.addAll()

The .addAll() method is used to add the contents of a collection to an instance of the ArrayList class.

arrayListInstance.addAll(index, collection);

The contents of collection are inserted into the arrayListInstance at the specified index. The collection must be of the same data type as arrayListInstance.

The following example creates two ArrayLists and inserts one into the other.

import java.util.ArrayList;

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

    // Create first ArrayList
    ArrayList<String> list1 = new ArrayList<String>();

    // Add items to first ArrayList
    list1.add("A");
    list1.add("B");
    list1.add("C");
    list1.add("D");

    // Create second ArrayList
    ArrayList<String> list2 = new ArrayList<String>();

    // Add items to second ArrayList
    list2.add("W");
    list2.add("X");
    list2.add("Y");
    list2.add("Z");

    // Print first ArrayList
    System.out.println("List 1:");
    System.out.println(list1);

    // Print second ArrayList
    System.out.println("List 2:");
    System.out.println(list2);

    // Add second ArrayList to first ArrayList at index 3
    list1.addAll(3, list2);

    // Print the combined ArrayList
    System.out.println("List 1 + List 2:");
    System.out.println(list1);
  }
}

The output would look like this:

List 1:
[A, B, C, D]
List 2:
[W, X, Y, Z]
List 1 + List 2:
[A, B, C, W, X, Y, Z, D]