HashSet retainAll() method in Java with Example
Last Updated :
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:
import java.util.*;
public class GFG1 {
public static void main(String[] argv) throws Exception
{
try {
HashSet<Integer>
arrset1 = new HashSet<Integer>();
arrset1.add( 1 );
arrset1.add( 2 );
arrset1.add( 3 );
arrset1.add( 4 );
arrset1.add( 5 );
System.out.println( "HashSet before "
+ "retainAll() operation : "
+ arrset1);
HashSet<Integer>
arrset2 = new HashSet<Integer>();
arrset2.add( 1 );
arrset2.add( 2 );
arrset2.add( 3 );
System.out.println( "Collection Elements"
+ " to be retained : "
+ arrset2);
arrset1.retainAll(arrset2);
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
import java.util.*;
public class GFG1 {
public static void main(String[] argv) throws Exception
{
try {
HashSet<Integer>
arrset1 = new HashSet<Integer>();
arrset1.add( 1 );
arrset1.add( 2 );
arrset1.add( 3 );
arrset1.add( 4 );
arrset1.add( 5 );
System.out.println( "HashSet before "
+ "retainAll() operation : "
+ arrset1);
HashSet<Integer>
arrset2 = null ;
System.out.println( "Collection Elements"
+ " to be retained : "
+ arrset2);
System.out.println( "\nTrying to pass "
+ "null as a specified element\n" );
arrset1.retainAll(arrset2);
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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