Example of Java Collection retainAll()
Example 1:
Below is the implementation of the above method:
Java
import java.util.ArrayList; import java.io.*; class GFG { public static void main (String[] args) { ArrayList<Integer> list1 = new ArrayList<Integer>(); list1.add(1); list1.add(2); list1.add(3); list1.add(4); ArrayList<Integer> list2 = new ArrayList<Integer>(); list2.add(2); list2.add(4); System.out.println("Original List:" + list1); System.out.println("List containing elements to be retained by the calling collection:" + list2); // Retains(or keep) only those elements which are specified inside the collection boolean modified = list1.retainAll(list2); System.out.println("Calling Collection Modified: " + modified); System.out.println("Original List(Elements Retained):" + list1); } } |
Output:
Original List:[1, 2, 3, 4]
List containing elements to be retained by the calling collection:[2, 4]
Calling Collection Modified: true
Original List(Elements Retained):[2, 4]
Example 2:
Java
import java.util.HashSet; import java.io.*; class GFG { public static void main (String[] args) { HashSet<String> hs1 = new HashSet<String>(); hs1.add("Pen"); hs1.add("Paper"); hs1.add("Pencil"); hs1.add("Rubber"); HashSet<String> hs2 = new HashSet<String>(); hs2.add("Pen"); hs2.add("Paper"); System.out.println("Original HashSet Collection:" + hs1); System.out.println("HashSet containing elements to be retained by the calling collection:" + hs2); // Retains(or keep) only those elements which are specified inside the collection boolean modified = hs1.retainAll(hs2); System.out.println("Calling Collection Modified: " + modified); System.out.println("Original HashSet Collection(Elements Retained):" + hs1); } } |
Output:
Original HashSet Collection:[Pen, Paper, Pencil, Rubber]
HashSet containing elements to be retained by the calling collection:[Pen, Paper]
Calling Collection Modified: true
Original HashSet Collection(Elements Retained):[Pen, Paper]
Time Complexity of the above method:
Time Complexity of retainAll(): O(N)
where N is the size of the larger of the two collections as it needs to iterate through both of the collections and look for which element to retain and which to remove.
Java Collection retainAll() Method
In Java, the retainAll() method of Java Collection retains or keeps only those elements present inside the collection which is given as an argument to the function.
Syntax for retainAll() method:
boolean retainAll(Collection<?> c);
Parameters: c is the collection containing elements retained or kept by the calling collection.
Return Type:
It returns a boolean value. It returns true if the calling collection is modified(i.e. any of the elements is removed) and returns false if no elements were removed.
Exception:
- Null Pointer Exception: If this collection is null or it holds one or more elements as null and invoked collection does not allow null elements.
- UnsupportedOperationException: If the retainAll() method is not supported by the collection.
- ClassCastException: If the elements are not mutually comparable(i.e. you have a mix of different object types), this exception can occur when trying to compare the elements.