This blog demonstrates diffrence between ArrayList remove() method
and Collection remove() method that we use and face below types of problem:–

import java.util.*;
public class UsingCollection{
	public static void main(String[] args)	{
		ArrayList<String> lst = new ArrayList<String>();
		Collection<String> col = lst;
		lst.add("one" );
		lst.add("two" );
		lst.add("three" );
		lst.remove(0);
		col.remove(0);
		System.out.println("Added three items, remove two, so 1 item to remain." );
		System.out.println("Number of elements is: " + lst.size());
		System.out.println("Number of elements is: " + col.size());
	}
}

The output from the previous code is as follows:
Added three items, remove two, so 1 item to remain.
Number of elements is: 2
Number of elements is: 2

Question : don’t make any change’s in elements why???
MyAnswer :
Reason :

Collection remove :
• boolean remove(Object o)
  Removes a single instance of the specified element from this collection, if it is present (optional operation).

ArrayList remove :
• boolean remove(Object o)
  Removes the first occurrence of the specified element from this list, if it is present.
• E remove(int index)
  Removes the element at the specified position in this list.

try this!!!!!!!!!

Number of elements is: 2