Java HashSet Example

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In this tutorial, we will explore the HashSet class in Java, which is a specialized implementation of the Set interface. The HashSet class provides the basic operations for a set, including adding, removing, and iterating over elements, while ensuring that there are no duplicate elements. This tutorial will demonstrate how to use HashSet with examples, covering all important operations and different ways for iteration using Java 8 features.

Table of Contents

1. Introduction
2. Prerequisites
3. Step-by-Step Guide
   1. Creating a HashSet
   2. Adding and Retrieving Elements
   3. Iterating Over the Set
   4. Removing Elements
   5. HashSet Methods
4. Complete Code Example
5. Conclusion

Introduction

HashSet is a part of Java's java.util package and implements the Set interface. It is backed by a HashMap, ensuring that the elements in the set are unique and unordered. HashSet is useful in scenarios where you need to store unique elements and do not care about the order of elements.

Prerequisites

Before we start, ensure you have the following:

• Java Development Kit (JDK) installed (latest version preferred)
• An Integrated Development Environment (IDE) such as IntelliJ IDEA or Eclipse

Step-by-Step Guide

Step 1: Creating a HashSet

First, let's create a HashSet and add some elements to it.

import java.util.HashSet; import java.util.Set; public class HashSetExample { public static void main(String[] args) { // Create a HashSet Set<String> hashSet = new HashSet<>(); // Add elements to the set hashSet.add("Ravi"); hashSet.add("Sita"); hashSet.add("Arjun"); hashSet.add("Lakshmi"); // Print the set System.out.println("HashSet: " + hashSet); } } 

Output:

HashSet: [Arjun, Lakshmi, Ravi, Sita] 

Step 2: Adding and Retrieving Elements

Let's add some elements to the HashSet and check if certain elements are present.

public class HashSetExample { public static void main(String[] args) { // Create a HashSet Set<String> hashSet = new HashSet<>(); // Add elements to the set hashSet.add("Ravi"); hashSet.add("Sita"); hashSet.add("Arjun"); hashSet.add("Lakshmi"); // Check if elements are present System.out.println("Contains Ravi: " + hashSet.contains("Ravi")); System.out.println("Contains Gopal: " + hashSet.contains("Gopal")); } } 

Output:

Contains Ravi: true Contains Gopal: false 

Step 3: Iterating Over the Set

We can iterate over the HashSet using a for-each loop, iterator, and Java 8 features like forEach and streams.

Using For-Each Loop

public class HashSetExample { public static void main(String[] args) { // Create a HashSet Set<String> hashSet = new HashSet<>(); // Add elements to the set hashSet.add("Ravi"); hashSet.add("Sita"); hashSet.add("Arjun"); hashSet.add("Lakshmi"); // Iterate over the set using for-each loop System.out.println("Iterating over HashSet using for-each loop:"); for (String element : hashSet) { System.out.println(element); } } } 

Output:

Iterating over HashSet using for-each loop: Arjun Lakshmi Ravi Sita 

Using Iterator

import java.util.Iterator; public class HashSetExample { public static void main(String[] args) { // Create a HashSet Set<String> hashSet = new HashSet<>(); // Add elements to the set hashSet.add("Ravi"); hashSet.add("Sita"); hashSet.add("Arjun"); hashSet.add("Lakshmi"); // Iterate over the set using iterator System.out.println("Iterating over HashSet using iterator:"); Iterator<String> iterator = hashSet.iterator(); while (iterator.hasNext()) { System.out.println(iterator.next()); } } } 

Output:

Iterating over HashSet using iterator: Arjun Lakshmi Ravi Sita 

Using forEach and Lambda Expression (Java 8)

public class HashSetExample { public static void main(String[] args) { // Create a HashSet Set<String> hashSet = new HashSet<>(); // Add elements to the set hashSet.add("Ravi"); hashSet.add("Sita"); hashSet.add("Arjun"); hashSet.add("Lakshmi"); // Iterate over the set using forEach and lambda System.out.println("Iterating over HashSet using forEach and lambda:"); hashSet.forEach(element -> System.out.println(element)); } } 

Output:

Iterating over HashSet using forEach and lambda: Arjun Lakshmi Ravi Sita 

Using Streams (Java 8)

import java.util.stream.Collectors; public class HashSetExample { public static void main(String[] args) { // Create a HashSet Set<String> hashSet = new HashSet<>(); // Add elements to the set hashSet.add("Ravi"); hashSet.add("Sita"); hashSet.add("Arjun"); hashSet.add("Lakshmi"); // Iterate over the set using streams System.out.println("Iterating over HashSet using streams:"); hashSet.stream().forEach(System.out::println); // Convert HashSet to a List using streams System.out.println("HashSet to List:"); hashSet.stream().collect(Collectors.toList()).forEach(System.out::println); } } 

Output:

Iterating over HashSet using streams: Arjun Lakshmi Ravi Sita HashSet to List: Arjun Lakshmi Ravi Sita 

Step 4: Removing Elements

Let's remove elements from the HashSet and demonstrate the use of the remove method.

public class HashSetExample { public static void main(String[] args) { // Create a HashSet Set<String> hashSet = new HashSet<>(); // Add elements to the set hashSet.add("Ravi"); hashSet.add("Sita"); hashSet.add("Arjun"); hashSet.add("Lakshmi"); // Remove an element hashSet.remove("Arjun"); // Print the set after removal System.out.println("HashSet after removal: " + hashSet); } } 

Output:

HashSet after removal: [Lakshmi, Ravi, Sita] 

Step 5: HashSet Methods

Here are some other important methods provided by the HashSet class:

• addAll(Collection<? extends E> c): Adds all of the elements in the specified collection to this set.
• clear(): Removes all of the elements from this set.
• isEmpty(): Returns true if this set contains no elements.
• size(): Returns the number of elements in this set.

import java.util.HashSet; import java.util.Set; public class HashSetMethodsExample { public static void main(String[] args) { // Create a HashSet Set<String> hashSet = new HashSet<>(); // Add elements to the set hashSet.add("Ravi"); hashSet.add("Sita"); hashSet.add("Arjun"); hashSet.add("Lakshmi"); // Use addAll method Set<String> anotherSet = new HashSet<>(); anotherSet.add("Ram"); anotherSet.add("Shyam"); hashSet.addAll(anotherSet); System.out.println("After addAll: " + hashSet); // Check if the set is empty System.out.println("Is HashSet empty? " + hashSet.isEmpty()); // Get the size of the set System.out.println("Size of HashSet: " + hashSet.size()); // Clear the set hashSet.clear(); System.out.println("After clear: " + hashSet); } } 

Output:

After addAll: [Arjun, Ram, Shyam, Lakshmi, Ravi, Sita] Is HashSet empty? false Size of HashSet: 6 After clear: [] 

Complete Code Example

Here's the complete code example demonstrating various operations with HashSet:

import java.util.HashSet; import java.util.Iterator; import java.util.Set; import java.util.stream.Collectors; public class HashSetExample { public static void main(String[] args) { // Create a HashSet Set<String> hashSet = new HashSet<>(); // Add elements to the set hashSet.add("Ravi"); hashSet.add("Sita"); hashSet.add("Arjun"); hashSet.add("Lakshmi"); // Check if elements are present System.out .println("Contains Ravi: " + hashSet.contains("Ravi")); System.out.println("Contains Gopal: " + hashSet.contains("Gopal")); // Iterate over the set using for-each loop System.out.println("Iterating over HashSet using for-each loop:"); for (String element : hashSet) { System.out.println(element); } // Iterate over the set using iterator System.out.println("Iterating over HashSet using iterator:"); Iterator<String> iterator = hashSet.iterator(); while (iterator.hasNext()) { System.out.println(iterator.next()); } // Iterate over the set using forEach and lambda System.out.println("Iterating over HashSet using forEach and lambda:"); hashSet.forEach(element -> System.out.println(element)); // Iterate over the set using streams System.out.println("Iterating over HashSet using streams:"); hashSet.stream().forEach(System.out::println); // Convert HashSet to a List using streams System.out.println("HashSet to List:"); hashSet.stream().collect(Collectors.toList()).forEach(System.out::println); // Remove an element hashSet.remove("Arjun"); System.out.println("HashSet after removal: " + hashSet); // Use addAll method Set<String> anotherSet = new HashSet<>(); anotherSet.add("Ram"); anotherSet.add("Shyam"); hashSet.addAll(anotherSet); System.out.println("After addAll: " + hashSet); // Check if the set is empty System.out.println("Is HashSet empty? " + hashSet.isEmpty()); // Get the size of the set System.out.println("Size of HashSet: " + hashSet.size()); // Clear the set hashSet.clear(); System.out.println("After clear: " + hashSet); } } 

Output:

Contains Ravi: true Contains Gopal: false Iterating over HashSet using for-each loop: Arjun Lakshmi Ravi Sita Iterating over HashSet using iterator: Arjun Lakshmi Ravi Sita Iterating over HashSet using forEach and lambda: Arjun Lakshmi Ravi Sita Iterating over HashSet using streams: Arjun Lakshmi Ravi Sita HashSet to List: Arjun Lakshmi Ravi Sita HashSet after removal: [Lakshmi, Ravi, Sita] After addAll: [Arjun, Ram, Shyam, Lakshmi, Ravi, Sita] Is HashSet empty? false Size of HashSet: 6 After clear: [] 

Conclusion

In this tutorial, we demonstrated how to use the HashSet class in Java. We covered creating a HashSet, adding and retrieving elements, iterating over the set using various methods, removing elements, and using some important HashSet methods. By following this guide, developers can effectively use HashSet in scenarios where the uniqueness of elements is important.