile an error
   Main myObj = new Main(); // Create an object of Main
   myObj.myPublicMethod(); // Call the public method on the object
 }
}
Note: You will learn more about these keywords (called modifiers) in the chapter.
Access Methods With an Object
Example
Create a Car object named myCar. Call the fullThrottle() and speed()
methods on the myCar object, and run the program:
// Create a Main class
public class Main {
Â
 // Create a fullThrottle() method
 public void fullThrottle() {
   System.out.println("The car is going as fast as it can!");
 }
 // Create a speed() method and add a parameter
 public void speed(int maxSpeed) {
   System.out.println("Max speed is: " + maxSpeed);
 }
 // Inside main, call the methods on the myCar object
 public static void main(String[] args) {
   Main myCar = new Main();  // Create a myCar object
   myCar.fullThrottle();     // Call the fullThrottle() method
   myCar.speed(200);        // Call the speed() method
 }
}
// The car is going as fast as it can!
// Max speed is: 200
Example explained
1) We created a custom Main class with the class keyword.
2) We created the fullThrottle() and
speed()
methods in the Main class.
3) The fullThrottle() method and the
speed()
method will print out some text, when they are called.
4) The speed()
method accepts an int parameter called
maxSpeed - we
will use this in 8).
5) In order to use the Main class and its
methods, we need to create an object of the
Main Class.
6) Then, go to the main() method, which you know by now is a built-in
Java method that runs your program (any code inside main is executed).
7) By using the new keyword we created an object with the name
myCar.
8) Then, we call the fullThrottle() and
speed()
methods on the
myCar object, and run the program using the name of the object (myCar), followed by a dot (.), followed by the name of the method (fullThrottle(); and
speed(200);).
Notice that we add an int parameter of 200 inside the
speed() method.
Remember that..
The dot (.) is used to access the object's attributes and methods.
To call a method in Java, write the method name followed by a set of parentheses (), followed by a semicolon (;).
A class must have a matching filename (Main and
Main.java).
Using Multiple Classes
Like we specified in the , it is
a good practice to create an object of a class and access it in another class.
Remember that the name of the java file should match the class name. In this
example, we have created two files in the same directory:
Main.java
Second.java
Main.java
public class Main {
 public void fullThrottle() {
   System.out.println("The car is going as fast as it can!");
 }
 public void speed(int maxSpeed) {
   System.out.println("Max speed is: " + maxSpeed);
 }
}
Second.java
class Second {
 public static void main(String[] args) {
   Main myCar = new Main();    // Create a myCar object
   myCar.fullThrottle();      // Call the fullThrottle() method
   myCar.speed(200);         // Call the speed() method
 }
}
The car is going as fast as it can!
Max speed is: 200
Java Constructors
A constructor in Java is a special method that is used to initialize objects. The constructor is called when an object of a class is created. It can be used to set initial values for object attributes:
Example
Create a constructor:
// Create a Main class
publicclassMain{
int x;// Create a class attribute
// Create a class constructor for the Main class
publicMain(){
x =5;// Set the initial value for the class attribute x
}
publicstaticvoidmain(String[] args){
Main myObj =newMain();// Create an object of class Main (This will call the constructor)
System.out.println(myObj.x);// Print the value of x
}
}
// Outputs 5
Note that the constructor name must match the class name, and it cannot have a return type (like void).
Also note that the constructor is called when the object is created.
All classes have constructors by default: if you do not create a class constructor yourself, Java creates one for you. However, then you are not able to set initial values for object attributes.
Constructor Parameters
Constructors can also take parameters, which is used to initialize attributes.
The following example adds an int y parameter to the constructor. Inside the constructor we set x to y (x=y). When we call the constructor, we pass a parameter to the constructor (5), which will set the value of x to 5:
Example
publicclassMain{
int x;
publicMain(int y){
x = y;
}
publicstaticvoidmain(String[] args){
Main myObj =newMain(5);
System.out.println(myObj.x);
}
}
// Outputs 5
You can have as many parameters as you want:
Example
publicclassMain{
int modelYear;
String modelName;
publicMain(int year,String name){
modelYear = year;
modelName = name<
Modifiers
By now, you are quite familiar with the public keyword that appears in almost all of our examples:
publicclassMain
The public keyword is an access modifier, meaning that it is used to set the access level for classes, attributes, methods and constructors.
We divide modifiers into two groups:
Access Modifiers - controls the access level
Non-Access Modifiers - do not control access level, but provides other functionality
Access Modifiers
For classes, you can use either public or default:
Modifier
Description
Try it
public
The class is accessible by any other class
Try it »
default
The class is only accessible by classes in the same package. This is used when you don't specify a modifier. You will learn more about packages in the Packages chapter
Try it »
For attributes, methods and constructors, you can use the one of the following:
Modifier
Description
Try it
public
The code is accessible for all classes
Try it »
private
The code is only accessible within the declared class
Try it »
default
The code is only accessible in the same package. This is used when you don't specify a modifier. You will learn more about packages in the Packages chapter
Try it »
protected
The code is accessible in the same package and subclasses. You will learn more about subclasses and superclasses in the Inheritance chapter
Try it »
Non-Access Modifiers
For classes, you can use either final or abstract:
Modifier
Description
Try it
final
The class cannot be inherited by other classes (You will learn more about inheritance in the Inheritance chapter)
Try it »
abstract
The class cannot be used to create objects (To access an abstract class, it must be inherited from another class. You will learn more about inheritance and abstraction in the Inheritance and Abstraction chapters)
Try it »
For attributes and methods, you can use the one of the following:
Modifier
Description
final
Attributes and methods cannot be overridden/modified
static
Attributes and methods belongs to the class, rather than an object
abstract
Can only be used in an abstract class, and can only be used on methods. The method does not have a body, for example abstract void run();. The body is provided by the subclass (inherited from). You will learn more about inheritance and abstraction in the Inheritance and Abstraction chapters
transient
Attributes and methods are skipped when serializing the object containing them
synchronized
Methods can only be accessed by one thread at a time
volatile
The value of an attribute is not cached thread-locally, and is always read from the "main memory"
Final
If you don't want the ability to override existing attribute values, declare attributes as final:
Example
publicclassMain{
finalint x =10;
finaldoublePI=3.14;
publicstaticvoidmain(String[] args){
Main myObj =newMain();
myObj.x =50;// will generate an error: cannot assign a value to a final variable
myObj.PI=25;// will generate an error: cannot assign a value to a final variable
System.out.println
Encapsulation
The meaning of Encapsulation, is to make sure that "sensitive" data is hidden from users. To achieve this, you must:
declare class variables/attributes as private
provide public get and set methods to access and update the value of a private variable
Get and Set
You learned from the previous chapter that private variables can only be accessed within the same class (an outside class has no access to it). However, it is possible to access them if we provide public get and set methods.
The get method returns the variable value, and the set method sets the value.
Syntax for both is that they start with either get or set, followed by the name of the variable, with the first letter in upper case:
Example
publicclassPerson{
privateString name;// private = restricted access
// Getter
publicStringgetName(){
return name;
}
// Setter
publicvoidsetName(String newName){
this.name = newName;
}
}
Example explained
The get method returns the value of the variable name.
The set method takes a parameter (newName) and assigns it to the name variable. The this keyword is used to refer to the current object.
However, as the name variable is declared as private, we cannot access it from outside this class:
Example
publicclassMain{
publicstaticvoidmain(String[] args){
Person myObj =newPerson();
myObj.name ="John";// error
System.out.println(myObj.name);// error
}
}
If the variable was declared as public, we would expect the following output:
John
However, as we try to access a private variable, we get an error:
MyClass.java:4: error: name has
private access in Person myObj.name
= "John"; ^ MyClass.java:5:
error: name has private access in Person System.out.println(myObj.name); ^ 2
errors
Instead, we use the getName() and setName() methods to access and update the variable:
Example
publicclassMain{
publicstaticvoidmain(String[] args){
Person myObj =newPerson();
myObj.setName("John");// Set the value of the name variable to "John"
System.out.println(myObj.getName());
}
}
// Outputs "John"
Why Encapsulation?
Better control of class attributes and methods
Class attributes can be made read-only (if you only use the get method), or write-only (if you only use the set method)
Flexible: the programmer can change one part of the code without affecting other parts
Increased security of data
Java Packages
Java Packages & API
A package in Java is used to group related classes. Think of it as
a folder in a file directory. We use packages to avoid name conflicts, and
to write a better maintainable code. Packages are divided into two categories:
Built-in Packages (packages from the Java API)
User-defined Packages (create your own packages)
Built-in Packages
The Java API is a library of prewritten classes, that are free to use, included in the
Java Development Environment.
The library contains components for managing input, database programming, and much much
more. The complete list can be found at Oracles website: .
The library is divided into packages and classes.
Meaning you can either import a single class (along with its methods and
attributes), or a whole package that contain
all the classes that belong to the specified package.
To use a class or a package from the library, you need to use the import
keyword:
Syntax
import package.name.Class; // Import a single class
import package.name.*; // Import the whole package
Import a Class
If you find a class you want to use, for example, the Scanner class, which is used to get
user input, write the following code:
Example
import java.util.Scanner;
In the example above, java.util is a package, while Scanner is a class of
the java.util package.
To use the Scanner class, create an object of the class and use any of the available methods found in the Scanner class documentation.
In our example, we will use the nextLine() method, which is used to read a
complete line:
Example
Using the Scanner class to get user input:
import java.util.Scanner;
class MyClass {
 public static void main(String[] args) {
   Scanner myObj = new Scanner(System.in);
   System.out.println("Enter username");
   String userName = myObj.nextLine();
   System.out.println("Username is: " + userName);
 }
}
Import a Package
There are many packages to choose from. In the previous example, we used the Scanner class from the java.util package. This package also contains date and time
facilities, random-number generator and other utility classes.
To import a whole package, end the sentence with an asterisk sign (*).
The following example
will import ALL the classes in the java.util package:
Example
import java.util.*;
User-defined Packages
To create your own package, you need to understand that Java uses a file system directory to store them. Just like folders on your computer:
package mypack;
class MyPackageClass {
 public static void main(String[] args) {
   System.out.println("This is my package!");
 }
}
Save the file as MyPackageClass.java, and compile it:
C:UsersYour Name>javac MyPackageClass.java
Then compile the package:
C:UsersYour Name>javac -d . MyPackageClass.java
This forces the compiler to create the "mypack" package.
The -d keyword specifies the destination for where to save the class file. You
can use any directory name, like c:/user (windows), or, if you want to keep
the package within the same directory, you can use the dot sign ".", like in
the example above.
Note: The package name should be written in lower case to avoid conflict with class names.
When we compiled the package in the example above, a new folder was created, called "mypack".
To run the MyPackageClass.java file, write the following:
C:UsersYour Name>java mypack.MyPackageClass
The output will be:
This is my package!
Java Inheritance (Subclass and Superclass)
In Java, it is possible to inherit attributes and methods from one class to another. We group the "inheritance concept" into two categories:
subclass (child) - the class that inherits from another class
superclass (parent) - the class being inherited from
To inherit from a class, use the extends keyword.
In the example below, the Car class (subclass) inherits the attributes and methods from the Vehicle class (superclass):
We set the brand attribute in Vehicle to a protectedaccess modifier. If it was set to private, the Car class would not be able to access it.
Why And When To Use "Inheritance"?
- It is useful for code reusability: reuse attributes and methods of an existing class when you create a new class.
Tip: Also take a look at the next chapter, Polymorphism, which uses inherited methods to perform different tasks.
The final Keyword
If you don't want other classes to inherit from a class, use the final keyword:
If you try to access a final class, Java will generate an error:
finalclassVehicle{
...
}
classCarextendsVehicle{
...
}
The output will be something like this:
Main.java:9: error: cannot inherit from final Vehicle class Main extends Vehicle { ^ 1 error)
Java Polymorphism
Polymorphism means "many forms", and it occurs when we have many classes that are related to each other by inheritance.
Like we specified in the previous chapter; Inheritance lets us inherit attributes and methods from another class. Polymorphism uses those methods to perform different tasks. This allows us to perform a single action in different ways.
For example, think of a superclass called Animal that has a method called animalSound(). Subclasses of Animals could be Pigs, Cats, Dogs, Birds - And they also have their own implementation of an animal sound (the pig oinks, and the cat meows, etc.):
Example
classAnimal{
publicvoidanimalSound(){
System.out.println("The animal makes a sound");
}
}
classPigextendsAnimal{
publicvoidanimalSound(){
System.out.println("The pig says: wee wee");
}
}
classDogextendsAnimal{
publicvoidanimalSound(){
System.out.println("The dog says: bow wow");
}
}
Remember from the Inheritance chapter that we use the extends keyword to inherit from a class.
Now we can create Pig and Dog objects and call the animalSound() method on both of them:
Example
classAnimal{
publicvoidanimalSound(){
System.out.println("The animal makes a sound");
}
}
classPigextendsAnimal{
publicvoidanimalSound(){
System.out.println("The pig says: wee wee");
}
}
classDogextendsAnimal{
publicvoidanimalSound(){
System.out.println("The dog says: bow wow");
}
}
classMain{
publicstaticvoidmain(String[] args){
Animal myAnimal =newAnimal();// Create a Animal object
Animal myPig =newPig();// Create a Pig object
Animal myDog =newDog();// Create a Dog object
myAnimal.animalSound();
myPig.animalSound();
myDog.animalSound();
}
}
Why And When To Use "Inheritance" and "Polymorphism"?
- It is useful for code reusability: reuse attributes and methods of an existing class when you create a new class.
Java Inner Classes
In Java, it is also possible to nest classes (a class within a class). The purpose of nested classes is to group classes that belong together, which makes your code more readable and maintainable.
To access the inner class, create an object of the outer class, and then create an object of the inner class:
Unlike a "regular" class, an inner class can be private or protected. If you don't want outside objects to access the inner class, declare the class as private:
If you try to access a private inner class from an outside class, an error occurs:
Main.java:13: error: OuterClass.InnerClass has private access in OuterClass OuterClass.InnerClass myInner = myOuter.new InnerClass(); ^
Static Inner Class
An inner class can also be static, which means that you can access it without creating an object of the outer class:
Example
Abstract Classes and Methods
Data abstraction is the process of hiding certain details and showing only essential information to the user. Abstraction can be achieved with either abstract classes or interfaces (which you will learn more about in the next chapter).
The abstract keyword is a non-access modifier, used for classes and methods:
Abstract class: is a restricted class that cannot be used to create objects (to access it, it must be inherited from another class).
Abstract method: can only be used in an abstract class, and it does not have a body. The body is provided by the subclass (inherited from).
An abstract class can have both abstract and regular methods:
abstract class Animal {
public abstract void animalSound();
public void sleep() {
System.out.println("Zzz");
}
}
From the example above, it is not possible to create an object of the Animal class:
Animal myObj = new Animal(); // will generate an error
To access the abstract class, it must be inherited from another class. Let's convert the Animal class we used in the Polymorphism chapter to an abstract class:
Remember from the Inheritance chapter that we use the extends keyword to inherit from a class.
ExampleGet your own Java Server
// Abstract class
abstract class Animal {
// Abstract method (does not have a body)
public abstract void animalSound();
// Regular method
public void sleep() {
System.out.println("Zzz");
}
}
// Subclass (inherit from Animal)
class Pig extends Animal {
public void animalSound() {
// The body of animalSound() is provided here
System.out.println("The pig says: wee wee");
}
}
class Main {
public static void main(String[] args) {
Pig myPig = new Pig(); // Create a Pig object
myPig.animalSound();
myPig.sleep();
}
}
Why And When To Use Abstract Classes and Methods?
To achieve security - hide certain details and only show the important details of an object.
Note: Abstraction can also be achieved with Interfaces, which you will learn more about in the next chapter.
Interfaces
Another way to achieve abstraction in Java, is with interfaces.
An interface is a completely "abstract class" that is used to group related methods with empty bodies:
Example
// interface
interfaceAnimal{
publicvoidanimalSound();// interface method (does not have a body)
publicvoidrun();// interface method (does not have a body)
}
To access the interface methods, the interface must be "implemented" (kinda like inherited) by another class with the implements keyword (instead of extends). The body of the interface method is provided by the "implement" class:
Example
// Interface
interfaceAnimal{
publicvoidanimalSound();// interface method (does not have a body)
publicvoidsleep();// interface method (does not have a body)
}
// Pig "implements" the Animal interface
classPigimplementsAnimal{
publicvoidanimalSound(){
// The body of animalSound() is provided here
System.out.println("The pig says: wee wee");
}
publicvoidsleep(){
// The body of sleep() is provided here
System.out.println("Zzz");
}
}
classMain{
publicstaticvoidmain(String[] args){
Pig myPig =newPig();// Create a Pig object
myPig.animalSound();
myPig.sleep();
}
}
Notes on Interfaces:
·Like abstract classes, interfaces cannot be used to create objects (in the example above, it is not possible to create an "Animal" object in the MyMainClass)
·Interface methods do not have a body - the body is provided by the "implement" class
·On implementation of an interface, you must override all of its methods
·Interface methods are by default abstract and public
·Interface attributes are by default public, static and final
·An interface cannot contain a constructor (as it cannot be used to create objects)
Why And When To Use Interfaces?
1) To achieve security - hide certain details and only show the important details of an object (interface).
2) Java does not support "multiple inheritance" (a class can only inherit from one superclass). However, it can be achieved with interfaces, because the class can implement multiple interfaces. Note: To implement multiple interfaces, separate them with a comma (see example below).
Multiple Interfaces
To implement multiple interfaces, separate them with a comma:
Example
interfaceFirstInterface{
Enums
An enum is a special "class" that represents a group of constants (unchangeable variables, like final variables).
To create an enum, use the enum keyword (instead of class or interface), and separate the constants with a comma. Note that they should be in uppercase letters:
Example
enumLevel{
LOW,
MEDIUM,
HIGH
}
You can access enum constants with the dot syntax:
Level myVar = Level.MEDIUM;
Enum is short for "enumerations", which means "specifically listed".
Enum inside a Class
You can also have an enum inside a class:
Example
publicclassMain{
enumLevel{
LOW,
MEDIUM,
HIGH
}
publicstaticvoidmain(String[] args){
Level myVar =Level.MEDIUM;
System.out.println(myVar);
}
}
The output will be:
MEDIUM
Enum in a Switch Statement
Enums are often used in switch statements to check for corresponding values:
Example
enumLevel{
LOW,
MEDIUM,
HIGH
}
publicclassMain{
publicstaticvoidmain(String[] args){
Level myVar =Level.MEDIUM;
switch(myVar){
caseLOW:
System.out.println("Low level");
break;
caseMEDIUM:
System.out.println("Medium level");
break;
caseHIGH:
System.out.println
Java User Input
The Scanner class is used to get user input, and it is found in the java.util package.
To use the Scanner class, create an object of the class and use any of the available methods found in the Scanner class documentation. In our example, we will use the nextLine() method, which is used to read Strings:
Example
import java.util.Scanner;// Import the Scanner class
classMain{
publicstaticvoidmain(String[] args){
Scanner myObj =newScanner(System.in);// Create a Scanner object
System.out.println("Enter username");
String userName = myObj.nextLine();// Read user input
System.out.println("Username is: "+ userName);// Output user input
}
}
If you don't know what a package is, read our Java Packages Tutorial.
Input Types
In the example above, we used the nextLine() method, which is used to read Strings. To read other types, look at the table below:
Method
Description
nextBoolean()
Reads a boolean value from the user
nextByte()
Reads a byte value from the user
nextDouble()
Reads a double value from the user
nextFloat()
Reads a float value from the user
nextInt()
Reads a int value from the user
nextLine()
Reads a String value from the user
nextLong()
Reads a long value from the user
nextShort()
Reads a short value from the user
In the example below, we use different methods to read data of various types:
Example
import java.util.Scanner;
classMain{
publicstaticvoidmain(String[] args){
Scanner myObj =newScanner(System.in);
System.out.println("Enter name, age and salary:");
// String input
String name = myObj.nextLine();
// Numerical input
Java Dates
Java does not have a built-in Date class, but we can import the java.time package to work with the date and time API. The package includes many date and time classes. For example:
Class
Description
LocalDate
Represents a date (year, month, day (yyyy-MM-dd))
LocalTime
Represents a time (hour, minute, second and nanoseconds (HH-mm-ss-ns))
LocalDateTime
Represents both a date and a time (yyyy-MM-dd-HH-mm-ss-ns)
DateTimeFormatter
Formatter for displaying and parsing date-time objects
If you don't know what a package is, read our Java Packages Tutorial.
Display Current Date
To display the current date, import the java.time.LocalDate class, and use its now() method:
Example
import java.time.LocalDate;// import the LocalDate class
publicclassMain{
publicstaticvoidmain(String[] args){
LocalDate myObj =LocalDate.now();// Create a date object
System.out.println(myObj);// Display the current date
}
}
The output will be:
2024-03-29
Display Current Time
To display the current time (hour, minute, second, and nanoseconds), import the java.time.LocalTime class, and use its now() method:
Example
import java.time.LocalTime;// import the LocalTime class
publicclassMain{
publicstaticvoidmain(String[] args){
LocalTime myObj =LocalTime.now();
System.out.println(myObj);
}
}
The output will be:
22:16:34.158328
Display Current Date and Time
To display the current date and time, import the java.time.LocalDateTime class, and use its now() method:
Example
import java.time.LocalDateTime;// import the LocalDateTime class
publicclassMain{
publicstaticvoidmain(String[] args){
LocalDateTime myObj =LocalDateTime.now();
System.out.println(myObj);
}
}
The output will be:
2024-03-29T22:16:34.158406
Java ArrayList
The ArrayList class is a resizable array, which can be found in the java.util package.
The difference between a built-in array and an ArrayList in Java, is that the size of an array cannot be modified (if you want to add or remove elements to/from an array, you have to create a new one). While elements can be added and removed from an ArrayList whenever you want. The syntax is also slightly different:
Example
Create an ArrayList object called cars that will store strings:
import java.util.ArrayList;// import the ArrayList class
ArrayList<String> cars =newArrayList<String>();// Create an ArrayList object
If you don't know what a package is, read our Java Packages Tutorial.
Add Items
The ArrayList class has many useful methods. For example, to add elements to the ArrayList, use the add() method:
Example
import java.util.ArrayList;
publicclassMain{
publicstaticvoidmain(String[] args){
ArrayList<String> cars =newArrayList<String>();
cars.add("Volvo");
cars.add("BMW");
cars.add("Ford");
cars.add("Mazda");
System.out.println(cars);
}
}
Access an Item
To access an element in the ArrayList, use the get() method and refer to the index number:
Example
cars.get(0);
Remember: Array indexes start with 0: [0] is the first element. [1] is the second element, etc.
Change an Item
To modify an element, use the set() method and refer to the index number:
Example
cars.set(0,"Opel");
Remove an Item
To remove an element, use the remove() method and refer to the index number:
Example
cars.remove(0);
To remove all the elements in the ArrayList, use the clear() method:
Example
cars.clear();
ArrayList Size
To find out how many elements an ArrayList have, use the size method:
Example
cars.size();
Loop Through an ArrayList
Loop through the elements of an ArrayList with a for loop, and use the size() method to specify how many times the loop should run:
Example
publicclassMain{
publicstaticvoid
Java LinkedList
In the previous chapter, you learned about the ArrayList class. The LinkedList class is almost identical to the ArrayList:
Example
// Import the LinkedList class
import java.util.LinkedList;
publicclassMain{
publicstaticvoidmain(String[] args){
LinkedList<String> cars =newLinkedList<String>();
cars.add("Volvo");
cars.add("BMW");
cars.add("Ford");
cars.add("Mazda");
System.out.println(cars);
}
}
ArrayList vs. LinkedList
The LinkedList class is a collection which can contain many objects of the same type, just like the ArrayList.
The LinkedList class has all of the same methods as the ArrayList class because they both implement the List interface. This means that you can add items, change items, remove items and clear the list in the same way.
However, while the ArrayList class and the LinkedList class can be used in the same way, they are built very differently.
How the ArrayList works
The ArrayList class has a regular array inside it. When an element is added, it is placed into the array. If the array is not big enough, a new, larger array is created to replace the old one and the old one is removed.
How the LinkedList works
The LinkedList stores its items in "containers." The list has a link to the first container and each container has a link to the next container in the list. To add an element to the list, the element is placed into a new container and that container is linked to one of the other containers in the list.
When To Use
Use an ArrayList for storing and accessing data, and LinkedList to manipulate data.
LinkedList Methods
For many cases, the ArrayList is more efficient as it is common to need access to random items in the list, but the LinkedList provides several methods to do certain operations more efficiently:
Method
Description
addFirst()
Adds an item to the beginning of the list.
addLast()
Add an item to the end of the list
removeFirst()
Remove an item from the beginning of the list.
removeLast()
Remove an item from the end of the list
getFirst()
Get the item at the beginning of the list
getLast()
Get the item at the end of the list
Java HashMap
In the ArrayList chapter, you learned that Arrays store items as an ordered collection, and you have to access them with an index number (int type). A HashMap however, store items in "key/value" pairs, and you can access them by an index of another type (e.g. a String).
One object is used as a key (index) to another object (value). It can store different types: String keys and Integer values, or the same type, like: String keys and String values:
Example
Create a HashMap object called capitalCities that will store Stringkeys and Stringvalues:
import java.util.HashMap;// import the HashMap class
To access a value in the HashMap, use the get() method and refer to its key:
Example
capitalCities.get("England");
Remove an Item
To remove an item, use the remove() method and refer to the key:
Example
capitalCities.remove("England");
To remove all items, use the clear() method:
Example
capitalCities.clear();
HashMap Size
To find out how many items there are, use the size() method:
Example
capitalCities.size();
Loop Through a HashMap
Loop through the items of a HashMap with a for-each loop.
Note:
Java HashSet
A HashSet is a collection of items where every item is unique, and it is found in the java.util package:
Example
Create a HashSet object called cars that will store strings:
import java.util.HashSet;// Import the HashSet class
HashSet<String> cars =newHashSet<String>();
Add Items
The HashSet class has many useful methods. For example, to add items to it, use the add() method:
Example
// Import the HashSet class
import java.util.HashSet;
publicclassMain{
publicstaticvoidmain(String[] args){
HashSet<String> cars =newHashSet<String>();
cars.add("Volvo");
cars.add("BMW");
cars.add("Ford");
cars.add("BMW");
cars.add("Mazda");
System.out.println(cars);
}
}
Note: In the example above, even though BMW is added twice it only appears once in the set because every item in a set has to be unique.
Check If an Item Exists
To check whether an item exists in a HashSet, use the contains() method:
Example
cars.contains("Mazda");
Remove an Item
To remove an item, use the remove() method:
Example
cars.remove("Volvo");
To remove all items, use the clear() method:
Example
cars.clear();
HashSet Size
To find out how many items there are, use the size method:
Example
cars.size();
Loop Through a HashSet
Loop through the items of an HashSet with a for-each loop:
Example
for(String i : cars){
System.out.println(i);
}
Other Types
Items in an HashSet are actually objects. In the examples above, we created items (objects) of type "String". Remember that a String in Java is an object (not a primitive type). To use other types, such as int, you must specify an equivalent wrapper class: Integer. For other primitive types, use: Boolean for boolean, Character for char, Double for double, etc:
Example
Use a HashSet that stores Integer objects:
Java Iterator
An Iterator is an object that can be used to loop through collections, like ArrayList and HashSet. It is called an "iterator" because "iterating" is the technical term for looping.
To use an Iterator, you must import it from the java.util package.
Getting an Iterator
The iterator() method can be used to get an Iterator for any collection:
Example
// Import the ArrayList class and the Iterator class
import java.util.ArrayList;
import java.util.Iterator;
publicclassMain{
publicstaticvoidmain(String[] args){
// Make a collection
ArrayList<String> cars =newArrayList<String>();
cars.add("Volvo");
cars.add("BMW");
cars.add("Ford");
cars.add("Mazda");
// Get the iterator
Iterator<String> it = cars.iterator();
// Print the first item
System.out.println(it.next());
}
}
Looping Through a Collection
To loop through a collection, use the hasNext() and next() methods of the Iterator:
Example
while(it.hasNext()){
System.out.println(it.next());
}
Removing Items from a Collection
Iterators are designed to easily change the collections that they loop through. The remove() method can remove items from a collection while looping.
Example
Use an iterator to remove numbers less than 10 from a collection:
Wrapper classes provide a way to use primitive data types (int, boolean, etc..) as objects.
The table below shows the primitive type and the equivalent wrapper class:
Primitive Data Type
Wrapper Class
byte
Byte
short
Short
int
Integer
long
Long
float
Float
double
Double
boolean
Boolean
char
Character
Sometimes you must use wrapper classes, for example when working with Collection objects, such as ArrayList, where primitive types cannot be used (the list can only store objects):
To create a wrapper object, use the wrapper class instead of the primitive type. To get the value, you can just print the object:
Example
publicclassMain{
publicstaticvoidmain(String[] args){
Integer myInt =5;
Double myDouble =5.99;
Character myChar ='A';
System.out.println(myInt);
System.out.println(myDouble);
System.out.println(myChar);
}
}
Since you're now working with objects, you can use certain methods to get information about the specific object.
For example, the following methods are used to get the value associated with the corresponding wrapper object: intValue(), byteValue(), shortValue(), longValue(), floatValue(), doubleValue(), charValue(), booleanValue().
This example will output the same result as the example above:
Example
publicclassMain{
publicstaticvoidmain(String[] args){
Integer myInt =5;
Double myDouble =5.99;
Character myChar ='A';
System.out.println(
Java Exceptions
When executing Java code, different errors can occur: coding errors made by the programmer, errors due to wrong input, or other unforeseeable things.
When an error occurs, Java will normally stop and generate an error message. The technical term for this is: Java will throw an exception (throw an error).
Java try and catch
The try statement allows you to define a block of code to be tested for errors while it is being executed.
The catch statement allows you to define a block of code to be executed, if an error occurs in the try block.
The try and catch keywords come in pairs:
Syntax
try{
//Block of code to try
}
catch(Exceptione){
//Block of code to handle errors
}
Consider the following example:
This will generate an error, because myNumbers[10] does not exist.
publicclassMain{
publicstaticvoidmain(String[] args){
int[] myNumbers ={1,2,3};
System.out.println(myNumbers[10]);// error!
}
}
The output will be something like this:
Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 10 at Main.main(Main.java:4)
If an error occurs, we can use try...catch to catch the error and execute some code to handle it:
Example
publicclassMain{
publicstaticvoidmain(String[] args){
try{
int[] myNumbers ={1,2,3};
System.out.println(myNumbers[10]);
}catch(Exception e){
System.out.println("Something went wrong.");
}
}
}
The output will be:
Something went wrong.
Finally
The finally statement lets you execute code, after try...catch, regardless of the result:
Example
publicclassMain{
publicstaticvoidmain(String[] args){
try{
int
What is a Regular Expression?
A regular expression is a sequence of characters that forms a search pattern. When you search for data in a text, you can use this search pattern to describe what you are searching for.
A regular expression can be a single character, or a more complicated pattern.
Regular expressions can be used to perform all types of text search and text replace operations.
Java does not have a built-in Regular Expression class, but we can import the java.util.regex package to work with regular expressions. The package includes the following classes:
Pattern Class - Defines a pattern (to be used in a search)
Matcher Class - Used to search for the pattern
PatternSyntaxException Class - Indicates syntax error in a regular expression pattern
Example
Find out if there are any occurrences of the word "w3schools" in a sentence:
In this example, The word "w3schools" is being searched for in a sentence.
First, the pattern is created using the Pattern.compile() method. The first parameter indicates which pattern is being searched for and the second parameter has a flag to indicates that the search should be case-insensitive. The second parameter is optional.
The matcher() method is used to search for the pattern in a string. It returns a Matcher object which contains information about the search that was performed.
The find() method returns true if the pattern was found in the string and false if it was not found.
Flags
Flags in the compile() method change how the search is performed. Here are a few of them:
Pattern.CASE_INSENSITIVE - The case of letters will be ignored when performing a search.
Pattern.LITERAL - Special characters in the pattern will not have any special meaning and will be treated as ordinary characters when performing a search.
Pattern.UNICODE_CASE - Use it together with the CASE_INSENSITIVE flag to also ignore the case of letters outside of the English alphabet
Regular Expression Patterns
The first parameter of the Pattern.compile() method is the pattern. It describes what is being searched for.
Brackets are used to find a range of characters:
Expression
Description
[abc]
Find one character from the options between the brackets
[^abc]
Find one character NOT between the brackets
[0-9]
Find one character from the range 0 to 9
Metacharacters
Metacharacters are characters with a special meaning:
Metacharacter
Description
|
Find a match for any one of the patterns separated by | as in: cat|dog|fish
.
Find just one instance of any character
^
Finds a match as the beginning of a string as in: ^Hello
$
Finds a match at the end of the string as in: World$
d
Find a digit
s
Find a whitespace character
b
Find a match at the beginning of a word like this: bWORD, or at the end of a word like this: WORDb
uxxxx
Find the Unicode character specified by the hexadecimal number xxxx
Quantifiers
Quantifiers define quantities:
Quantifier
Description
n+
Matches any string that contains at least one n
n*
Java Threads
Threads allows a program to operate more efficiently by doing multiple things at the same time.
Threads can be used to perform complicated tasks in the background without interrupting the main program.
Creating a Thread
There are two ways to create a thread.
It can be created by extending the Thread class and overriding its run() method:
Extend Syntax
publicclassMainextendsThread{
publicvoidrun(){
System.out.println("This code is running in a thread");
}
}
Another way to create a thread is to implement the Runnable interface:
Implement Syntax
publicclassMainimplementsRunnable{
publicvoidrun(){
System.out.println("This code is running in a thread");
}
}
Running Threads
If the class extends the Thread class, the thread can be run by creating an instance of the class and call its start() method:
Extend Example
publicclassMainextendsThread{
publicstaticvoidmain(String[] args){
Main thread =newMain();
thread.start();
System.out.println("This code is outside of the thread");
}
publicvoidrun(){
System.out.println("This code is running in a thread");
}
}
If the class implements the Runnable interface, the thread can be run by passing an instance of the class to a Thread object's constructor and then calling the thread's start() method:
Implement Example
publicclassMainimplementsRunnable{
publicstaticvoidmain(String[] args){
Main obj =newMain();
Thread thread =newThread(obj);
thread.start();
System.out.println
Java Threads
Threads allows a program to operate more efficiently by doing multiple things at the same time.
Threads can be used to perform complicated tasks in the background without interrupting the main program.
Creating a Thread
There are two ways to create a thread.
It can be created by extending the Thread class and overriding its run() method:
Extend Syntax
publicclassMainextendsThread{
publicvoidrun(){
System.out.println("This code is running in a thread");
}
}
Another way to create a thread is to implement the Runnable interface:
Implement Syntax
publicclassMainimplementsRunnable{
publicvoidrun(){
System.out.println("This code is running in a thread");
}
}
Running Threads
If the class extends the Thread class, the thread can be run by creating an instance of the class and call its start() method:
Extend Example
publicclassMainextendsThread{
publicstaticvoidmain(String[] args){
Main thread =newMain();
thread.start();
System.out.println("This code is outside of the thread");
}
publicvoidrun(){
System.out.println("This code is running in a thread");
}
}
If the class implements the Runnable interface, the thread can be run by passing an instance of the class to a Thread object's constructor and then calling the thread's start() method:
Implement Example
publicclassMainimplementsRunnable{
publicstaticvoidmain(String[] args){
Main obj =newMain();
Thread thread =newThread(obj);
thread.start();
System.out.
File handling is an important part of any application.
Java has several methods for creating, reading, updating, and deleting files.
Java File Handling
The File class from the java.io package, allows us to work with files.
To use the File class, create an object of the class, and specify the filename or directory name:
Example
import java.io.File;// Import the File class
File myObj =newFile("filename.txt");// Specify the filename
If you don't know what a package is, read our Java Packages Tutorial.
The File class has many useful methods for creating and getting information about files. For example:
Method
Type
Description
canRead()
Boolean
Tests whether the file is readable or not
canWrite()
Boolean
Tests whether the file is writable or not
createNewFile()
Boolean
Creates an empty file
delete()
Boolean
Deletes a file
exists()
Boolean
Tests whether the file exists
getName()
String
Returns the name of the file
getAbsolutePath()
String
Returns the absolute pathname of the file
length()
Long
Returns the size of the file in bytes
list()
String[]
Returns an array of the files in the directory
mkdir()
Boolean
Creates a directory
Create a File
To create a file in Java, you can use the createNewFile() method. This method returns a boolean value: true if the file was successfully created, and false if the file already exists. Note that the method is enclosed in a try...catch block. This is necessary because it throws an IOException if an error occurs (if the file cannot be created for some reason):
Example
import java.io.File;// Import the File class
import java.io.IOException;// Import the IOException class to handle errors
To create a file in a specific directory (requires permission), specify the path of the file and use double backslashes to escape the "" character (for Windows). On Mac and Linux you can just write the path, like: /Users/name/filename.txt
In the following example, we use the FileWriter class together with its write() method to write some text to the file we created in the example above. Note that when you are done writing to the file, you should close it with the close() method:
Example
import java.io.FileWriter;// Import the FileWriter class
import java.io.IOException;// Import the IOException class to handle errors
publicclassWriteToFile{
publicstaticvoidmain(String[] args){
try{
FileWriter myWriter =newFileWriter("filename.txt"
Read a File
In the previous chapter, you learned how to create and write to a file.
In the following example, we use the Scanner class to read the contents of the text file we created in the previous chapter:
Example
import java.io.File;// Import the File class
import java.io.FileNotFoundException;// Import this class to handle errors
import java.util.Scanner;// Import the Scanner class to read text files
publicclassReadFile{
publicstaticvoidmain(String[] args){
try{
File myObj =newFile("filename.txt");
Scanner myReader =newScanner(myObj);
while(myReader.hasNextLine()){
String data = myReader.nextLine();
System.out.println(data);
}
myReader.close();
}catch(FileNotFoundException e){
System.out.println("An error occurred.");
e.printStackTrace();
}
}
}
The output will be:
Files in Java might be tricky, but it is fun enough!
Get File Information
To get more information about a file, use any of the File methods:
To delete a file in Java, use the delete() method:
Example
import java.io.File;// Import the File class
publicclassDeleteFile{
publicstaticvoidmain(String[] args){
File myObj =newFile("filename.txt");
if(myObj.delete()){
System.out.println("Deleted the file: "+ myObj.getName());
}else{
System.out.println("Failed to delete the file.");
}
}
}
The output will be:
Deleted the file: filename.txt
Delete a Folder
You can also delete a folder. However, it must be empty:
Example
import java.io.File;
publicclassDeleteFolder{
publicstaticvoidmain(String[] args){
File myObj =newFile("C:\Users\MyName\Test");
if(myObj.delete()){
System.out.println("Deleted the folder: "+ myObj.getName());
}else{
System.out.println("Failed to delete the folder.");
}
}
}
The output will be:
Deleted the folder: Test
Add Two Numbers
Learn how to add two numbers in Java:
Example
int x =5;
int y =6;
int sum = x + y;
System.out.println(sum);// Print the sum of x + y
Add Two Numbers with User Input
Learn how to add two numbers with user input:
Example
import java.util.Scanner;// Import the Scanner class
classMyClass{
publicstaticvoidmain(String[] args){
int x, y, sum;
Scanner myObj =newScanner(System.in);// Create a Scanner object
System.out.println("Type a number:");
x = myObj.nextInt();// Read user input
System.out.println("Type another number:");
y = myObj.nextInt();// Read user input
sum = x + y;// Calculate the sum of x + y
System.out.println("Sum is: "+ sum);// Print the sum
}
}
Java How To Count Words
Count Number of Words in a String
You can easily count the number of words in a string with the following example:
Example
String words = "One Two Three Four";
int countWords = words.split("\s").length;
System.out.println(countWords);
Java How To Reverse a String
Reverse a String
You can easily reverse a string by characters with the following example:
Example
String originalStr = "Hello";
String reversedStr = "";
for (int i = 0; i < originalStr.length(); i++) {
reversedStr = originalStr.charAt(i) + reversedStr;
}
System.out.println("Reversed string: "+ reversedStr);
Java How To Calculate the Sum of Elements
Calculate the Sum of an Array
Get the sum of array elements:
Example
int[] myArray = {1, 5, 10, 25};
int sum = 0;
int i;
// Loop through the array elements and store the sum in the sum variable
for (i = 0; i < myArray.length; i++) {
sum += myArray[i];
}
System.out.println("The sum is: " + sum);
Java How To Get the Area of a Rectangle
Area of Rectangle
The area of a rectangle can be found by multiplying the length of the
rectangle by the width:
Example
int length = 5;
int width = 2;
int area = length * width;
System.out.println("Area of rectangle: " + area);
Java How To Find Even or Odd Numbers
Check Whether a Number is Even or Odd
Find out if a number is even or odd:
Example
int number = 5;
// Find out if the number above is even or odd
if (number % 2 == 0) {
System.out.println(number + " is even.");
} else {
System.out.println(number + " is odd.");
}
Java Reserved Keywords
Java has a set of keywords that are reserved words that cannot be used as variables, methods, classes, or any other identifiers:
Keyword
Description
abstract
A non-access modifier. Used for classes and methods: An abstract class cannot be used to create objects (to access it, it must be inherited from another class). An abstract method can only be used in an abstract class, and it does not have a body. The body is provided by the subclass (inherited from)
assert
For debugging
boolean
A data type that can only store true or false values
break
Breaks out of a loop or a switch block
byte
A data type that can store whole numbers from -128 and 127
case
Marks a block of code in switch statements
catch
Catches exceptions generated by try statements
char
A data type that is used to store a single character
class
Defines a class
continue
Continues to the next iteration of a loop
const
Defines a constant. Not in use - use final instead
default
Specifies the default block of code in a switch statement
do
Used together with while to create a do-while loop
double
A data type that can store whole numbers from 1.7e−308 to 1.7e+308
else
Used in conditional statements
enum
Declares an enumerated (unchangeable) type
exports
Exports a package with a module. New in Java 9
extends
Extends a class (indicates that a class is inherited from another class)
final
A non-access modifier used for classes, attributes and methods, which makes them non-changeable (impossible to inherit or override)
finally
Used with exceptions, a block of code that will be executed no matter if there is an exception or not
float
A data type that can store whole numbers from 3.4e−038 to 3.4e+038
for
Create a for loop
goto
Not in use, and has no function
if
Makes a conditional statement
implements
Implements an interface
import
Used to import a package, class or interface
instanceof
Checks whether an object is an instance of a specific class or an interface
int
A data type that can store whole numbers from -2147483648 to 2147483647
interface
Used to declare a special type of class that only contains abstract methods
long
A data type that can store whole numbers from -9223372036854775808 to 9223372036854775808
module
Declares a module. New in Java 9
native
Specifies that a method is not implemented in the same Java source file (but in another language)
new
Creates new objects
package
Declares a package
private
An access modifier used for attributes, methods and constructors, making them only accessible within the declared class
protected
An access modifier used for attributes, methods and constructors, making them accessible in the same package and subclasses
public
An access modifier used for classes, attributes, methods and constructors, making them accessible by any other class
requires
Specifies required libraries inside a module. New in Java 9
return
Finished the execution of a method, and can be used to return a value from a method
short
A data type that can store whole numbers from -32768 to 32767
static
A non-access modifier used for methods and attributes. Static methods/attributes can be accessed without creating an object of a class
strictfp
Restrict the precision and rounding of floating point calculations
super
Refers to superclass (parent) objects
switch
Selects one of many code blocks to be executed
synchronized
A non-access modifier, which specifies that methods can only be accessed by one thread at a time
this
Refers to the current object in a method or constructor
throw
Creates a custom error
throws
Indicates what exceptions may be thrown by a method
transient
A non-accesss modifier, which specifies that an attribute is not part of an object's persistent state
try
Creates a try...catch statement
var
Declares a variable. New in Java 10
void
Specifies that a method should not have a return value
volatile
Indicates that an attribute is not cached thread-locally, and is always read from the "main memory"
while
Creates a while loop
Note:
ExampleGet your own Java Server
An abstract method belongs to an abstract class, and it does not have a body. The body is provided by the subclass:
// Code from filename: Main.java
// abstract class abstract class Main {
publicString fname ="John";
publicint age =24;
publicabstractvoidstudy();// abstract method
}
// Subclass (inherit from Main)
classStudentextendsMain{
publicint graduationYear =2018;
publicvoidstudy(){// the body of the abstract method is provided here
System.out.println("Studying all day long");
}
}
// End code from filename: Main.java
// Code from filename: Second.java
classSecond{
publicstaticvoidmain(String[] args){
// create an object of the Student class (which inherits attributes and methods from Main)
The abstract keyword is a non-access modifier, used for classes and methods.
Class: An abstract class is a restricted class that cannot be used to create objects (to access it, it must be inherited from another class).
Method: An abstract method can only be used in an abstract class, and it does not have a body. The body is provided by the subclass (inherited from).
Related Pages
Read more about modifiers in our Java Modifiers Tutorial.
Example
A boolean data type with true or false values:
boolean isJavaFun = true;
boolean isFishTasty = false;
System.out.println(isJavaFun);Â Â Â Â // Outputs true
System.out.println(isFishTasty);Â Â // Outputs false
Definition and Usage
The boolean keyword is a data type that can only take the values true or false.
Boolean values are mostly used for conditional testing (read the for more information).
Related Pages
Read more about data types in our .
Read more about booleans in our .
Example
End the loop when i is equal to 4:
for(int i =0; i <10; i++){
if(i ==4){
break;
}
System.out.println(i);
}
Definition and Usage
The break keyword is used to break out a for loop, a while loop or a switch block.
More Examples
Example
Break out of a while loop:
int i =0; while(i <10){
System.out.println(i);
i++;
if(i ==4){
break;
}
}
Related Pages
Use the continue keyword to end the current iteration in a loop, but continue with the next.
Read more about for loops in our Java For Loops Tutorial.
Read more about while loops in our Java While Loops Tutorial.
Read more about break and continue in our Java Break Tutorial.
Example
byte myNum =100;
System.out.println(myNum);
Definition and Usage
The byte keyword is a data type that can store whole numbers from -128 to 127.
Related Pages
Read more about data types in our Java Data Types Tutorial.
Example
Calculate the weekday name:
int day =4;
switch(day){
case1:
System.out.println("Monday");
break;
case2:
System.out.println("Tuesday");
break;
case3:
System.out.println("Wednesday");
break;
case4:
System.out.println("Thursday");
break;
case5:
System.out.println("Friday");
break;
case6:
System.out.println("Saturday");
break;
case7:
System.out.println("Sunday");
break;
}
// Outputs "Thursday" (day 4)
Definition and Usage
The case keyword marks a block of code in a switch statement.
Related Pages
Read more about the switch statement in our Java Switch Tutorial.
Example
If an error occur, use try...catch to catch the error and execute some code to handle it:
try{
int[] myNumbers ={1,2,3};
System.out.println(myNumbers[10]);
}catch(Exception e){
System.out.println("Something went wrong.");
}
Definition and Usage
The catch keyword catches exceptions generated by try statements.
The catch statement allows you to define a block of code to be executed, if an error occurs in the try block.
Related Pages
Read more about exceptions in our Java Try..Catch Tutorial.
Example
char myGrade ='B';
System.out.println(myGrade);
Definition and Usage
The char keyword is a data type that is used to store a single character.
A char value must be surrounded by single quotes, like 'A' or 'c'.
Related Pages
Read more about data types in our Java Data Types Tutorial.
Example
char myGrade ='B';
System.out.println(myGrade);
Definition and Usage
The char keyword is a data type that is used to store a single character.
A char value must be surrounded by single quotes, like 'A' or 'c'.
Related Pages
Read more about data types in our Java Data Types Tutorial.
Example
Skip the iteration if the variable i is 4, but continue with the next iteration:
for(int i =0; i <10; i++){
if(i ==4){
continue;
}
System.out.println(i);
}
Definition and Usage
The continue keyword is used to end the current iteration in a for loop (or a while loop), and continues to the next iteration.
More Examples
Example
Use the continue keyword in a while loop
int i =0; while(i <10){
if(i ==4){
i++;
continue;
}
System.out.println(i);
i++;
}
Related Pages
Use the break keyword to break out of a loop.
Read more about for loops in our Java For Loops Tutorial.
Read more about while loops in our Java While Loops Tutorial.
Read more about break and continue in our Java Break Tutorial.
Example
Specify some code to run if there is no case match in a switch block:
int day =4;
switch(day){
case6:
System.out.println("Today is Saturday");
break;
case7:
System.out.println("Today is Sunday");
break;
default:
System.out.println("Looking forward to the Weekend");
}
/Outputs"Looking forward to the Weekend"
Definition and Usage
The default keyword the default block of code in a switch statement.
The default keyword specifies some code to run if there is no case match in the switch.
Note: if the default keyword is used as the last statement in a switch block, it does not need a break.
Related Pages
Read more about the switch statement in our Java Switch Tutorial.
Example
The following loop will always be executed at least once, even if the condition is false, because the code block is executed before the condition is tested:
int i =0;
do{
System.out.println(i);
i++;
}
while(i <5);
Definition and Usage
The do keyword is used together with while to create a do-while loop.
The while loop loops through a block of code as long as a specified condition is true:
The do/while loop is a variant of the while loop. This loop will execute the code block once, before checking if the condition is true, then it will repeat the loop as long as the condition is true.
Note: Do not forget to increase the variable used in the condition, otherwise the loop will never end!
Related Pages
Read more about while loops in our Java While Loop Tutorial.
Example
double myNum =19.99d;
System.out.println(myNum);
Definition and Usage
The double keyword is a data type that can store fractional numbers from 1.7e−308 to 1.7e+308.
Note that you should end the value with a "d":
Related Pages
Read more about data types in our Java Data Types Tutorial.
Example
Use the else statement to specify a block of code to be executed if the condition is false.
int time =20;
if(time <18){
System.out.println("Good day.");
}else{
System.out.println("Good evening.");
}
// Outputs "Good evening."
Definition and Usage
The else statement specifies a block of Java code to be executed if a condition is false in an if statement.
Java has the following conditional statements:
Use if to specify a block of code to be executed, if a specified condition is true
Use else to specify a block of code to be executed, if the same condition is false
Use else if to specify a new condition to test, if the first condition is false
Use switch to specify many alternative blocks of code to be executed
More Examples
Example
Use the else if statement to specify a new condition if the first condition is false.
int time =22;
if(time <10){
System.out.println("Good morning.");
}elseif(time <20){
System.out.println("Good day.");
}else{
System.out.println("Good evening.");
}
// Outputs "Good evening."
Related Pages
Read more about conditions in our Java If...Else Tutorial.
Example
Use the else statement to specify a block of code to be executed if the condition is false.
int time =20;
if(time <18){
System.out.println("Good day.");
}else{
System.out.println("Good evening.");
}
// Outputs "Good evening."
Definition and Usage
The else statement specifies a block of Java code to be executed if a condition is false in an if statement.
Java has the following conditional statements:
Use if to specify a block of code to be executed, if a specified condition is true
Use else to specify a block of code to be executed, if the same condition is false
Use else if to specify a new condition to test, if the first condition is false
Use switch to specify many alternative blocks of code to be executed
More Examples
Example
Use the else if statement to specify a new condition if the first condition is false.
int time =22;
if(time <10){
System.out.println("Good morning.");
}elseif(time <20){
System.out.println("Good day.");
}else{
System.out.println("Good evening.");
}
// Outputs "Good evening."
Related Pages
Read more about conditions in our Java If...Else Tutorial.
Example
The Car class (subclass) inherits the attributes and methods from the Vehicle class (superclass):
The extends keyword extends a class (indicates that a class is inherited from another class).
In Java, it is possible to inherit attributes and methods from one class to another. We group the "inheritance concept" into two categories:
subclass (child) - the class that inherits from another class
superclass (parent) - the class being inherited from
To inherit from a class, use the extends keyword.
Related Pages
Read more about inheritance in our Java Inheritance Tutorial.
Example
Set a variable to final, to prevent it from being overridden/modified:
publicclassMain{
finalint x =10;
publicstaticvoidmain(String[] args){
Main myObj =newMain();
myObj.x =25;// will generate an error: cannot assign a value to a final variable
System.out.println(myObj.x);
}
}
Definition and Usage
The final keyword is a non-access modifier used for classes, attributes and methods, which makes them non-changeable (impossible to inherit or override).
The final keyword is useful when you want a variable to always store the same value, like PI (3.14159...).
The final keyword is called a "modifier". You will learn more about these in the Java Modifiers Chapter.
Related Pages
Read more about attributes our Java Class Attributes Tutorial.
Example
Execute code, after try...catch, regardless of the result:
try{
int[] myNumbers ={1,2,3};
System.out.println(myNumbers[10]);
}catch(Exception e){
System.out.println("Something went wrong.");
}finally{
System.out.println("The 'try catch' is finished.");
}
Definition and Usage
The finally keyword is used to execute code (used with exceptions - try..catch statements) no matter if there is an exception or not.
Related Pages
Read more about exceptions in our Java Try..Catch Tutorial.
Example
float myNum =5.75f;
System.out.println(myNum);
Definition and Usage
The float keyword is a data type that can store fractional numbers from 3.4e−038 to 3.4e+038.
The for loop loops through a block of code a number of times.
From the example above:
Statement 1 sets a variable before the loop starts (int i = 0).
Statement 2 defines the condition for the loop to run (i must be less than 5). If the condition is true, the loop will start over again, if it is false, the loop will end.
Statement 3 increases a value (i++) each time the code block in the loop has been executed.
More Examples
There is also a "for-each" loop, which is used exclusively to loop through elements in an array:
The following example outputs all elements in the cars array, using a "for-each" loop:
Example
String[] cars ={"Volvo","BMW","Ford","Mazda"};
for(String i : cars){
System.out.println(i);
}
Related Pages
Read more about for loops in our Java For Loop Tutorial.
Example
Test two values to find out if 20 is greater than 18. If the condition is true, print some text:
if(20>18){
System.out.println("20 is greater than 18");
}
Definition and Usage
The if statement specifies a block of Java code to be executed if a condition is true.
Java has the following conditional statements:
Use if to specify a block of code to be executed, if a specified condition is true
Use else to specify a block of code to be executed, if the same condition is false
Use else if to specify a new condition to test, if the first condition is false
Use switch to specify many alternative blocks of code to be executed
More Examples
Example
Use the if statement to test variables:
int x =20;
int y =18;
if(x > y){
System.out.println("x is greater than y");
}
Example
Use the else statement to specify a block of code to be executed if the condition is false.
int time =20;
if(time <18){
System.out.println("Good day.");
}else{
System.out.println("Good evening.");
}
// Outputs "Good evening."
Example
Use the else if statement to specify a new condition if the first condition is false.
int time =22;
if(time <10){
System.out.println("Good morning.");
}elseif(time <20){
System.out.println("Good day.");
}else{
System.out.println("Good evening.");
}
// Outputs "Good evening."
Related Pages
Read more about conditions in our Java If...Else Tutorial.
Example
An interface is an abstract "class" that is used to group related methods with "empty" bodies:
To access the interface methods, the interface must be "implemented" (kinda like inherited) by another class with the implements keyword (instead of extends). The body of the interface method is provided by the "implement" class:
// interface
interfaceAnimal{
publicvoidanimalSound();// interface method (does not have a body)
publicvoidsleep();// interface method (does not have a body)
}
// Pig "implements" the Animal interface
classPigimplementsAnimal{
publicvoidanimalSound(){
// The body of animalSound() is provided here
System.out.println("The pig says: wee wee");
}
publicvoidsleep(){
// The body of sleep() is provided here
System.out.println("Zzz");
}
}
classMyMainClass{
publicstaticvoidmain(String[] args){
Pig myPig =newPig();// Create a Pig object
myPig.animalSound();
myPig.sleep();
}
}
Definition and Usage
The implements keyword is used to implement an interface.
The interface keyword is used to declare a special type of class that only contains abstract methods.
To access the interface methods, the interface must be "implemented" (kinda like inherited) by another class with the implements keyword (instead of extends). The body of the interface method is provided by the "implement" class.
Notes on Interfaces:
It cannot be used to create objects (in the example above, it is not possible to create an "Animal" object in the MyMainClass)
Interface methods does not have a body - the body is provided by the "implement" class
On implementation of an interface, you must override all of its methods
Interface methods are by default abstract and public
Interface attributes are by default public, static and final
An interface cannot contain a constructor (as it cannot be used to create objects)
Why And When To Use Interfaces?
To achieve security - hide certain details and only show the important details of an object (interface).
Java does not support "multiple inheritance" (a class can only inherit from one superclass). However, it can be achieved with interfaces, because the class can implement multiple interfaces. Note: To implement multiple interfaces, separate them with a comma (see example below).
Multiple Interfaces
To implement multiple interfaces, separate them with a comma:
Example
interfaceFirstInterface{
publicvoidmyMethod();// interface method
}
interfaceSecondInterface{
publicvoidmyOtherMethod();// interface method
}
// DemoClass "implements" FirstInterface and SecondInterface
The instanceof keyword checks whether an object is an instance of a specific class or an interface.
The instanceof keyword compares the instance with type. The return value is either true or false.
Related Pages
Read more about objects in our Java Classes/Objects Tutorial.
Example
int myNum =100000;
System.out.println(myNum);
Definition and Usage
The int keyword is a data type that can store whole numbers from -2147483648 to 2147483647.
Related Pages
Read more about data types in our Java Data Types Tutorial.
ExampleGet your own Java Server
An interface is an abstract "class" that is used to group related methods with "empty" bodies:
To access the interface methods, the interface must be "implemented" (kinda like inherited) by another class with the implements keyword (instead of extends). The body of the interface method is provided by the "implement" class:
// interface
interface Animal {
public void animalSound(); // interface method (does not have a body)
public void sleep(); // interface method (does not have a body)
}
// Pig "implements" the Animal interface
class Pig implements Animal {
public void animalSound() {
// The body of animalSound() is provided here
System.out.println("The pig says: wee wee");
}
public void sleep() {
// The body of sleep() is provided here
System.out.println("Zzz");
}
}
class MyMainClass {
public static void main(String[] args) {
Pig myPig = new Pig(); // Create a Pig object
myPig.animalSound();
myPig.sleep();
}
}
Try it Yourself »
Definition and Usage
The interface keyword is used to declare a special type of class that only contains abstract methods.
To access the interface methods, the interface must be "implemented" (kinda like inherited) by another class with the implements keyword (instead of extends). The body of the interface method is provided by the "implement" class.
Notes on Interfaces:
It cannot be used to create objects (in the example above, it is not possible to create an "Animal" object in the MyMainClass)
Interface methods does not have a body - the body is provided by the "implement" class
On implementation of an interface, you must override all of its methods
Interface methods are by default abstract and public
Interface attributes are by default public, static and final
An interface cannot contain a constructor (as it cannot be used to create objects)
Why And When To Use Interfaces?
To achieve security - hide certain details and only show the important details of an object (interface).
Java does not support "multiple inheritance" (a class can only inherit from one superclass). However, it can be achieved with interfaces, because the class can implement multiple interfaces. Note: To implement multiple interfaces, separate them with a comma (see example below).
Multiple Interfaces
To implement multiple interfaces, separate them with a comma:
Example
interface FirstInterface {
public void myMethod(); // interface method
}
interface SecondInterface {
public void myOtherMethod(); // interface method
}
// DemoClass "implements" FirstInterface and SecondInterface
class DemoClass implements FirstInterface, SecondInterface {
public void myMethod() {
System.out.println("Some text..");
}
public void myOtherMethod() {
System.out.println("Some other text...");
}
}
class MyMainClass {
public static void main(String[] args) {
DemoClass myObj = new DemoClass();
myObj.myMethod();
myObj.myOtherMethod();
}
}
Try it Yourself »
Related Pages
Read more about interfaces in our Java Interface Tutorial.
Example
long myNum =15000000000L;
System.out.println(myNum);
Definition and Usage
The long keyword is a data type that can store whole numbers from -9223372036854775808 to 9223372036854775808.
Note that you should end the value with an "L":
Related Pages
Read more about data types in our Java Data Types Tutorial.
Example
Create an object called "myObj" and print the value of x:
publicclassMain{
int x =5;
publicstaticvoidmain(String[] args){
MainmyObj=newMain();
System.out.println(myObj.x);
}
}
Definition and Usage
The new keyword creates new objects.
Related Pages
Read more about objects in our Java Classes/Objects Tutorial.
MyPackageClass.java
packagemypack;
classMyPackageClass{
publicstaticvoidmain(String[] args){
System.out.println("This is my package!");
}
}
Definition and Usage
The package keyword creates a package.
Related Pages
Read more about packages in our Java Packages Tutorial.
The protected keyword is an access modifier used for attributes, methods and constructors, making them accessible in the same package and subclasses.
Related Pages
Read more about modifiers in our Java Modifiers Tutorial.
Example
Second accesses a public
Main class with public attributes:
/* Code from filename: Main.java
public class Main {
 public String fname = "John";
 public String lname = "Doe";
 public String email = "";
 public int age = 24;
}
*/
class Second {
 public static void main(String[] args) {
   Main myObj = new Main();
   System.out.println("Name: " + myObj.fname + " " + myObj.lname);
   System.out.println("Email: " + myObj.email);
   System.out.println("Age: " + myObj.age);
 }
}
Definition and Usage
The public keyword is an access modifier used for classes, attributes, methods and constructors, making them accessible by any other class.
Related Pages
Read more about modifiers in our .
Example
A method with a return value:
public class Main {
 static int myMethod(int x) {
   return 5 + x;
 }
 public static void main(String[] args) {
   System.out.println(myMethod(3));
 }
}
// Outputs 8 (5 + 3)
Definition and Usage
The return keyword finished the execution of a method, and can be used to return a value from a method.
More Examples
Tip: Use the keyword to specify that a method should not have a return value:
Example
A method without any return values:
public class Main {
 static void myMethod() {
   System.out.println("I just got executed!");
 }
 public static void main(String[] args) {
   myMethod();
 }
}
Related Pages
Read more about methods in our .
Example
short myNum =5000;
System.out.println(myNum);
Definition and Usage
The short keyword is a data type that can store whole numbers from -32768 to 32767.
A static method can be accessed without creating an object of the class first:
publicclassMain{
// Static method
staticvoidmyStaticMethod(){
System.out.println("Static methods can be called without creating objects");
}
// Public method
publicvoidmyPublicMethod(){
System.out.println("Public methods must be called by creating objects");
}
// Main method
publicstaticvoidmain(String[] args){
myStaticMethod();// Call the static method
// myPublicMethod(); This would output an error
Main myObj =newMain();// Create an object of Main
myObj.myPublicMethod();// Call the public method
}
}
Definition and Usage
The static keyword is a non-access modifier used for methods and attributes. Static methods/attributes can be accessed without creating an object of a class.
Related Pages
Read more about modifiers in our Java Modifiers Tutorial.
Example
Using super to call the superclass of Dog (subclass):
classAnimal{// Superclass (parent)
publicvoidanimalSound(){
System.out.println("The animal makes a sound");
}
}
classDogextendsAnimal{// Subclass (child)
publicvoidanimalSound(){
super.animalSound();// Call the superclass method
System.out.println("The dog says: bow wow");
}
}
publicclassMain{
publicstaticvoidmain(String args[]){
Animal myDog =newDog();// Create a Dog object
myDog.animalSound();// Call the method on the Dog object
}
}
Definition and Usage
The super keyword refers to superclass (parent) objects.
It is used to call superclass methods, and to access the superclass constructor.
The most common use of the super keyword is to eliminate the confusion between superclasses and subclasses that have methods with the same name.
To understand the super keyword, you should have a basic understanding of Inheritance and Polymorphism.
Related Pages
Read more about inheritance (subclasses and superclasses) in our Java Inheritance Tutorial.
Read more about polymorphism in our Java Polymorphism Tutorial.
Example
Use the switch statement to calculate the weekday name:
int day =4;
switch(day){
case1:
System.out.println("Monday");
break;
case2:
System.out.println("Tuesday");
break;
case3:
System.out.println("Wednesday");
break;
case4:
System.out.println("Thursday");
break;
case5:
System.out.println("Friday");
break;
case6:
System.out.println("Saturday");
break;
case7:
System.out.println("Sunday");
break;
}
// Outputs "Thursday" (day 4)
Definition and Usage
The switch keyword selects one of many code blocks to be executed.
From the example above, it works like this:
The switch expression is evaluated once.
The value of the expression is compared with the values of each case.
If there is a match, the associated block of code is executed.
The break keyword is used to break out of the switch block when a match is found
The this keyword refers to the current object in a method or constructor.
The most common use of the this keyword is to eliminate the confusion between class attributes and parameters with the same name (because a class attribute is shadowed by a method or constructor parameter). If you omit the keyword in the example above, the output would be "0" instead of "5".
this can also be used to:
Invoke current class constructor
Invoke current class method
Return the current class object
Pass an argument in the method call
Pass an argument in the constructor call
Related Pages
Read more about objects in our Java Classes/Objects Tutorial.
Read more about constructors in our Java Constructors Tutorial.
Read more about methods in our Java Methods Tutorial.
Example
Throw an exception if age is below 18 (print "Access denied"). If age is 18 or older, print "Access granted":
publicclassMain{
staticvoidcheckAge(int age){
if(age <18){
thrownewArithmeticException("Access denied - You must be at least 18 years old.");
}
else{
System.out.println("Access granted - You are old enough!");
}
}
publicstaticvoidmain(String[] args){
checkAge(15);// Set age to 15 (which is below 18...)
}
}
Definition and Usage
The throw keyword is used to create a custom error.
The throw statement is used together with an exception type. There are many exception types available in Java: ArithmeticException, ClassNotFoundException, ArrayIndexOutOfBoundsException, SecurityException, etc.
The exception type is often used together with a custom method, like in the example above.
Differences between throw and throws:
throw
throws
Used to throw an exception for a method
Used to indicate what exception type may be thrown by a method
Cannot throw multiple exceptions
Can declare multiple exceptions
Syntax:
throw is followed by an object (new type)
used inside the method
Syntax:
throws is followed by a class
and used with the method signature
Related Pages
Read more about exceptions in our Java Try..Catch Tutorial.
Example
Throw an exception if age is below 18 (print "Access denied"). If age is 18 or older, print "Access granted":
thrownewArithmeticException("Access denied - You must be at least 18 years old.");
}
else{
System.out.println("Access granted - You are old enough!");
}
}
publicstaticvoidmain(String[] args){
checkAge(15);// Set age to 15 (which is below 18...)
}
}
Definition and Usage
The throws keyword indicates what exception type may be thrown by a method.
There are many exception types available in Java: ArithmeticException, ClassNotFoundException, ArrayIndexOutOfBoundsException, SecurityException, etc.
Differences between throw and throws:
throw
throws
Used to throw an exception for a method
Used to indicate what exception type may be thrown by a method
Cannot throw multiple exceptions
Can declare multiple exceptions
Syntax:
throw is followed by an object (new type)
used inside the method
Syntax:
throws is followed by a class
and used with the method signature
Related Pages
Read more about exceptions in our Java Try..Catch Tutorial.
Example
If an error occur, use try...catch to catch the error and execute some code to handle it:
try{
int[] myNumbers ={1,2,3};
System.out.println(myNumbers[10]);
}catch(Exception e){
System.out.println("Something went wrong.");
}
Definition and Usage
The try keyword creates a try...catch statement.
The try statement allows you to define a block of code to be tested for errors while it is being executed.
The catch statement allows you to define a block of code to be executed, if an error occurs in the try block.
Related Pages
Read more about exceptions in our Java Try..Catch Tutorial.
Example
A method without any return values:
publicclassMain{
staticvoidmyMethod(){
System.out.println("I just got executed!");
}
publicstaticvoidmain(String[] args){
myMethod();
}
}
Definition and Usage
The void keyword specifies that a method should not have a return value.
More Examples
Tip: If you want a method to return a value, you can use a primitive data type (such as int, char, etc.) instead of void, and use the return keyword inside the method:
In the example below, the code in the loop will run, over and over again, as long as a variable i is less than 5:
int i =0;
while(i <5){
System.out.println(i);
i++;
}
Definition and Usage
The while loop loops through a block of code as long as a specified condition is true.
Note: Do not forget to increase the variable used in the condition, otherwise the loop will never end!
More Examples
The do/while loop is a variant of the while loop. This loop will execute the code block once, before checking if the condition is true, then it will repeat the loop as long as the condition is true:
Example
int i =0;
do{
System.out.println(i);
i++;
}
while(i <5);
Related Pages
Read more about while loops in our Java While Loop Tutorial.
All String Methods
The String class has a set of built-in methods that you can use on strings.
Method
Description
Return Type
charAt()
Returns the character at the specified index (position)
char
codePointAt()
Returns the Unicode of the character at the specified index
int
codePointBefore()
Returns the Unicode of the character before the specified index
int
codePointCount()
Returns the number of Unicode values found in a string.
int
compareTo()
Compares two strings lexicographically
int
compareToIgnoreCase()
Compares two strings lexicographically, ignoring case differences
int
concat()
Appends a string to the end of another string
String
contains()
Checks whether a string contains a sequence of characters
boolean
contentEquals()
Checks whether a string contains the exact same sequence of characters of the specified CharSequence or StringBuffer
boolean
copyValueOf()
Returns a String that represents the characters of the character array
String
endsWith()
Checks whether a string ends with the specified character(s)
boolean
equals()
Compares two strings. Returns true if the strings are equal, and false if not
boolean
equalsIgnoreCase()
Compares two strings, ignoring case considerations
boolean
format()
Returns a formatted string using the specified locale, format string, and arguments
String
getBytes()
Encodes this String into a sequence of bytes using the named charset, storing the result into a new byte array
byte[]
getChars()
Copies characters from a string to an array of chars
void
hashCode()
Returns the hash code of a string
int
indexOf()
Returns the position of the first found occurrence of specified characters in a string
int
intern()
Returns the canonical representation for the string object
String
isEmpty()
Checks whether a string is empty or not
boolean
lastIndexOf()
Returns the position of the last found occurrence of specified characters in a string
int
length()
Returns the length of a specified string
int
matches()
Searches a string for a match against a regular expression, and returns the matches
boolean
offsetByCodePoints()
Returns the index within this String that is offset from the given index by codePointOffset code points
int
regionMatches()
Tests if two string regions are equal
boolean
replace()
Searches a string for a specified value, and returns a new string where the specified values are replaced
String
replaceFirst()
Replaces the first occurrence of a substring that matches the given regular expression with the given replacement
String
replaceAll()
Replaces each substring of this string that matches the given regular expression with the given replacement
String
split()
Splits a string into an array of substrings
String[]
startsWith()
Checks whether a string starts with specified characters
boolean
subSequence()
Returns a new character sequence that is a subsequence of this sequence
CharSequence
substring()
Returns a new string which is the substring of a specified string
String
toCharArray()
Converts this string to a new character array
char[]
toLowerCase()
Converts a string to lower case letters
String
toString()
Returns the value of a String object
String
toUpperCase()
Converts a string to upper case letters
String
trim()
Removes whitespace from both ends of a string
String
valueOf()
Returns the string representation of the specified value
String
The Java Math class has many methods that allows you to perform mathematical tasks on numbers.
All Math Methods
A list of all Math methods can be found in the table below:
Method
Description
Return Type
abs(x)
Returns the absolute value of x
double|float|int|long
acos(x)
Returns the arccosine of x, in radians
double
asin(x)
Returns the arcsine of x, in radians
double
atan(x)
Returns the arctangent of x as a numeric value between -PI/2 and PI/2 radians
double
atan2(y,x)
Returns the angle theta from the conversion of rectangular coordinates (x, y) to polar coordinates (r, theta).
double
cbrt(x)
Returns the cube root of x
double
ceil(x)
Returns the value of x rounded up to its nearest integer
double
copySign(x, y)
Returns the first floating point x with the sign of the second floating point y
double
cos(x)
Returns the cosine of x (x is in radians)
double
cosh(x)
Returns the hyperbolic cosine of a double value
double
exp(x)
Returns the value of Ex
double
expm1(x)
Returns ex -1
double
floor(x)
Returns the value of x rounded down to its nearest integer
double
getExponent(x)
Returns the unbiased exponent used in x
int
hypot(x, y)
Returns sqrt(x2 +y2) without intermediate overflow or underflow
double
IEEEremainder(x, y)
Computes the remainder operation on x and y as prescribed by the IEEE 754 standard
double
log(x)
Returns the natural logarithm (base E) of x
double
log10(x)
Returns the base 10 logarithm of x
double
log1p(x)
Returns the natural logarithm (base E) of the sum of x and 1
double
max(x, y)
Returns the number with the highest value
double|float|int|long
min(x, y)
Returns the number with the lowest value
double|float|int|long
nextAfter(x, y)
Returns the floating point number adjacent to x in the direction of y
double|float
nextUp(x)
Returns the floating point value adjacent to x in the direction of positive infinity
double|float
pow(x, y)
Returns the value of x to the power of y
double
random()
Returns a random number between 0 and 1
double
round(x)
Returns the value of x rounded to its nearest integer
int
rint(x)
Returns the double value that is closest to x and equal to a mathematical integer
double
signum(x)
Returns the sign of x
double
sin(x)
Returns the sine of x (x is in radians)
double
sinh(x)
Returns the hyperbolic sine of a double value
double
sqrt(x)
Returns the square root of x
double
tan(x)
Returns the tangent of an angle
double
tanh(x)
Returns the hyperbolic tangent of a double value
double
toDegrees(x)
Converts an angle measured in radians to an approx. equivalent angle measured in degrees
double
toRadians(x)
Converts an angle measured in degrees to an approx. angle measured in radians
double
ulp(x)
Returns the size of the unit of least precision (ulp) of x
double|float
Note: All Math methods are static.
Java Examples
Java Syntax
Java Comments
Java Variables
Java Data Types
Java Type Casting
Java Operators
Java Strings
Java Math
Java Booleans
Java If...Else (Conditions)
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Java Modifiers
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Java Regular Expressions
Java Threads
Java Files
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Java Operators
Operators are used to perform operations on variables and values.
In the example below, we use the +operator to add together two values:
Example
int x =100+50;
Although the + operator is often used to add together two values, like in the example above, it can also be used to add together a variable and a value, or a variable and another variable:
Example
int sum1 =100+50;// 150 (100 + 50)
int sum2 = sum1 +250;// 400 (150 + 250)
int sum3 = sum2 + sum2;// 800 (400 + 400)
Java divides the operators into the following groups:
Arithmetic operators
Assignment operators
Comparison operators
Logical operators
Bitwise operators
Arithmetic Operators
Arithmetic operators are used to perform common mathematical operations.
Operator
Name
Description
Example
+
Addition
Adds together two values
x + y
-
Subtraction
Subtracts one value from another
x - y
*
Multiplication
Multiplies two values
x * y
/
Division
Divides one value by another
x / y
%
Modulus
Returns the division remainder
x % y
++
Increment
Increases the value of a variable by 1
++x
--
Decrement
Decreases the value of a variable by 1
--x
Java Assignment Operators
Assignment operators are used to assign values to variables.
In the example below, we use the assignment operator (=) to assign the value 10 to a variable called x:
Example
int x =10;
The addition assignment operator (+=) adds a value to a variable:
