How do you add two lists in Java?

The addAll() method of the List interface can be used to combine two lists in Java. It comes in two variants − one appends elements at the end, and another inserts elements at a specific index.

addAll() Without Index

Appends all elements from the specified collection to the end of the list −

boolean addAll(Collection<? extends E> c)

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

addAll() With Index

Inserts all elements from the specified collection at the specified position, shifting existing elements to the right −

boolean addAll(int index, Collection<? extends E> c)

The index parameter specifies the position at which the first element from the collection will be inserted.

Example

The following example demonstrates both variants of addAll() ?

import java.util.ArrayList;
import java.util.List;

public class CollectionsDemo {
    public static void main(String[] args) {
        List<String> list = new ArrayList<>();
        list.add("A");
        list.add("B");
        list.add("C");
        System.out.println("List: " + list);

        List<String> list1 = new ArrayList<>();
        list1.add("D");
        list1.add("E");
        list1.add("F");
        System.out.println("List1: " + list1);

        // addAll() without index - appends at end
        list.addAll(list1);
        System.out.println("After addAll(): " + list);

        List<String> list2 = new ArrayList<>();
        list2.add("G");
        list2.add("H");
        list2.add("I");

        // addAll() with index - inserts at position 0
        list2.addAll(0, list);
        System.out.println("List2 after addAll(0, list): " + list2);
    }
}

The output of the above code is ?

List: [A, B, C]
List1: [D, E, F]
After addAll(): [A, B, C, D, E, F]
List2 after addAll(0, list): [A, B, C, D, E, F, G, H, I]

Conclusion

Use addAll(collection) to append one list to the end of another, or addAll(index, collection) to insert a list at a specific position. Both methods modify the original list and return true if elements were added successfully.