Retain the Elemets of a Collection

boolean retainAll(Collection<?> c)

    Retains only the elements in this set that are contained in the specified collection (optional operation). Return true if this set changed as a result of the call.

import java.util.*;
class SetRetainAll{
 public static void main(String args[]){
  Set<Integer> mySet1 = new HashSet<Integer> ();
  Set<Integer> mySet2 = new HashSet<Integer> ();
  Set<Integer> mySet3 = new HashSet<Integer> ();
  
  List<Integer> myList1 = new ArrayList<Integer> ();
  List<Integer> myList2 = new ArrayList<Integer> ();
  List<Integer> myList3 = new ArrayList<Integer> ();
  
  /*Adding Elements to the mySet1 */
  for(int i=0; i < 10; i++)
   mySet1.add(i);
   
  /* Copy data from mySet1 to mySet2 */
  mySet2.addAll(mySet1);
  mySet3.addAll(mySet1);
  myList3.addAll(mySet1);
  
  /* Add Elements to myList1 */
  for(int i=0; i < 20; i+=2)
   myList1.add(i);
   
  /* Add Elements to myList2 */
  for(int i=15; i< 25; i++)
   myList2.add(i);
   
  /* Display the Data */
  System.out.println("Data in mySet1 is \n" + mySet1); 
  System.out.println("\nData in mySet2 is \n" + mySet2);
  System.out.println("\nData in myList1 is \n" + myList1);
  System.out.println("\nData in myList2 is \n" + myList2);
  
  /* Retain the data of myList1 from mySet1 */
  System.out.println("\nIs mySet1 changed after retaining " + mySet1.retainAll(myList1));
  
  /* Retain the data of myList2 from mySet2 */
  System.out.println("\nIs mySet2 changed after retaining " + mySet2.retainAll(myList2));
  
  /* Retain the data of myList3 from mySet3 */
  System.out.println("\nIs mySet2 changed after retaining " + mySet3.retainAll(myList3));
  
  System.out.println("\nElements in mySet1 After retaining is \n" +mySet1);
  System.out.println("\nElements in mySet2 After retaining is \n" +mySet2);
  System.out.println("\nElements in mySet3 After retaining is \n" +mySet3);
   
 }
}

Output

Data in mySet1 is
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

Data in mySet2 is
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

Data in myList1 is
[0, 2, 4, 6, 8, 10, 12, 14, 16, 18]

Data in myList2 is
[15, 16, 17, 18, 19, 20, 21, 22, 23, 24]

Is mySet1 changed after retaining true

Is mySet2 changed after retaining true

Is mySet2 changed after retaining false

Elements in mySet1 After retaining is
[0, 2, 4, 6, 8]

Elements in mySet2 After retaining is
[]

Elements in mySet3 After retaining is
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

1. retainAll throws UnsupportedOperationException if the retainAll operation is not supported by this set.

   Example

   If program tries to perform retainAll on un modifiable Set, then Jave run time  throws UnsupportedOperationException.

import java.util.*;
class SetRetainAllUnsupported{
 public static void main(String args[]){
  Set<Integer> mySet1 = new HashSet<Integer> ();  
  List<Integer> myList1 = new ArrayList<Integer> ();
  Set<Integer> unmodifySet;

  /*Adding Elements to the mySet1 */
  for(int i=0; i < 10; i++)
   mySet1.add(i);
 
  /* Add Elements to myList1 */
  for(int i=0; i < 20; i+=2)
   myList1.add(i);
   
  unmodifySet = Collections.unmodifiableSet(mySet1);
   
  /* Trying to Perform retainAll on unmodifySet */
  unmodifySet.retainAll(myList1);
 }
}

Output

Exception in thread "main" java.lang.UnsupportedOperationException
        at java.util.Collections$UnmodifiableCollection.retainAll(Unknown Source)
        at SetRetainAllUnsupported.main(SetRetainAllUnsupported.java:19)

2. throws ClassCastException if the class of an element of this set is incompatible with the specified collection

3. throws NullPointerException if this set contains a null element and the specified collection does not permit null elements

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