Incase it doesn’t show up:
Say List is an interface.
You have implementations like ArrayList and LinkedList.
Many of those method implementations will differ. But some will be identical. The identical ones go in the abstract base class, so you can share method implementation inheritance without duplicating code.
That’s why.
If the lists have shared components then that can be solved with composition. It’s semantically the same as using abstract classes, but with the difference that this code dependency doesn’t need to be exposed to the outside. This makes the dependency more loosely coupled.
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.