The retainAll() method of java.util.Stack class is used to retain from this stack 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 stack.
Returns Value: This method returns true if this stack changed as a result of the call.
Exception: This method throws NullPointerException if this stack 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 Stack<Integer>            Stack<Integer>                stack1 = new Stack<Integer>();              // Populating stack1            stack1.add(1);            stack1.add(2);            stack1.add(3);            stack1.add(4);            stack1.add(5);              // print stack1            System.out.println("Stack before "                               + "retainAll() operation : "                               + stack1);              // Creating another object of Stack<Integer>            Stack<Integer>                stack2 = new Stack<Integer>();            stack2.add(1);            stack2.add(2);            stack2.add(3);              // print stack2            System.out.println("Collection Elements"                               + " to be retained : "                               + stack2);              // Removing elements from stack            // specified in stack2            // using retainAll() method            stack1.retainAll(stack2);              // print stack1            System.out.println("Stack after "                               + "retainAll() operation : "                               + stack1);        }          catch (NullPointerException e) {            System.out.println("Exception thrown : " + e);        }    }} |
Output:
Stack before retainAll() operation : [1, 2, 3, 4, 5] Collection Elements to be retained : [1, 2, 3] Stack 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 Stack<Integer>            Stack<Integer>                stack1 = new Stack<Integer>();              // Populating stack1            stack1.add(1);            stack1.add(2);            stack1.add(3);            stack1.add(4);            stack1.add(5);              // print stack1            System.out.println("Stack before "                               + "retainAll() operation : "                               + stack1);              // Creating another object of Stack<Integer>            Stack<Integer>                stack2 = null;              // print stack2            System.out.println("Collection Elements"                               + " to be retained : "                               + stack2);              System.out.println("\nTrying to pass "                               + "null as a specified element\n");              // Removing elements from stack            // specified in stack2            // using retainAll() method            stack1.retainAll(stack2);              // print stack1            System.out.println("Stack after "                               + "retainAll() operation : "                               + stack1);        }          catch (NullPointerException e) {            System.out.println("Exception thrown : " + e);        }    }} |
Output:
Stack 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
