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

have a look at this post

https://stackoverflow.com/questions/4380796/what-is-public-abstract-interface-in-java/4381308#4381308

interface is %100 abstract class.

the keyword abstract is redundant here

Answer 2

Where did you come across the chunk of code you have posted, any old java code base ?
This is what the JLS has to say :

9.1.1.1 abstract Interfaces:
Every interface is implicitly abstract. This modifier is obsolete and should not be used in new programs.

9.4 Abstract Method Declarations:
For compatibility with older versions of the Java platform, it is permitted but discouraged, as a matter of style, to redundantly specify the abstract modifier for methods declared in interfaces.

Answer 3

"Why declare an interface as abstract?"-I got the same question and thought that abstract is redundant. But had to rethink when I saw the Map interface in java 1.8.May be this has to be changed in java

//  (version 1.8 : 52.0, no super bit)
// Signature: <K:Ljava/lang/Object;V:Ljava/lang/Object;>Ljava/lang/Object;
public abstract interface java.util.Map {

  // Method descriptor #1 ()I
  public abstract int size();
}

Answer 4

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.

Answer 5

The default behavior of an interface is essentially equivalent to what you have in your example. Defining it as abstract is just redundant.