Java ArrayList retainAll()

And, all those elements that are not present in the specified collection are removed from the arraylist.

The syntax of the retainAll() method is:

arraylist.retainAll(Collection c);

Here, arraylist is an object of the ArrayList class.

retainAll() Parameters

The retainAll() method takes a single parameter.

• collection -only those elements present in collection are retained in the arraylist.

retainAll() Return Value

• returns true if elements are removed from the arraylist
• throws ClassCastException if the class of elements present in arraylist is incompatible with the class of elements in the specified collection
• throws NullPointerException if the arraylist contains null element and the specified collection does not allow null elements

Example 1: Java ArrayList retainAll()

import java.util.ArrayList;

class Main {
    public static void main(String[] args) {
        // create an ArrayList
        ArrayList<String> languages1 = new ArrayList<>();

        // insert element to the ArrayList
        languages1.add("JavaScript");
        languages1.add("Python");
        languages1.add("Java");
        System.out.println("ArrayList 1: " + languages1);

        // create another ArrayList
        ArrayList<String> languages2 = new ArrayList<>();

        // add elements to ArrayList
        languages2.add("English");
        languages2.add("Java");
        languages2.add("Python");
        System.out.println("ArrayList 2: " + languages2);

        // retain the common element
        languages1.retainAll(languages2);
        System.out.println("Common Elements: " + languages1);
    }
}

Output

ArrayList 1: [JavaScript, Python, Java]
ArrayList 2: [English, Java, Python]
Common Elements: [Python, Java]

In the above example, we have created two arraylists named languages1 and languages2. Notice the line,

languages1.retainAll(languages2);

Here, we are passing the arraylist languages2 as an argument to the retainAll() method. The method removes all elements from languages1 that are not present in languages2. Hence, only common elements are retained.

Example 2: Show Common Elements Between ArrayList and HashSet

import java.util.ArrayList;
import java.util.HashSet;

class Main {
    public static void main(String[] args) {
        // create an ArrayList
        ArrayList<Integer> numbers = new ArrayList<>();

        // add element to ArrayList
        numbers.add(1);
        numbers.add(2);
        numbers.add(3);
        System.out.println("ArrayList: " + numbers);

        // create a HashSet
        HashSet<Integer> primeNumbers = new HashSet<>();

        // add elements to HashSet
        primeNumbers.add(2);
        primeNumbers.add(3);
        primeNumbers.add(5);
        System.out.println("HashSet: " + primeNumbers);

        // retain common elements in the ArrayList
        numbers.retainAll(primeNumbers);
        System.out.println("Common Elements: " + numbers);
    }
}

Output

ArrayList: [1, 2, 3]
HashSet: [2, 3, 5]
Common Elements: [2, 3]

In the above example, we have created an arraylist named numbers and a hashset named primeNumbers. Notice the line,

numbers.retainAll(primeNumbers);

Here, the retainAll() method removes all those elements from numbers that are not present in primeNumbers. And, only keeps the common elements. Hence, 2 and 3 are retained in the arraylist numbers.