Java .retainAll()

The .retainAll() method of ArrayList retains only the elements in the ArrayList that are common to another specified collection. It removes all elements from the ArrayList that are not present in the specified collection.

Syntax

B.retainAll(Collection c);

• B: The ArrayList whose elements will be retained.
• c: The collection whose elements will be retained in the ArrayList B.

Note: Following are the exceptions when using .retainAll() method:

• ClassCastException: If the class of an element of this ArrayList is incompatible with the Passed collection.
• NullPointerException: If the list contains a null element and the passed collection does not permit null elements, or if the specified collection is null.

Example

The below example demonstrates the use of .retainAll().

// File: RetainAllExample.java
import java.util.ArrayList;
public class RetainAllExample{
    public static void main(String[] args) {
        ArrayList<String> list1 = new ArrayList<>();
        list1.add("apple");
        list1.add("banana");
        list1.add("orange");

        ArrayList<String> list2 = new ArrayList<>();
        list2.add("banana");
        list2.add("grape");
        list2.add("mango");

        list1.retainAll(list2);

        System.out.println("List 1 after retainAll : " + list1);
        System.out.println("List 2 after retainAll : "+list2);
    }
}

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The above example gives the following output:

List 1 after retainAll : [banana]
List 2 after retainAll : [banana, grape, mango]

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