In your example, the declaration of ArrayList look like:

public class ArrayList extends AbstractList implements List {
}

The dependence on AbstractList is public. Any public method in AbstractList is also accessible from the outside.

Compare it with my solution:

public class ArrayList implements List {
    private AbstractList = new AbstractList();
}

Nothing about the internals of ArrayList is exposed. You’re free to change the internals however you want. There’s no chance any outside code will depend on this implementation detail.