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Stack retainAll() method in Java with Example

Last Updated : 24 Dec, 2018
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:
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
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
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