HashSet in Java

HashSet in Java implements the Set interface of the Collections Framework. It is used to store the unique elements, and it doesn't maintain any specific order of elements.

• HashSet does not allow duplicate elements.
• Uses HashMap internally which is an implementation of hash table data structure.
• Also implements Serializable and Cloneable interfaces.
• HashSet is not thread-safe. To make it thread-safe, synchronization is needed externally.

import java.util.*;

class GFG 
{
    public static void main(String[] args) 
    {
        // Instantiate an object of HashSet
        HashSet<Integer> hs = new HashSet<>();

      	// Adding elements 
        hs.add(1);
        hs.add(2);
        hs.add(1);

        System.out.println("HashSet Size: " + hs.size());
        System.out.println("Elements in HashSet: " + hs);
    }
}

HashSet Size: 2
Elements in HashSet: [1, 2]

Hierarchy Diagram of HashSet

It implements the Set interface, which is a sub-interface of the Collection interface.