HashSet retainAll() method in Java with Example

24 Dec, 2018
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);        }    }}
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);        }    }}
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

Java - util package
Java-Collections
Java-Functions
java-hashset
Java
Java
Java-Collections
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