The retainAll() method of java.util.Set interface 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.
Return 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 Set
Set<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( "Set before retainAll() operation : "
+ arrset1);
// Creating another object of Set
Set<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( "Set after retainAll() operation : "
+ arrset1);
}
catch (NullPointerException e) {
System.out.println( "Exception thrown : " + e);
}
}
} |
Set before retainAll() operation : [1, 2, 3, 4, 5] Collection Elements to be retained : [1, 2, 3] Set 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 Set<Integer>
Set<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( "Set before retainAll() operation : "
+ arrset1);
// Creating another object of Set<Integer>
Set<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( "Set after retainAll() operation : "
+ arrset1);
}
catch (NullPointerException e) {
System.out.println( "Exception thrown : " + e);
}
}
} |
Set 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
Reference: https://docs.oracle.com/javase/7/docs/api/java/util/Set.html#retainAll(java.util.Collection)