Java TreeSet - addAll() Method

The java.util.TreeSet.addAll() method is used to add all of the elements in the specified collection to this set.

Syntax

public boolean addAll(Collection<? extends E> c)

Here, E is the type of element maintained by the container.

Parameters

Return Value

Returns true if the TreeSet changed as a result of the call.

Exception

• Throws ClassCastException, if the elements provided cannot be compared with the elements currently in the set.
• Throws NullPointerException, if the specified collection is null or if any element is null and this set uses natural ordering, or its comparator does not permit null elements.

Example:

In the example below, the java.util.TreeSet.addAll() method is used to append all elements of the TreeSet Set2 at the end of TreeSet Set1.

import java.util.*;

public class MyClass {
  public static void main(String[] args) {
    //creating a TreeSet
    TreeSet<Integer> Set1 = new TreeSet<Integer>();
    TreeSet<Integer> Set2 = new TreeSet<Integer>();

    //populating Set1 
    Set1.add(10);
    Set1.add(20);
    Set1.add(30);

    //populating Set2 
    Set2.add(100);
    Set2.add(200);

    //printing Set1
    System.out.println("Before method call, Set1 contains: " + Set1); 

    //appending all elements of Set2 into Set1
    Set1.addAll(Set2);
    
    //printing TreeSet
    System.out.println("After method call, Set1 contains: " + Set1);      
  }
}

The output of the above code will be:

Before method call, Set1 contains: [10, 20, 30]
After method call, Set1 contains: [10, 20, 30, 100, 200]