Why declare an interface as abstract?

Question

What\'s point of declaring an interface as abstract? Same thing for an interface method. Is there a point to it?

eg.

public abstract interface Presen         


        

Answer 1

The abstract modifier for an interface method is always redundant as well as the public modifier.

The abstract modifier on the interface itself may be redundant for a strict technical reason as an interface can never be instantiated using the new operator and the interface will always be abstract if asked via reflection.

However, there can be a semantic reason for declaring an interface abstract (that is also supported by various UML tools): You might want to express that an interface is explicitly declared abstract in the way that a non-abstract class may not implement the interface directly but only via a sub-interface. So e.g. you might consider the interface Node as semantically abstract while the sub-interfaces Folder and File that extend Node are semantically not abstract. You will never have an instance that is only a Node - it will be either a Folder or a File.

Even further there are frameworks that allow "instantiation" of interfaces (technically via dynamic proxies). There some interface (e.g. a predefined base interface) are not allowed to supply as argument. For documentation purpose it can make sense in the source code to use the abstract modifier to express such information.