Can I declare normal functions in an Interface?

Question

i was studied Java in oracle website. on that, i saw the example like

public class Horse {
   public String identifyMyself() {
      return "I am a horse.";
   }
}
public interface Flyer {
   default public String identifyMyself() {
       return "I am able to fly.";
   }
}
public interface Mythical {
   default public String identifyMyself() {
       return "I am a mythical creature.";
   }
}
public class Pegasus extends Horse implements Flyer, Mythical {
   public static void main(String... args) {
       Pegasus myApp = new Pegasus();
       System.out.println(myApp.identifyMyself());
   }
}

Can i write interface like this? I hope that i can only write abstract functions in interface. then why in oracle website they are gave example like this?

http://docs.oracle.com/javase/tutorial/java/IandI/override.html this is the link — user3345682, Feb 24 '14 at 07:54
See also: http://stackoverflow.com/questions/18286235/default-implementation-of-method-define-in-interface-default?rq=1 , http://stackoverflow.com/questions/16764791/how-does-java-8-new-default-interface-model-works-incl-diamond-multiple-inhe — user2864740, Feb 24 '14 at 07:56
I didn't know this. Interfaces with implementations. I feel sick. — weston, Feb 24 '14 at 07:58
@weston You should feel happy for Java 8, not sick. Java has needed a kick-start for a good while. — user2864740, Feb 24 '14 at 07:58

Subhrajyoti Majumder · Accepted Answer · 2014-02-24T09:23:56.670

7

This feature is avaialble in java 8, it is called default method or defender method.

Default methods enable you to add new functionality to the interfaces of your libraries and ensure binary compatibility with code written for older versions of those interfaces.

Find more about default method here.

java 8 snapshoot version is available jdk8 Build b129 .

There is one common question that people ask about default methods when they hear about the new feature for the first time: "What if the class implements two interfaces and both those interfaces define a default method with the same signature?".

but it is handled during compile time, get more explanation with example Here

edited Feb 24 '14 at 09:23

answered Feb 24 '14 at 07:52

Subhrajyoti Majumder

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oh k... so i can declare normal function also in interface ah? – user3345682 Feb 24 '14 at 07:53
are you thinking about diamond problem? – Subhrajyoti Majumder Feb 24 '14 at 07:56
Yup. Now I have no idea which function will be called. With the pure interfaces and single inheritance I always knew. – Bathsheba Feb 24 '14 at 07:57
It is handled on compilation. Get an explantion [here](http://zeroturnaround.com/rebellabs/java-8-explained-default-methods/) – Subhrajyoti Majumder Feb 24 '14 at 07:58
1

There is no diamond problem with defender methods. Interfaces can't have state, defender methods can' be final, and you'll be forced by the compiler to choose which implementation you want if you implement two interfaces with a common defender method. – JB Nizet Feb 24 '14 at 08:01
In this case here, `Horse.identifyMyself()` should win as it is a non-`interface` method. Without it, the method invocation was ambiguous. – Holger Feb 28 '14 at 18:12

score 1 · Answer 2 · answered Feb 24 '14 at 07:53

In Java 8, this is possible. And, it's called as default methods in the interfaces.

Default methods enable you to add new functionality to the interfaces of your libraries and ensure binary compatibility with code written for older versions of those interfaces.

Check here for more details

score 0 · Answer 3 · answered Feb 24 '14 at 07:54

0

As other SO users saying its available from java 8 .

but otherwise ..

No you cant , in the interface all the methods ar abstract methods by default .

Hope that Helps .

answered Feb 24 '14 at 07:54

Can I declare normal functions in an Interface?

3 Answers3