LinkedHashSet retainAll() method in Java with Example

The retainAll() method of java.util.LinkedHashSet class is used to retain from this set all of its elements that are contained in the specified collection.

Syntax:

public boolean retainAll(Collection c)

Parameters: This method takes collection c as a parameter containing elements to be retained from this set.

Returns Value: This method returns true if this set changed as a result of the call.

Exception: This method throws NullPointerException if this set contains a null element and the specified collection does not permit null elements (optional), or if the specified collection is null.

Below are the examples to illustrate the retainAll() method.

Example 1:

// Java program to demonstrate
// retainAll() method for Integer value

import java.util.*;

public class GFG1 {
    public static void main(String[] argv) throws Exception
    {

        try {

            // Creating object of LinkedHashSet<Integer>
            LinkedHashSet<Integer>
                set1 = new LinkedHashSet<Integer>();

            // Populating set1
            set1.add(1);
            set1.add(2);
            set1.add(3);
            set1.add(4);
            set1.add(5);

            // print set1
            System.out.println("LinkedHashSet before "
                               + "retainAll() operation : "
                               + set1);

            // Creating another object of  LinkedHashSet<Integer>
            LinkedHashSet<Integer>
                set2 = new LinkedHashSet<Integer>();
            set2.add(1);
            set2.add(2);
            set2.add(3);

            // print set2
            System.out.println("Collection Elements"
                               + " to be retained : "
                               + set2);

            // Removing elements from set
            // specified in set2
            // using retainAll() method
            set1.retainAll(set2);

            // print set1
            System.out.println("LinkedHashSet after "
                               + "retainAll() operation : "
                               + set1);
        }

        catch (NullPointerException e) {
            System.out.println("Exception thrown : " + e);
        }
    }
}

Output:

LinkedHashSet before retainAll() operation : [1, 2, 3, 4, 5]
Collection Elements to be retained : [1, 2, 3]
LinkedHashSet after retainAll() operation : [1, 2, 3]

Example 2: For NullPointerException

// Java program to demonstrate
// retainAll() method for Integer value

import java.util.*;

public class GFG1 {
    public static void main(String[] argv) throws Exception
    {

        try {

            // Creating object of LinkedHashSet<Integer>
            LinkedHashSet<Integer>
                set1 = new LinkedHashSet<Integer>();

            // Populating set1
            set1.add(1);
            set1.add(2);
            set1.add(3);
            set1.add(4);
            set1.add(5);

            // print set1
            System.out.println("LinkedHashSet before "
                               + "retainAll() operation : "
                               + set1);

            // Creating another object of  LinkedHashSet<Integer>
            LinkedHashSet<Integer>
                set2 = null;

            // print set2
            System.out.println("Collection Elements"
                               + " to be retained : "
                               + set2);

            System.out.println("\nTrying to pass "
                               + "null as a specified element\n");

            // Removing elements from set
            // specified in set2
            // using retainAll() method
            set1.retainAll(set2);

            // print set1
            System.out.println("LinkedHashSet after "
                               + "retainAll() operation : "
                               + set1);
        }

        catch (NullPointerException e) {
            System.out.println("Exception thrown : " + e);
        }
    }
}

Output:

LinkedHashSet before retainAll() operation : [1, 2, 3, 4, 5]
Collection Elements to be retained : null

Trying to pass null as a specified element

Exception thrown : java.lang.NullPointerException