Java Interfaces – Should You Implement an Interface Directly or Have the Superclass Do It?

1. Java classes are similar to C++ as described in the third paragraph. Java interfaces are a different concept.
2. In Java, a class implicitly specifies both an interface in the GoF sense and the implementation. By contrast a Java interface explicitly specifies an interface in the GoF sense and doesn't specify an implementation. In both cases inheritance implies interface inheritance. Only in the case of class inheritance does it imply implementation inheritance.

For a better example of the distinction, I highly recommend learning just enough Ruby to understand how a class can use method_missing to proxy off of an unrelated object. This provides a very explicit example of interface inheritance without implementation inheritance. (Bonus, Ruby's OO model is the same as Smalltalk's while having a more familiar syntax, which gives you a leg up on the rest of the book.)

A rule of thumb is that:

• Interfaces define the public API (contract) or a certain functionality if you will,
• Abstract classes (may) provide a private API (for extended classes to use), such as shared functionality.

So i am confused which methods should go where

If a method should be part of the public API, add it to the interface and update dependencies where necessary (i.e. Abstract and / or Generic class).

Otherwise, implement the method as high up as is needed based on how generic or specific it is.

Sometimes i see class A extends Abstractuserclass and sometimes class A extends Abstractuserclass implements useraInterface. what is the difference if Abstractclass already implements Userinterface

There's no difference in that case; you don't have to specify an interface twice if the ancestor already implements it, because the interpreter will throw an error if you try to break the contract.

This would only make sense if the child class implements another interface altogether. And this is a strength of interfaces; since you're defining a piece of functionality rather than a specific implementation, you can make any class implement any interface if you wanted to. This is a great help when writing mock classes for testing.