How To use super to default implementation in an interface

Question

It will awesome if I can apply a refer to a default implementation to the interface in the following program using super. Like Alpha.super.reset() so, plz tell us where this statement will be use.

interface Alpha {
  default void reset() {
    System.out.println("This is alpha version of default");
  }
}

interface Beta {
  default void reset() {
    System.out.println("This is beta version of default");
  }
}

class MyClass implements Alpha, Beta {
  void display() {
    System.out.println("This is not default");
  }
}

class MainClass {
  public static void main(String args[]) {
    MyClass ob = new MyClass();
    ob.reset();
    ob.display();
  }  
}

For example if we want to use the beta version of reset() method, then we have to extend one of them two interfaces. For example : interface Beta extends Alpha then it will use the beta version of reset(). But here we want to use the super and where it will be use.

`interface` does not allow implementation. What you are looking for seems to be `trait`s in scala? — shinkou, Dec 26 '14 at 04:21
Like `Alpha.super.reset();` That's exactly the way you refer to it. — Sergey Kalinichenko, Dec 26 '14 at 04:22
@vikeng21 "default" was introduced in Java 8. That is why the interface contains code. — Philipp Claßen, Dec 26 '14 at 04:23
@community downvoting without comments is conter-productive, we have a new user here — Alexander Malakhov, Dec 26 '14 at 04:23
For example if we wants to use the beta version of reset() method, then we have to extend one of them two interfaces,,,for example : 'interface beta extends alpha' then it will use the beta version of "reset()". but here we want to use the super and where it will be use. — Manohar Kumar, Dec 26 '14 at 04:25
@Philipp Claßen thanks did not about know this feature in Java 8. have removed comment — vikeng21, Dec 26 '14 at 04:26

score 2 · Accepted Answer · edited Apr 19 '18 at 13:29

2

The syntax for calling the default method implementation that could be coming from two different interfaces uses the name of the interface followed by .super as follows:

class MyClass implements Alpha, Beta {
    public void reset() {
        Alpha.super.reset();
        Beta.super.reset();
    }
}

This implementation of the reset() method calls the default method implementations from both interfaces.

edited Apr 19 '18 at 13:29

Vukašin Manojlović

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answered Dec 26 '14 at 04:24

Sergey Kalinichenko

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When i trying to apply the above code what you write in answer than i get error, that `MyClass cannot implement reset` .Plz edit your answer. – Manohar Kumar Dec 26 '14 at 04:39
@ManoharKumar That's because the method is supposed to be public. – Sergey Kalinichenko Dec 26 '14 at 04:44

score 0 · Answer 2 · answered Dec 26 '14 at 04:43

Compiling your code would produce an error:

java: class MyClass inherits unrelated defaults for reset() from types alpha and beta

because the compiler cannot select a declaration to use in this conflicting situation.

Here is an article for your reference.

How To use super to default implementation in an interface

2 Answers2