immutability of a class when an instance variable present as arraylist

问题

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?

回答1:

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.

回答2:

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.

回答3:

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.

来源：https://stackoverflow.com/questions/9006040/immutability-of-a-class-when-an-instance-variable-present-as-arraylist