Immutable Object with ArrayList member variable - why can this variable be changed?

Question

I have got one class with various member variables. There is a constructor and there are getter-methods, but no setter-methods. In fact, this object should be immutable.

public class Example {
   private ArrayList<String> list; 
}

Now I noticed the following: when I get the variable list with a getter-method, I can add new values and so on - I can change the ArrayList. When I call the next time get() for this variable, the changed ArrayList is returned. How can this be? I didn't set it again, I just worked on it! With a String this behaviour isn't possible. So what is the difference here?

Answer 1

Just because the reference to the list is immutable doesn't mean that the list it refers to is immutable.

Even if list was made final this would be allowed

// changing the object which list refers to
example.getList().add("stuff");

but this would not allowed:

// changing list
example.list = new ArrayList<String>();   // assuming list is public

---

In order to make the list immutable (prevent also the first line), I suggest you use Collections.unmodifiableList:

public class Example {
    final private ArrayList<String> list;

    Example(ArrayList<String> listArg) {
        list = Collections.unmodifiableList(listArg);
    }
}

(Note that this creates an unmodifiable view of the list. If someone is holding on to the original reference, then the list can still be modified through that.)

---

With a String this behaviour isnt possible. So what is the difference here?

That is because a String is already immutable (unmodifiable) just as the list would be if you turned it into an unmodifiableList.

Comparison:

              String data structure  | List data structure
           .-------------------------+------------------------------------.
Immutable  | String                  | Collection.unmodifiableList(...)   |
-----------+-------------------------+------------------------------------|
Mutable    | StringBuffer            | ArrayList                          |
           '-------------------------+------------------------------------'