immutability of a class when an instance variable

Question

I have a class which is immutable

Suppose I have a getter method for a member variable of type ArrayList. In that case when I get a reference to that variable, I can add or remove an element from it. In that case immutability seems to get violated.

Can anyone explain this concept in details?

Answer 1

You shouldn't provide variable of type ArrayList. Provide just List and make sure the getter does one of the next:

1. returns a copy of your list
2. or returns unmodifiable list

or both.

Answer 2

if you create a new instance of that changed object of type X, then instance of X would be considered as immutable. It is easier to understand if you consider your ArrayList. YOu've methods to alter this list. Each altering method make a copy of this list and add/removes/updates on the new copy and return back.

Answer 3

You are indeed right. Immutability is violated.

To make a class immutable, you need to make sure that all of its getters return safe copies of any class whose state could otherwise change.