The retainAll() method of java.util.HashSet 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 HashSet<Integer>            HashSet<Integer>                arrset1 = new HashSet<Integer>();              // Populating arrset1            arrset1.add(1);            arrset1.add(2);            arrset1.add(3);            arrset1.add(4);            arrset1.add(5);              // print arrset1            System.out.println("HashSet before "                               + "retainAll() operation : "                               + arrset1);              // Creating another object of HashSet<Integer>            HashSet<Integer>                arrset2 = new HashSet<Integer>();            arrset2.add(1);            arrset2.add(2);            arrset2.add(3);              // print arrset2            System.out.println("Collection Elements"                               + " to be retained : "                               + arrset2);              // Removing elements from arrset            // specified in arrset2            // using retainAll() method            arrset1.retainAll(arrset2);              // print arrset1            System.out.println("HashSet after "                               + "retainAll() operation : "                               + arrset1);        }          catch (NullPointerException e) {            System.out.println("Exception thrown : " + e);        }    }} |
Output:
HashSet before retainAll() operation : [1, 2, 3, 4, 5] Collection Elements to be retained : [1, 2, 3] HashSet 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 HashSet<Integer>            HashSet<Integer>                arrset1 = new HashSet<Integer>();              // Populating arrset1            arrset1.add(1);            arrset1.add(2);            arrset1.add(3);            arrset1.add(4);            arrset1.add(5);              // print arrset1            System.out.println("HashSet before "                               + "retainAll() operation : "                               + arrset1);              // Creating another object of HashSet<Integer>            HashSet<Integer>                arrset2 = null;              // print arrset2            System.out.println("Collection Elements"                               + " to be retained : "                               + arrset2);              System.out.println("\nTrying to pass "                               + "null as a specified element\n");              // Removing elements from arrset            // specified in arrset2            // using retainAll() method            arrset1.retainAll(arrset2);              // print arrset1            System.out.println("HashSet after "                               + "retainAll() operation : "                               + arrset1);        }          catch (NullPointerException e) {            System.out.println("Exception thrown : " + e);        }    }} |
Output:
HashSet 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
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