In your example, the declaration of ArrayList look like:
public classArrayListextendsAbstractListimplementsList{
}
The dependence on AbstractList is public. Any public method in AbstractList is also accessible from the outside. It opens up for tricky dependencies that can be difficult to unravel.
Compare it with my solution:
public classArrayListimplementsList{
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.
In my example, how is the code dependency exposed to the outside? The caller only knows about the List interface in my example.
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. It opens up for tricky dependencies that can be difficult to unravel.
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.