HashSet in Java

What is a HashSet in Java

HashSet in Java is a class that implements the Set interface and stores data in a hashtable. It is part of the java.util package. It uses the hashing technique to store and retrieve the elements from the HashSet. During the hashing process, it uses the generated hashcode value to store the elements at the required index.

Features of Java HashSet

• It stores only unique value which means it does not allow duplicate values.
• It does not maintain any insertion order since it stores the data based on the hashcode value.
• HashSet allows storing null values.
• It is non-synchronized.
• Java HashSet class can be used as the best option for search operations since its performance is faster.

Hierarchy of HashSet

Constructors in HashSet

Methods in HashSet

Below are the methods supported by the Java HashSet class.

Example: Add elements

Below is an example to add elements to the Java HashSet using the add() method. We can also add a collection of elements to the Set using the addAll() method.

import java.util.HashSet;

public class AddHashSetElements {

  public static void main(String[] args) {
    HashSet<Integer> hs = new HashSet<Integer>();
    hs.add(30);
    hs.add(10);
    hs.add(20);
    hs.add(40);
    
    System.out.println("Elements in the HashSet after add operation: " + hs);
    
    HashSet<Integer> h = new HashSet<Integer>();
    h.add(60);
    h.add(50);
    
    hs.addAll(h);
    
    System.out.println("ELements in the HashSet after addAll operation: " + hs);
    

  }

}

Elements in the HashSet after add operation: [20, 40, 10, 30]
ELements in the HashSet after addAll operation: [50, 20, 40, 10, 60, 30]

Example: Remove elements

The below example shows how to remove the elements from the HashSet in Java using the remove() method. To remove the collection of elements, we can use the removeAll() method, and to retain only the collection elements we can use the retainAll() method.

import java.util.HashSet;

public class RemoveHashSetElements {

  public static void main(String[] args) {
    HashSet<Integer> hs = new HashSet<Integer>();
    hs.add(30);
    hs.add(10);
    hs.add(20);
    hs.add(40);
    
    HashSet<Integer> h = new HashSet<Integer>();
    h.add(60);
    h.add(50);
    hs.addAll(h);
    
    System.out.println("Elements in the HashSet: " + hs);
    
    hs.remove(30);
    System.out.println("Elements in the HashSet after remove method: " + hs);
    
    hs.retainAll(h);
    System.out.println("Elements in the HashSet after retainAll method: " + hs);

    hs.removeAll(h);
    System.out.println("Elements in the HashSet after removeAll method: " + hs);

  }

}

Elements in the HashSet: [50, 20, 40, 10, 60, 30]
Elements in the HashSet after remove method: [50, 20, 40, 10, 60]
ELements in the HashSet after retainAll method: [50, 60]
Elements in the HashSet after removeAll method: []

Example: Clear the HashSet and check if empty

We can clear the HashSet by using the clear() method which removes all the elements from the set. The isEmpty() method checks if the HashSet is empty or not.

import java.util.HashSet;

public class RemoveHashSetElements {

  public static void main(String[] args) {
    HashSet<Integer> hs = new HashSet<Integer>();
    hs.add(30);
    hs.add(10);
    hs.add(20);
    hs.add(40);
    
    System.out.println("Elements in the HashSet: " + hs);
    System.out.println("Size of the HashSet: " + hs.size());
    
    hs.clear();
    System.out.println("Is HashSet empty: " + hs.isEmpty());

  }

}

Elements in the HashSet: [20, 40, 10, 30]
Size of the HashSet: 4
Is HashSet empty: true

Example: Check if elements exist

The below example uses the contains() method to check if the HashSet contains the specified element. We can use the containsAll() method to check if the set contains a collection of elements.

import java.util.HashSet;
public class CheckElement {

  public static void main(String[] args) {
    HashSet<String> city = new HashSet<String>();
    city.add("Bangalore");
    city.add("Chennai");
    city.add("Delhi");
    city.add("Mumbai");
    
    System.out.println("Elements in the HashSet: " + city);
    System.out.println("Check if Chennai exist: " + city.contains("Chennai"));
    System.out.println("Check if Hyderabad exist: " + city.contains("Hyderabad"));
    
    HashSet<String> c = new HashSet<String>();
    c.add("Hyderabad");
    c.add("Jaipur");
    
    city.addAll(c);
    System.out.println("Elements after addAll method: " + city);
    System.out.println("Check if collection exist: " + city.containsAll(c));
  }

}

Elements in the HashSet: [Delhi, Chennai, Mumbai, Bangalore]
Check if Chennai exist: true
Check if Hyderabad exist: false
Elements after addAll method: [Delhi, Chennai, Jaipur, Mumbai, Hyderabad, Bangalore]
Check if collection exist: true

Example: Iterate over HashSet elements

We can traverse through the elements in the HashSet in Java using the iterator() method. Below is an example to illustrate the same.

import java.util.HashSet;
import java.util.Iterator;
public class IterateHashSet {

  public static void main(String[] args) {
    HashSet<String> names = new HashSet<String>();
    names.add("Ravi");
    names.add("Rakesh");
    names.add("Suresh");
    names.add("Dinesh");
    
    Iterator<String> it = names.iterator();
    while(it.hasNext())
      System.out.println(it.next());

  }

}

Suresh
Ravi
Dinesh
Rakesh

Reference