Java HashSet

Overview

Java HashSet class is a member of Java collections framework. HashSet implements Set interface and extends AbstractSet.

For official documentation on HashSet, please visit here

Some of the key points of HashSet include :

• HashSet cannot contain duplicate values.
• HashSet is an unordered collection. It does not maintain the order in which the elements are inserted.
• HashSet allows null value.
• HashSet internally uses a HashMap to store its elements.
• HashSet is not thread-safe. If multiple threads try to modify a HashSet at the same time, then the final outcome is not-deterministic. You must explicitly synchronize concurrent access to a HashSet in a multi-threaded environment.
• HashSet also implements Searlizable and Cloneable interfaces.

HashSet Declaration

Here is the declaration for HashSet

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

HashSet Constructors

Methods & Description

Apart from the methods inherited from its parent classes, HashSet defines following methods

Creating a HashSet

The example below shows how to create a HashSet using the HashSet() constructor

import java.util.HashSet;
import java.util.Set;
public class CreateHashSetExample {
    public static void main(String[] args) {
        // Creating a HashSet
        Set<String> set1 = new HashSet<>();
        Set<String> set2 = new HashSet<>(set1);
        // creating a hashset with capacity
        Set<String> set3 = new HashSet<>(10);
        Set<String> set4 = new HashSet<>(5, 0.75);
    }
}

Adding elements to HashSet

Here is the example that shows how to add elements to HashSet

package com.techijournal.HashSet;

import java.util.HashSet;
import java.util.Set;

public class HashSetExample {
	public static void main(String[] args) {
		// Creating a HashSet
		Set<String> set1 = new HashSet<>();
		// Adding new elements to the HashSet
		set1.add("TechiJournal");
		set1.add("Callicoder");
		set1.add("GeeksForGeeks");
		set1.add("TutorialsPoint");
		set1.add("JavaTpoint");
		// Adding duplicate elements will be ignored
		set1.add("BeginnersBook");
		System.out.println(set1);
	}
}

Output : 
[Callicoder, TechiJournal, BeginnersBook, TutorialsPoint, GeeksForGeeks, JavaTpoint]

Creating a HashSet from another collection

The below example shows how to create a Set from other collection

package com.techijournal.HashSet;

import java.util.ArrayList;
import java.util.HashSet;
import java.util.List;
import java.util.Set;

public class AddSet {

	public static void main(String[] args) {
		List<Integer> list1 = new ArrayList<>();
		list1.add(5);
		list1.add(10);
		list1.add(15);
		List<Integer> list2 = new ArrayList<>();
		list2.add(3);
		list2.add(6);
		list2.add(9);
		// Creating a HashSet from another collection (ArrayList)
		Set<Integer> set1 = new HashSet<>(list1);
		// Adding all the elements from an existing collection to a HashSet
		set1.addAll(list2);
		System.out.println(set1);
	}

}

Output : 
[3, 5, 6, 9, 10, 15]

Removing elements from a HashSet

Here is the example that shows :

• Remove all the elements that exist in a given collection from the HashSet.
• Remove an element from a HashSet.
• Remove all the elements that satisfy a given predicate from the HashSet.
• Clear the HashSet completely by removing all the elements.

package com.techijournal.HashSet;

import java.util.ArrayList;
import java.util.HashSet;
import java.util.List;
import java.util.Set;

public class RemoveSet {
	public static void main(String[] args) {
		Set<String> set1 = new HashSet<>();
		set1.add("Sumanth");
		set1.add("Durgesh");
		set1.add("Lokesh");
		set1.add("Vamsi");
		set1.add("Deepthi");
		set1.add("Prasanth");
		System.out.println("names : " + set1);
		// Remove an element from a HashSet (The remove() method returns false if the
		// element does not exist in the HashSet)
		boolean isRemoved = set1.remove("Deepthi");
		System.out.println("After remove(Deepthi) => " + set1);

		// Remove all elements belonging to a given collection from a HashSet
		List<String> duplicateNames = new ArrayList<>();
		duplicateNames.add("Prasanth");
		duplicateNames.add("Riya");
		set1.removeAll(duplicateNames);
		System.out.println("After removeAll(perfectSquares) => " + set1);
		// Remove all elements matching a given predicate
		set1.removeIf(name -> name.equalsIgnoreCase("Prasanth"));
		System.out.println("After removeIf() => " + set1);
		// Remove all elements from HashSet (clear it completely)
		set1.clear();
		System.out.println("After clear() => " + set1);
	}
}

Output : 
names : [Durgesh, Vamsi, Sumanth, Prasanth, Lokesh, Deepthi]
After remove(Deepthi) => [Durgesh, Vamsi, Sumanth, Prasanth, Lokesh]
After removeAll(perfectSquares) => [Durgesh, Vamsi, Sumanth, Lokesh]
After removeIf() => [Durgesh, Vamsi, Sumanth, Lokesh]
After clear() => []

Iterating over HashSet

The following example shows different ways of iterating over a HashSet

package com.techijournal.HashSet;

import java.util.HashSet;
import java.util.Iterator;
import java.util.Set;

public class IterateHashSet {
	public static void main(String[] args) {
		Set<String> namesSet = new HashSet<>();
		namesSet.add("Sumanth");
		namesSet.add("Durgesh");
		namesSet.add("Vamsi");
		namesSet.add("Sneha");
		namesSet.add("Deepthi");
		namesSet.add("Sachin");
		System.out.println("=== Iterate over a HashSet using Java 8 forEach and lambda ===");
		namesSet.forEach(name -> {
			System.out.println(name);
		});
		System.out.println("=== Iterate over a HashSet using iterator() ===");
		Iterator<String> namesIterator = namesSet.iterator();
		while (namesIterator.hasNext()) {
			String name = namesIterator.next();
			System.out.println(name);
		}
		System.out.println("=== Iterate over a HashSet using iterator() and Java 8 forEachRemaining() method ===");
		namesIterator = namesSet.iterator();
		namesIterator.forEachRemaining(name -> {
			System.out.println(name);
		});
		System.out.println("=== Iterate over a HashSet using simple for-each loop ===");
		for (String name : namesSet) {
			System.out.println(name);
		}
	}
}

Output : 
=== Iterate over a HashSet using Java 8 forEach and lambda ===
Durgesh
Vamsi
Sumanth
Sneha
Deepthi
Sachin
=== Iterate over a HashSet using iterator() ===
Durgesh
Vamsi
Sumanth
Sneha
Deepthi
Sachin
=== Iterate over a HashSet using iterator() and Java 8 forEachRemaining() method ===
Durgesh
Vamsi
Sumanth
Sneha
Deepthi
Sachin
=== Iterate over a HashSet using simple for-each loop ===
Durgesh
Vamsi
Sumanth
Sneha
Deepthi
Sachin

Userdefined Objects in HashSet

This example shows how to create a HashSet of user defined objects.

package com.techijournal.HashSet;

import java.util.HashSet;
import java.util.Objects;
import java.util.Set;

class Movie {
	private long movieId;
	private String movieName;

	public Movie(long movieId, String movieName) {
		this.movieId = movieId;
		this.movieName = movieName;
	}

	public long getMovieId() {
		return movieId;
	}

	public void setMovieId(long movieId) {
		this.movieId = movieId;
	}

	public String getMovieName() {
		return movieName;
	}

	public void setMovieName(String movieName) {
		this.movieName = movieName;
	}

	@Override
	public boolean equals(Object o) {
		if (this == o)
			return true;
		if (o == null || getClass() != o.getClass())
			return false;
		Movie movie = (Movie) o;
		return movieId == movie.movieId;
	}

	@Override
	public int hashCode() {
		return Objects.hash(Long.valueOf(movieId));
	}

	@Override
	public String toString() {
		return "Movie{" + "id=" + movieId + ", name='" + movieName + '\'' + '}';
	}
}

public class HashSetUserDefined {
	public static void main(String[] args) {
		Set<Movie> movies = new HashSet<>();
		movies.add(new Movie(1, "Bahubali"));
		movies.add(new Movie(2, "Dangal"));
		movies.add(new Movie(3, "Ratsasan"));
		movies.add(new Movie(1, "Bahubali"));
		System.out.println(movies);
	}
}

Output : 
[Movie{id=1, name='Bahubali'}, Movie{id=2, name='Dangal'}, Movie{id=3, name='Ratsasan'}]

Conclusion

In this article, you learned about HashSet, its constructors and methods, how to add elements to a HashSet, how to remove elements from the HashSet, how to iterate over a HashSet, and how to create a HashSet of user defined objects.

Thanks, Note

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