boolean addAll(Collection C)

// Java code to illustrate addAll()
  
import java.io.*;
import java.util.*;
  
public class TreeSetDemo {
    public static void main(String args[])
    {
        // Creating an empty Set
        SortedSet<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);
    }
}

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

// Java code to illustrate addAll()
  
import java.io.*;
import java.util.*;
  
public class SetDemo {
    public static void main(String args[])
    {
        // Creating an empty Set
        SortedSet<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);
    }
}

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