Why retainAll in ArrayList throws an Exception

Question

I have created a new ArrayList using subList Method.Now when I try to perform intersection operation using retainAll it Throws following exception

retainAll() Method works for Below Code

List<Integer> arrNums1 = new ArrayList<Integer>();
arrNums1.add(1);
arrNums1.add(2);
arrNums1.add(3);

List<Integer> arrNums2 = arrNums1.subList(0, 1);
arrNums2.retainAll(arrNums1);

But when i try to apply retainAll for Below code it generates Exception as Below

Java Code

public class Generics1
{   
 public static void main(String[] args)
 {
       List<Fruits> arrFruits = new ArrayList<Fruits>();

        Fruits objApple  = new Apple();
        Fruits objOrange = new Orange();
        Fruits objMango  = new Mango();

        arrFruits.add(objApple);
        arrFruits.add(objOrange);
        arrFruits.add(objMango);

        List<Fruits> arrNewFruits = arrFruits.subList(0, 1);

        System.out.println(arrFruits.retainAll(arrNewFruits));
  }
}

class Fruits {}

class Apple extends Fruits {}

class Orange extends Fruits {}

class Mango extends Fruits {}

ERROR

Answer 1

When you use List#subList():

Returns a view of the portion of this list between the specified fromIndex, inclusive, and toIndex, exclusive. (If fromIndex and toIndex are equal, the returned list is empty.) The returned list is backed by this list, so non-structural changes in the returned list are reflected in this list, and vice-versa. The returned list supports all of the optional list operations supported by this list.

You are allowed to mutate elements within it but not change the structure of the list.

The doc further says :

The semantics of the list returned by this method become undefined if the backing list (i.e., this list) is structurally modified in any way other than via the returned list. (Structural modifications are those that change the size of this list, or otherwise perturb it in such a fashion that iterations in progress may yield incorrect results.)

The retainAll() function uses an iterator to delete the non intersecting values , this causes ConcurrentModificationException. Note what the documenation says :

Note that this exception does not always indicate that an object has been concurrently modified by a different thread. If a single thread issues a sequence of method invocations that violates the contract of an object, the object may throw this exception.

Make a copy of the List and then perform retainAll():

List<Fruits> arrNewFruits = new ArrayList<>(arrFruits.subList(0, 1));