How to Copy or Append HashSet to Another HashSet in Java?

HashSet is used to store distinct values in Java. HashSet stores the elements in random order, so there is no guarantee of the elements’ order. The HashSet class implements the Set interface, backed by a hash table which is actually a HashMap instance. 

We can copy or append a HashSet to another HashSet. There is a couple of ways to copy HashSet or append HashSet to another HashSet in Java:

• Using HashSet constructor
• Using clone() method
• Using the addAll() method

Method 1: Using the HashSet constructor

Using the constructor, we can copy the original HashSet to another HashSet bypassing the original HashSet to the constructor.

// passing the original HashSet to the constructor

HashSet<Integer> copySet = new HashSet<>(originalSet)

Code:

Java

// Java program to copy a HashSet to
// another HashSet using the constructor

import java.util.*;

public class GFG {
    public static void main(String[] args)
    {

        // New HashSet
        HashSet<Integer> set = new HashSet<>();

        // Add elements to original set
        set.add(10);
        set.add(20);
        set.add(30);
        set.add(10);
        set.add(50);
        set.add(20);

        // Make a new HashSet and copy all elements of
        // original HashSet using constructor
        HashSet<Integer> copyOfSet = new HashSet<>(set);

        // Print original HashSet
        System.out.println("Original HashSet: " + set);

        // Print Copy HashSet
        System.out.println("Copy HashSet: " + copyOfSet);
    }
}

Output

Original HashSet: [50, 20, 10, 30]
Copy HashSet: [50, 20, 10, 30]

Method 2: Using the clone method

Using the clone method, we can copy all elements of the original HashSet to another HashSet in Java.

Note: The order of elements may be the same or may not be the same. So there is no guarantee of the order.

Code:

Java

// Java program to copy a HashSet to
// another HashSet using clone method in Java

import java.util.*;

public class GFG {
    public static void main(String[] args)
    {

        // New empty HashSet
        HashSet<Integer> set = new HashSet<>();

        // Add elements to set
        set.add(10);
        set.add(20);
        set.add(30);
        set.add(10);
        set.add(50);
        set.add(20);

        // Create a new HashSet to copy the original HashSet
        HashSet copyOfSet = new HashSet();

        // Copy HashSet using clone method
        copyOfSet = (HashSet)set.clone();

        // Print original HashSet
        System.out.println("Original HashSet: " + set);

        // Print Copy HashSet
        System.out.println("Copy HashSet: " + copyOfSet);
    }
}

Output

Original HashSet: [50, 20, 10, 30]
Copy HashSet: [50, 10, 20, 30]

Method 3: Using the addAll method

We can use the addAll() method to copy or to append a HashSet to a another HashSet. addAll method in Java add all elements to the HashSet.  

Note: The order of elements may be the same or may not be the same. So there is no guarantee of the order.

Code:

Java

// Java program to copy a HashSet to
// another HashSet using addAll method in Java

import java.util.*;

public class GFG {
  public static void main(String[] args) {

    // New empty HashSet
    HashSet<Integer> set = new HashSet<>();

    // Add elements to set
    set.add(10);
    set.add(20);
    set.add(30);
    set.add(10);
    set.add(50);
    set.add(20);

    // Create a new HashSet to copy the original HashSet
    HashSet<Integer> copyOfSet = new HashSet<>();

    // Copy HashSet using addAll method
    copyOfSet.addAll(set);

    // Print original HashSet
    System.out.println("Original HashSet: " + set);

    // Print Copy HashSet
    System.out.println("Copy HashSet: " + copyOfSet);

  }
}

Output

Original HashSet: [50, 20, 10, 30]
Copy HashSet: [50, 20, 10, 30]

Append using addAll method in the already existing HashSet:

Code:

Java

// Java program to copy a HashSet to another
// HashSet using addAll method in Java

import java.util.*;

public class GFG {
    public static void main(String[] args)
    {

        // New empty HashSet
        HashSet<Integer> set = new HashSet<>();

        // Add elements to set
        set.add(10);
        set.add(20);
        set.add(30);
        set.add(10);
        set.add(50);
        set.add(20);

        // Create a new HashSet to append a HashSet
        HashSet<Integer> appendedSet = new HashSet<>();

        // Add elements to appendedSet
        appendedSet.add(100);
        appendedSet.add(200);

        System.out.println("Before appending :");

        // Print original HashSet
        System.out.println("Original HashSet: " + set);

        // Print appendedSet before append
        System.out.println("Appended HashSet: "
                           + appendedSet);

        // Add all elements of set HashSet to appendedSet
        // using addAll method
        appendedSet.addAll(set);

        System.out.println("After appending");

        // Print appendedSet after append
        System.out.println("Appended HashSet: "
                           + appendedSet);
    }
}

Output

Before appending :
Original HashSet: [50, 20, 10, 30]
Appended HashSet: [100, 200]
After appending
Appended HashSet: [50, 100, 20, 200, 10, 30]