Set addAll() method in Java with Examples

The java.util.Set.addAll(Collection C) method is used to append all of the elements from the mentioned collection to the existing set. The elements are added randomly without following any specific order.

Syntax:

boolean addAll(Collection C)

Parameters: The parameter C is a collection of any type that is to be added to the set.

Return Value: The method returns true if it successfully appends the elements of the collection C to this Set otherwise it returns False.

Below programs illustrate the Java.util.Set.addAll() method:

Program 1 : Appending a tree set.

// Java code to illustrate addAll()
import java.io.*;
import java.util.*;

public class TreeSetDemo {
    public static void main(String args[])
    {
        // Creating an empty Set
        Set<String> st1 = new TreeSet<String>();

        // Use add() method to add elements into the Set
        st1.add("Welcome");
        st1.add("To");
        st1.add("Geeks");
        st1.add("4");
        st1.add("Geeks");
        st1.add("TreeSet");

        // Displaying the Set
        System.out.println("Set: " + st1);

        // Creating another Set
        Set<String> st2 = new TreeSet<String>();

        // Use add() method to add elements into the Set
        st2.add("Hello");
        st2.add("World");

        // Using addAll() method to Append
        st1.addAll(st2);

        // Displaying the final Set
        System.out.println("Set: " + st1);
    }
}

Output:

Set: [4, Geeks, To, TreeSet, Welcome]
Set: [4, Geeks, Hello, To, TreeSet, Welcome, World]

Program 2 : Appending an ArrayList.

// Java code to illustrate addAll()
import java.io.*;
import java.util.*;

public class SetDemo {
    public static void main(String args[])
    {
        // Creating an empty Set
        Set<String> st1 = new TreeSet<String>();

        // Use add() method to add elements into the Set
        st1.add("Welcome");
        st1.add("To");
        st1.add("Geeks");
        st1.add("4");
        st1.add("Geeks");
        st1.add("Set");

        // Displaying the Set
        System.out.println("Initial Set: " + st1);

        // An array collection is created
        ArrayList<String> collect = new ArrayList<String>();
        collect.add("A");
        collect.add("Computer");
        collect.add("Portal");

        // Using addAll() method to Append
        st1.addAll(collect);

        // Displaying the final Set
        System.out.println("Final Set: " + st1);
    }
}

Output:

Initial Set: [4, Geeks, Set, To, Welcome]
Final Set: [4, A, Computer, Geeks, Portal, Set, To, Welcome]

Reference: https://docs.oracle.com/javase/7/docs/api/java/util/Set.html#addAll(java.util.Collection)