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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 = new HashSet<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;

 

public class Main {

  public static void main(String[] args) {

    HashSet<String> cars = new HashSet<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:

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