Java HashSet Class

In this tutorial, we will learn about the Java HashSet class. We will learn about various hash set techniques and tasks with the help of examples.

The HashSet class of the Java Collections system gives the functionalities of the hash table data structure.

It implements the Set interface.

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Making a HashSet

To make a hash set, we should import the java.util.HashSet package first.

When we import the package, here is the manner by which we can make hash sets in Java.

// HashSet with 8 capacity and 0.75 load factor
HashSet<Integer> numbers = new HashSet<>(8, 0.75);

Here, we have created a hash set named numbers.

Notice, the part new HashSet<>(8, 0.75). Here, the first parameter is capacity, and the second parameter is loadFactor.

• capacity – The capacity of this hash set is 8. Meaning, it can store 8 elements.
• loadFactor – The load factor of this hash set is 0.6. This means, whenever our hash set is filled by 60%, the elements are moved to a new hash table of double the size of the original hash table.

Default capacity and load factor

It’s possible to create a hash table without defining its capacity and load factor. For example,

// HashSet with default capacity and load factor
HashSet<Integer> numbers1 = new HashSet<>();

By default,

• the capacity of the hash set will be 16
• the load factor will be 0.75

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Methods of HashSet

The HashSet class provides various methods that allow us to perform various operations on the set.

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Insert Elements to HashSet

• add() – inserts the specified element to the set
• addAll() – inserts all the elements of the specified collection to the set

For example,

import java.util.HashSet;

class Main {
    public static void main(String[] args) {
        HashSet<Integer> evenNumber = new HashSet<>();

        // Using add() method
        evenNumber.add(2);
        evenNumber.add(4);
        evenNumber.add(6);
        System.out.println("HashSet: " + evenNumber);

        HashSet<Integer> numbers = new HashSet<>();
        
        // Using addAll() method
        numbers.addAll(evenNumber);
        numbers.add(5);
        System.out.println("New HashSet: " + numbers);
    }
}

Output

HashSet: [2, 4, 6]
New HashSet: [2, 4, 5, 6]

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Access HashSet Elements

To access the elements of a hash set, we can use the iterator() method. In order to use this method, we must import the java.util.Iterator package. For example,

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

class Main {
    public static void main(String[] args) {
        HashSet<Integer> numbers = new HashSet<>();
        numbers.add(2);
        numbers.add(5);
        numbers.add(6);
        System.out.println("HashSet: " + numbers);

        // Calling iterator() method
        Iterator<Integer> iterate = numbers.iterator();
        System.out.print("HashSet using Iterator: ");
        // Accessing elements
        while(iterate.hasNext()) {
            System.out.print(iterate.next());
            System.out.print(", ");
        }
    }
}

Output

HashSet: [2, 5, 6]
HashSet using Iterator: 2, 5, 6,

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Remove Elements

• remove() – removes the specified element from the set
• removeAll() – removes all the elements from the set

For example,

import java.util.HashSet;

class Main {
    public static void main(String[] args) {
        HashSet<Integer> numbers = new HashSet<>();
        numbers.add(2);
        numbers.add(5);
        numbers.add(6);
        System.out.println("HashSet: " + numbers);

        // Using remove() method
        boolean value1 = numbers.remove(5);
        System.out.println("Is 5 removed? " + value1);

        boolean value2 = numbers.removeAll(numbers);
        System.out.println("Are all elements removed? " + value2);
    }
}

Output

HashSet: [2, 5, 6]
Is 5 removed? true
Are all elements removed? true

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Set Operations

The various methods of the HashSet class can also be used to perform various set operations.

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Union of Sets

Two perform the union between two sets, we can use the addAll() method. For example,

import java.util.HashSet;

class Main {
    public static void main(String[] args) {
        HashSet<Integer> evenNumbers = new HashSet<>();
        evenNumbers.add(2);
        evenNumbers.add(4);
        System.out.println("HashSet1: " + evenNumbers);

        HashSet<Integer> numbers = new HashSet<>();
        numbers.add(1);
        numbers.add(3);
        System.out.println("HashSet2: " + numbers);

        // Union of two set
        numbers.addAll(evenNumbers);
        System.out.println("Union is: " + numbers);
    }
}

Output

HashSet1: [2, 4]
HashSet2: [1, 3]
Union is: [1, 2, 3, 4]

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Intersection of Sets

To perform the intersection between two sets, we can use the retainAll() method. For example

import java.util.HashSet;

class Main {
    public static void main(String[] args) {
        HashSet<Integer> primeNumbers = new HashSet<>();
        primeNumbers.add(2);
        primeNumbers.add(3);
        System.out.println("HashSet1: " + primeNumbers);

        HashSet<Integer> evenNumbers = new HashSet<>();
        evenNumbers.add(2);
        evenNumbers.add(4);
        System.out.println("HashSet2: " + evenNumbers);

        // Intersection of two sets
        evenNumbers.retainAll(primeNumbers);
        System.out.println("Intersection is: " + evenNumbers);
    }
}

Output

HashSet1: [2, 3]
HashSet2: [2, 4]
Intersection is: [2]

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Difference of Sets

To calculate the difference between the two sets, we can use the removeAll() method. For example,

import java.util.HashSet;

class Main {
    public static void main(String[] args) {
        HashSet<Integer> primeNumbers = new HashSet<>();
        primeNumbers.add(2);
        primeNumbers.add(3);
        primeNumbers.add(5);
        System.out.println("HashSet1: " + primeNumbers);

        HashSet<Integer> oddNumbers = new HashSet<>();
        oddNumbers.add(1);
        oddNumbers.add(3);
        oddNumbers.add(5);
        System.out.println("HashSet2: " + oddNumbers);

        // Difference between HashSet1 and HashSet2
        primeNumbers.removeAll(oddNumbers);
        System.out.println("Difference : " + primeNumbers);
    }
}

Output

HashSet1: [2, 3, 5]
HashSet2: [1, 3, 5]
Difference: [2]

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Subset

To check if a set is a subset of another set or not, we can use the containsAll() method. For example,

import java.util.HashSet;

class Main {
    public static void main(String[] args) {
        HashSet<Integer> numbers = new HashSet<>();
        numbers.add(1);
        numbers.add(2);
        numbers.add(3);
        numbers.add(4);
        System.out.println("HashSet1: " + numbers);

        HashSet<Integer> primeNumbers = new HashSet<>();
        primeNumbers.add(2);
        primeNumbers.add(3);
        System.out.println("HashSet2: " + primeNumbers);

        // Check if primeNumbers is a subset of numbers
        boolean result = numbers.containsAll(primeNumbers);
        System.out.println("Is HashSet2 is subset of HashSet1? " + result);
    }
}

Output

HashSet1: [1, 2, 3, 4]
HashSet2: [2, 3]
Is HashSet2 is a subset of HashSet1? true

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Other Methods Of HashSet

Method	Description
clone()	Creates a copy of the HashSet
contains()	Searches the HashSet for the specified element and returns a boolean result
isEmpty()	Checks if the HashSet is empty
size()	Returns the size of the HashSet
clear()	Removes all the elements from the HashSet

To learn more about HashSet methods, visit Java HashSet (official Java documentation).

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Why HashSet?

In Java, HashSet is ordinarily used if we need to get to components arbitrarily. It is on the grounds that components in a hash table are gotten to using hash codes.

The hashcode of a component is of a kind character that assists with distinguishing the component in a hash table.

HashSet can’t contain copy components. Henceforth, each hash set component has an extraordinary hashcode.

Note: HashSet isn’t synchronized. That is if numerous strings access the hash set simultaneously and one of the strings changes the hash set. At that point, it must be externally synchronized.

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Java Set