Can a data abstraction use a class as a field, parameter, or return type?

Question

I think that everyone will agree that the following class achieves data abstraction (hiding of its internal representation):

class A {
    private List x = new ArrayList();

    public List getX() {
        return this.x;
    }

    public void setX(List x) {
        this.x = x;
    }
}

But could we say the same about the following one, which uses a class (instead of an interface like in the previous class) as a field type, parameter type, and return type?

class B {
    private ArrayList x = new ArrayList();

    public ArrayList getX() {
        return this.x;
    }

    public void setX(ArrayList x) {
        this.x = x;
    }
}

Answer 1

Sounds subjective to me. Data abstraction is not a black-and-white binary property; it is a matter of shades of grey. Any abstraction hides some information and reveals others.

Another way to put it is that these two classes expose a different amount of functionality and make a different set of promises. So, what do you want the client to be aware of, and what do you want them to not have to know about? What future changes do you want to allow yourself to make to the class (without having to change clients), and what future changes are not important to be able to make? The answers to those questions will determine which of these two implementations you choose. Each could be a reasonable choice that is consistent with the principle of data abstraction in different circumstances.

Also, remember that abstraction is a means to an end, not a goal in and of itself.

See, e.g., https://www.joelonsoftware.com/2002/11/11/the-law-of-leaky-abstractions/.