A class implements two interfaces. Which interface does a method belong to?

Question

There are two interfaces B and C each having the same method public m1()

class A implements B and C

If class A has to implement method m1(), the implemented method would be of which interface?

try out with simple program and check with the output. It seems to be interview question. — Hariharan, Nov 18 '13 at 10:18
See http://stackoverflow.com/questions/2598009/method-name-collision-in-interface-implementation-java — ice-phoenix, Nov 18 '13 at 10:19
Good question? I have trouble figuring out what "would be of which interface" is supposed to mean? I mean, what difference does it make? — arne.b, Nov 18 '13 at 10:21
I had asked similar question but it was interface and abstract class http://stackoverflow.com/questions/11296789/same-method-in-interface-and-abstract-class — Nandkumar Tekale, Nov 18 '13 at 10:22

Evgeniy Dorofeev · Accepted Answer · 2013-11-18T10:38:56.193

7

I think we can say that A.m1 implements both B.m1 and C.m1. Because both

B b = new A();
b.m1();

and

C c = new A();
c.m1();

will work

edited Nov 18 '13 at 10:38

answered Nov 18 '13 at 10:20

Evgeniy Dorofeev

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score 3 · Answer 2 · answered Nov 18 '13 at 10:21

3

This is a common problem, this is why having clear instructional method names is important. And good OOP design that will make same methods be abstract.

It is also the reason things are separated out in to classes.

Animal.eat()

Fish extends Animal
    Fish.eat()
Dog extends Animal
    Dog.eat()

answered Nov 18 '13 at 10:21

Peter_James

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4
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1

How does that deal with the question of one method defined in two interfaces? – arne.b Nov 18 '13 at 10:26
@arne.b It shows that in good designs and throughout design structure, this cross of method names will be minimized. A common example is `getSauce(...)` and `setSauce(...)` without the get/set it would just be `sauce(...)` which could mean either, or or both. – Peter_James Nov 18 '13 at 10:29

score 2 · Answer 3 · answered Nov 18 '13 at 10:24

Interface have does not have method body,So it hardly matters which method is implemented

See the following example

package test;
public class a implements i,j{

    @Override
    public void run() {
        // TODO Auto-generated method stub

    }

}
package test;

public interface i {
public void run();
}
package test;

public interface j {
public void run();
}

In the class a run() is overridden but does it matter if it is from interface i or j

score 1 · Answer 4 · answered Nov 18 '13 at 10:22

1

Since interfaces do not have the implementation, it doesn't matter. There are no deadly diamond of death sort of issues here.

answered Nov 18 '13 at 10:22

Kashif Nazar

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score 1 · Answer 5 · edited Jul 05 '14 at 00:36

1

You have to add only one public m1() method. It will be for both the intefaces. And if both the interfaces have the same parameters,the method declaration will be public m1().

edited Jul 05 '14 at 00:36

Utsav T

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answered Nov 18 '13 at 10:22

Kuldeep Choudhary

765
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score 1 · Answer 6 · answered Nov 18 '13 at 10:27

There will be no problem as long as declarations of m1 in B and C are "compatible", i.e. have the same return value.

E.g.

    public interface B {
    void doit();
}

public interface C {
    void doit();
}

public class A implements B, C {

    @Override
    public void doit() {
        // TODO Auto-generated method stub

    }
}

but if the return type differ then it's not clear which is to be called and that will result in compile error like "The return type is incompatible with B.doit()"

A class implements two interfaces. Which interface does a method belong to?

6 Answers6

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