The Java HashSet class is used to create a collection that stores data in a hash table. It derives from AbstractSet and implements the Set interface.

This class allows for the null element. The class also provides constant time performance for fundamental operations such as add, delete, contains, and size, assuming the hash function properly distributes the elements throughout the buckets, as we will see later in the article.

Declaration of the Java HashSet class:

public class HashSet<E>extends AbstractSet<E>implements Set<E>, Cloneable, Serializable

The following are the key features of the Java HashSet class:

• HashSet uses a process known as hashing to store the elements.
• HashSet only contains unique elements.
• HashSet accepts null values.
• The HashSet class does not support synchronization.
• The insertion order is not preserved by HashSet. Elements are put here based on their hashcode.

HashSet Constructors:

• HashSet(): It is used to construct a default HashSet.
• HashSet(Collection c): It is used to populate the hash set with elements from the collection c.
• HashSet(int capacity): It is used to set the hash’s capacity to the specified integer value capacity. As elements are added to the HashSet, the capacity grows automatically.

HashSet Method:

Modifier & Type	Method	Description
boolean	add(E e)	If the supplied element does not already exist in this set, it is added.
void	clear()	It is used to remove all of the set’s elements.
object	clone()	It is used to return a shallow replica of this HashSet instance, with no cloning of the elements.
boolean	contains(Object o)	It is used to return true if the supplied element is present in this set.
boolean	isEmpty()	It returns true if this set includes no elements.
Iterator<E>	iterator()	It is used to return an iterator across the set’s elements.
boolean	remove(Object o)	If the specified element is present, it is removed from this set.
int	size()	It returns the number of elements in the set.
Spliterator<E>	spliterator()	It is used to construct a late-binding and fail-fast Spliterator across the set’s elements.

Example: Adding Elements to HashSet

import java.io.*;
import java.util.*;

// Main class
// AddingElementsToHashSet
class Coderz {

	public static void main(String[] args)
	{
		// Creating an empty HashSet of string entities
		HashSet<String> hobj = new HashSet<String>();

		// Adding elements using add() method
		hobj.add("coderzpy.com");
		hobj.add("For");
		hobj.add("coding");

		// Printing all string el=ntries inside the Set
		System.out.println("HashSet elements : " + hobj);
	}
}

Output:

HashSet elements : [coding, For, coderzpy.com]

Example: Removal Of Elements of HashSet

import java.io.*;
import java.util.*;

class Coderz {

	// Main driver method
	public static void main(String[] args)
	{
		// Creating an
		HashSet<String> hobj = new HashSet<String>();

		// Adding elements to above Set
		// using add() method
		hobj.add("Apple");
		hobj.add("Banana");
		hobj.add("Cat");
		hobj.add("A");
		hobj.add("B");
		hobj.add("Z");

		// Printing the elements of HashSet elements
		System.out.println("Initial HashSet " + hobj);

		// Removing the element B
		hobj.remove("B");

		// Printing the updated HashSet elements
		System.out.println("After removing element " + hobj);

		// Returns false if the element is not present
		System.out.println("Element AC exists in the Set : "
						+ hobj.remove("AC"));
	}
}

Output:

Initial HashSet [A, Apple, B, Cat, Z, Banana]
After removing element [A, Apple, Cat, Z, Banana]
Element AC exists in the Set : false

Example: Iteration Over HashSet

import java.io.*;
import java.util.*;

class Coderz {

	// Main driver method
	public static void main(String[] args)
	{

		// Creating an empty HashSet of string entries
		HashSet<String> hobj = new HashSet<String>();

		// Adding elements to above Set
		// using add() method
		hobj.add("Coderzpy.com");
		hobj.add("is fun");
		hobj.add("for coding");
		hobj.add("A");
		hobj.add("B");
		hobj.add("Z");

		// Iterating though the HashSet using iterators
		Iterator itr = hobj.iterator();

		// Holds true till there is single element
		// remaining in Set
		while (itr.hasNext())

			// Traversing elements and printing them
			System.out.print(itr.next() + ", ");
		System.out.println();

		// Using enhanced for loop for traversal
		for (String s : hobj)

			// Traversing elements and printing them
			System.out.print(s + ", ");
		System.out.println();
	}
}

Output:

A, B, Coderzpy.com, for coding, is fun, Z, 
A, B, Coderzpy.com, for coding, is fun, Z, 

Note: also read about the Map Interface in Java Collection Framework

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