Set retainAll() Method in Java

Last Updated :11 Jul, 2025

In Java, theretainAll()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 demonstrateshow the retainAll() method retains only the common elements between two sets.

Java

// Java Program to demonstrate// the working of retainAll() with Setsimportjava.util.*;publicclassGeeks{publicstaticvoidmain(String[]args){// Create two sets of integersSet<Integer>s1=newHashSet<>(Arrays.asList(1,2,3,4,5));Set<Integer>s2=newHashSet<>(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 s2s1.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() methodmodifies 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 Setsimportjava.util.*;publicclassGeeks{publicstaticvoidmain(String[]args){// Create two sets of integersSet<Integer>s1=newHashSet<>(Arrays.asList(1,2,3,4,5));Set<Integer>s2=newHashSet<>(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 valuebooleanb=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 s2s1=newHashSet<>(Arrays.asList(3,4,5));// Now retain elements common between s1 and s2b=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 thatcalling retainAll() with a null collection as a parameter throws a NullPointerException.

Java

// Java Program to demonstrates retainAll()// method throws NullPointerExceptionimportjava.util.*;publicclassGeeks{publicstaticvoidmain(String[]args){// Create a set with null valuesSet<Integer>s1=newHashSet<>(Arrays.asList(1,2,3,null));Set<Integer>s2=null;s1.retainAll(s2);}}