HTML Attributes


HTML attributes provide additional information about HTML elements.


HTML Attributes

  • All HTML elements can have attributes
  • Attributes provide additional information about elements
  • Attributes are always specified in the start tag
  • Attributes usually come in name/value pairs like: name="value"

The href Attribute

The <a> tag defines a hyperlink. The href attribute specifies the URL of the page the link goes to:

Example

<a href="https://www.w3schools.com">Visit W3Schools</a>

You will learn more about links in our .


The src Attribute

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:

<!DOCTYPE html>
<html lang="en">
<body>
...
</body>
</html>

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:

<!DOCTYPE html>
<html lang="en-US">
<body>
...
</body>
</html>

You can see all the language codes in our .


The title Attribute

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

public class Main {

  public static void main(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:

  1. Go to "System Properties" (Can be found on Control Panel > System and Security > System > Advanced System Settings)
  2. Click on the "Environment variables" button under the "Advanced" tab
  3. Then, select the "Path" variable in System variables and click on the "Edit" button
  4. 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
  5. 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

public class Main {

  public static void main(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

public class Main {

  public static void main(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:

public static void main(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:

public static void main(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 Systemout 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:

Example

int myNum = 5;
float myFloatNum = 5.99f;
char myLetter = 'D';
boolean myBool = true;
String myText = "Hello";

You will learn more about in the next section.


Test Yourself With Exercises

Exercise:

Create a variable named carName and assign the value Volvo to it.

  = ;



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:

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);


Test Yourself With Exercises

Exercise:

Fill in the missing parts to create three variables of the same type, using a comma-separated list:

 x = 5 y = 6 z = 50;



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


Java Data Types


Java Data Types

As explained in the previous chapter, a in Java must be a specified data type:

Example

int myNum = 5;               // Integer (whole number)
float myFloatNum = 5.99f;    // Floating point number
char myLetter = 'D';         // Character
boolean myBool = true;       // Boolean
String myText = "Hello";     // 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

Test Yourself With Exercises

Exercise:

Add the correct data type for the following variables:

 myNum = 9;
 myFloatNum = 8.99f;
 myLetter = 'A';
 myBool = false;
 myText = "Hello World";



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 byteshortint 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);     // Outputs true
System.out.println(isFishTasty);   // 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

public class Main {

  public static void main(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

public class Main {

  public static void main(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.

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());   // Outputs "HELLO WORLD"

System.out.println(txt.toLowerCase());   // Outputs "hello world"

Try it Yourself »


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):

Example

String txt = "Please locate where 'locate' occurs!";

System.out.println(txt.indexOf("locate")); // Outputs 7

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());   // Outputs "HELLO WORLD"
System.out.println(txt.toLowerCase());   // 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):

Example

String txt = "Please locate where 'locate' occurs!";
System.out.println(txt.indexOf("locate")); // Outputs 7

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.


Test Yourself With Exercises

Exercise:

Fill in the missing part to create a greeting variable of type String and assign it the value Hello.

 greeting = ;



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);     // Outputs true
System.out.println(isFishTasty);   // 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 to 18. 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.


Test Yourself With Exercises

Exercise:

Fill in the missing parts to print the values true and false:

 isJavaFun = true;
 isFishTasty = false;
System.out.println(isJavaFun);
System.out.println(isFishTasty);



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
} else if (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.");
} else if (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:

Syntax

variable = (condition) ? expressionTrue :  expressionFalse;

Instead of writing:

Example

int time = 20;
if (time < 18) {
  System.out.println("Good day.");
} else {
  System.out.println("Good evening.");
}

You can simply write:

Example

int time = 20;
String result = (time < 18) ? "Good day." : "Good evening.";
System.out.println(result);

 

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:

Syntax

variable = (condition) ? expressionTrue :  expressionFalse;

Instead of writing:

Example

int time = 20;
if (time < 18) {
  System.out.println("Good day.");
} else {
  System.out.println("Good evening.");
}

You can simply write:

Example

int time = 20;
String result = (time < 18) ? "Good day." : "Good evening.";
System.out.println(result);

 

Java Switch Statements

Instead of writing many if..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) {

  case 1:

    System.out.println("Monday");

    break;

  case 2:

    System.out.println("Tuesday");

    break;

  case 3:

    System.out.println("Wednesday");

    break;

  case 4:

    System.out.println("Thursday");

    break;

  case 5:

    System.out.println("Friday");

    break;

  case 6:

    System.out.println("Saturday");

    break;

  case 7:

    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++)