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Set retainAll() Method in Java

Last Updated : 11 Jul, 2025

In Java, the retainAll() method is used to retain only the elements in a collection that are also present in another collection. It modifies the current collection by removing elements that are not in the specified collection.

Example 1: This example demonstrates how the retainAll() method retains only the common elements between two sets.

Java


 // Java Program to demonstrate 
// the working of retainAll() with Sets
import java.util.*;

public class Geeks {
    public static void main(String[] args)
    {
        // Create two sets of integers
        Set<Integer> s1 = new HashSet<>(Arrays.asList(1, 2, 3, 4, 5));
        Set<Integer> s2 = new HashSet<>(Arrays.asList(3, 4, 5, 6, 7));

        System.out.println("Set 1: " + s1);
        System.out.println("Set 2: " + s2);

        // Retain only the elements in 
        // s1 that are also in s2
        s1.retainAll(s2);

        System.out.println(
            "Modified Set 1 after retainAll: " + s1);
    }
}

Output

Set 1: [1, 2, 3, 4, 5]
Set 2: [3, 4, 5, 6, 7]
Modified Set 1 after retainAll: [3, 4, 5]

Syntax of retianAll() Method

boolean retainAll(Collection<?> c)

Parameter: This method takes collection as a parameter
Return Type: This method returns "true" if the current collection was modified by retaining common elements otherwise return "false".

Example 2: This example demonstrates how the retainAll() method modifies a set by retaining only the common elements with another set, and shows the return value indicating whether the set was modified.

Java


 // Java program to demonstrate how retainAll() 
// method returns boolean value with Sets
import java.util.*;

public class Geeks {
    public static void main(String[] args)
    {
        // Create two sets of integers
        Set<Integer> s1
            = new HashSet<>(Arrays.asList(1, 2, 3, 4, 5));
        Set<Integer> s2
            = new HashSet<>(Arrays.asList(3, 4, 5, 6, 7));

        System.out.println("Set 1: " + s1);
        System.out.println("Set 2: " + s2);

        // Call retainAll and check the return value
        boolean b = s1.retainAll(s2);
        System.out.println(
            "Set 1 after retainAll (common elements): "
            + s1);
        System.out.println("Was the set modified? " + b);

        // Modify s1 to contain all elements from s2
        s1 = new HashSet<>(Arrays.asList(3, 4, 5));

        // Now retain elements common between s1 and s2
        b = s1.retainAll(s2);
        System.out.println(
            "Set 1 after retainAll (common elements): "
            + s1);
        System.out.println("Was the set modified? " + b);
    }
}

Output:

Example 3: This example demonstrates that calling retainAll() with a null collection as a parameter throws a NullPointerException.

Java


 // Java Program to demonstrates retainAll() 
// method throws NullPointerException
import java.util.*;

public class Geeks {
    public static void main(String[] args) {
        
        // Create a set with null values
        Set<Integer> s1 = new HashSet<>(Arrays.asList(1, 2, 3, null));
        Set<Integer> s2 = null;

        s1.retainAll(s2);
    }
}

Output:

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