Why can an abstract class omit the implementation and declaration of the methods of the implemented interface?

Question

OOP

Yes, this happens, after several years of coding I noticed such a feature, I had never used it before, and now I still have no idea how and where it can be used wisely. Previously did an abstract class or interfaces. I came across this in java public abstract class ArrayList implements List{and then I realized that you can skip methods from the interface, because IMHO there are a lot of them.
So, why and why so? How, where and when can and should it be applied?

Answer 1

The only difference between an interface and an abstract class is that an interface cannot have state (fields, variables). Thus, interfaces do not need to solve the problem of diamond-shaped inheritance.
There are no objective reasons for the rest of the differences - it's just syntactic sugar.

Answer 2

An abstract class does not contain descriptions of methods. In it, you can add your methods to the interface, and the description of the functionality will be handled by a real class that inherits the created abstract one.