Conflicting interface methods in Java

Question

Possible Duplicate:
Method name collision in interface implementation - Java

What do we do if we need to implement two interfaces both of which contain a method with the same name and parameters, but different return types? For example:

interface A {
    public int foo();
}

interface B {
    public double foo();
}

class C implements A, B {
    public int foo() {...}  // compilation error
}

Is there an easy way to overcome this issue?

Why would you have two interfaces like that in the first place?? Or are you getting it from somewhere else? — Rohit Jain, Oct 08 '12 at 13:13
Use `Facade` pattern to enclose invokation of one equal-named method? As you can't declare two methods with equal names(even with diffirent return type) within one class. — Yegoshin Maxim, Oct 08 '12 at 13:17

Peter Lawrey · Accepted Answer · 2012-10-08T13:27:10.137

14

The simplest solution is to always return double in A as it can store every possible int value.

If you is not an option you need to use an alternative to inheritance.

class C {
    public A getA();
    public B getB();
}

C c = new C();
int a = c.getA().foo();
double b = c.getB().foo();

edited Oct 08 '12 at 13:27

answered Oct 08 '12 at 13:15

Peter Lawrey

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what if return type is an object(any) and int? we can not do same. – Nandkumar Tekale Oct 08 '12 at 13:20
2

Simply returning double won't solve the problem... There will still be a compiler error. You would also need to change the return type of `foo()` in A (which might not be possible if A was a previously existing interface). Your other proposed solution is probably better. – Alderath Oct 08 '12 at 13:23
@NandkumarTekale You have to find a super type for both methods. Or use the second approach. – Peter Lawrey Oct 08 '12 at 13:29
Another idea: Using Number instead of double. – Grim Dec 09 '12 at 07:57

score 13 · Answer 2 · answered Oct 08 '12 at 13:14

13

You cant. Java uniquely identifies methods by their name and their parameters, not their return type.

answered Oct 08 '12 at 13:14

Samuel

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This is one of the rules of overriding or overloading. – Prasanth Oct 08 '12 at 13:20

score 4 · Answer 3 · answered Oct 08 '12 at 13:27

You can write an Adapter class to implement one of the interfaces.

Example implementation:

class AdapterA implements A{
     AdapterA(C c){impl = c;}
     private final C impl;
     public int foo(){return c.fooReturningInt();}
}
class C implements B{

   public double foo(){...}
   public int fooReturningInt(){...}
}

score 3 · Answer 4 · answered Oct 08 '12 at 13:16

3

Use Number instead of double and int in interface A and B.

answered Oct 08 '12 at 13:16

Grim

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score 3 · Answer 5 · edited Oct 08 '12 at 13:22

3

A method in Java is uniquely defined by its signature. From http://docs.oracle.com/javase/tutorial/java/javaOO/methods.html

Definition: Two of the components of a method declaration comprise the method signature—the method's name and the parameter types.

edited Oct 08 '12 at 13:22

maba

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answered Oct 08 '12 at 13:16

manub

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Actually, same name and argument with different return type are illegal in java code, but legal in bytecode. – Matthieu Sep 19 '17 at 14:11

score 2 · Answer 6 · answered Oct 08 '12 at 13:15

Your foo() method is clearly not well defined. Likely there should be a parent interface with a public Number foo(), which is extended by A and B who override that to a more specific type. There isn't really a sensible way your class can implement both those interfaces unless you rename one of the foo methods.

Conflicting interface methods in Java

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