Tutorials References Exercises Videos Menu
Free Website Get Certified Pro

Java Tutorial

Java HOME Java Intro Java Get Started Java Syntax Java Output Java Comments Java Variables Java Data Types Java Type Casting Java Operators Java Strings Java Math Java Booleans Java If...Else Java Switch Java While Loop Java For Loop Java Break/Continue Java Arrays

Java Methods

Java Methods Java Method Parameters Java Method Overloading Java Scope Java Recursion

Java Classes

Java OOP Java Classes/Objects Java Class Attributes Java Class Methods Java Constructors Java Modifiers Java Encapsulation Java Packages / API Java Inheritance Java Polymorphism Java Inner Classes Java Abstraction Java Interface Java Enums Java User Input Java Date Java ArrayList Java LinkedList Java HashMap Java HashSet Java Iterator Java Wrapper Classes Java Exceptions Java RegEx Java Threads Java Lambda

Java File Handling

Java Files Java Create/Write Files Java Read Files Java Delete Files

Java How To

Add Two Numbers Count Words Reverse a String

Java Reference

Java Keywords Java String Methods Java Math Methods

Java Examples

Java Examples Java Compiler Java Exercises Java Quiz Java Certificate


Java HashSet


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);
  }
}

Try it Yourself »

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");

Try it Yourself »


Remove an Item

To remove an item, use the remove() method:

Example

cars.remove("Volvo");

Try it Yourself »

To remove all items, use the clear() method:

Example

cars.clear();

Try it Yourself »



HashSet Size

To find out how many items there are, use the size method:

Example

cars.size();

Try it Yourself »


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);
}

Try it Yourself »


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:

import java.util.HashSet;

public class Main {
  public static void main(String[] args) {

    // Create a HashSet object called numbers
    HashSet<Integer> numbers = new HashSet<Integer>();

    // Add values to the set
    numbers.add(4);
    numbers.add(7);
    numbers.add(8);

    // Show which numbers between 1 and 10 are in the set
    for(int i = 1; i <= 10; i++) {
      if(numbers.contains(i)) {
        System.out.println(i + " was found in the set.");
      } else {
        System.out.println(i + " was not found in the set.");
      }
    }
  }
}

Try it Yourself »