HashSet retainAll() method in Java with Example Last Updated : 24 Dec, 2018 Summarize Comments Improve Suggest changes Share Like Article Like Report 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 // 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 // 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 Comment More infoAdvertise with us Next Article Vector retainAll() method in Java with Examples C code_r Follow Improve Article Tags : Java Java-Collections Java - util package Java-Functions java-hashset +1 More Practice Tags : JavaJava-Collections Similar Reads HashSet removeAll() method in Java with Example The removeAll() method of java.util.HashSet class is used to remove from this set all of its elements that are contained in the specified collection.Syntax: public boolean removeAll(Collection c) Parameters: This method takes collection c as a parameter containing elements to be removed from this se 3 min read List retainAll() Method in Java with Examples This method is used to retain all the elements present in the collection from the specified collection into the list. 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