Java 8 - 两个接口包含具有相同方法签名但返回类型不同的默认方法，如何覆盖？

Question

I understand that if a class implements multiple interfaces containing default methods of same name, then we need to override that method in the child class so as to explicitly define what my method will do.
Problem is, see the below code :

interface A {
    default void print() {
        System.out.println(" In interface A ");
    }
}

interface B {
    default String print() {
        return "In interface B";
    }
}

public class C implements A, B {

    @Override
    public String print() {
        return "In class C";
    }

    public static void main(String arg[]) {
        // Other funny things
    }
}

Now interface A and B both have a default method with name 'print' but I want to override the print method of interface B - the one that returns a string and leave A's print as is. But this code doesn't compile giving this :

Overrides A.print
The return type is incompatible with A.print()

Clearly compiler is trying to override A's print method, and I have no idea why !

Answer 1

This is not possible.

8.4.8.3 :

If a method declaration d 1 with return type R 1 overrides or hides the declaration of another method d 2 with return type R 2 , then d 1 must be return-type-substitutable for d 2 , or a compile-time error occurs.

8.4.5 :

A method declaration d 1 with return type R 1 is return-type-substitutable for another method d 2 with return type R 2 iff any of the following is true:

• If R 1 is void then R 2 is void .

• If R 1 is a primitive type then R 2 is identical to R 1 .

• If R 1 is a reference type then one of the following is true:

  • R 1 , adapted to the type parameters of d 2 , is a subtype of R 2 .

  • R 1 can be converted to a subtype of R 2 by unchecked conversion.

  • d 1 does not have the same signature as d 2 , and R 1 = |R 2 | .

In other words, void , primitive- and reference-returning methods may only override and be overridden by methods of that same respective category. A void method may only override another void method, a reference-returning method may only override another reference-returning method, and so on.

One possible solution to the problem you're having could be to use composition instead of inheritance:

class C {
    private A a = ...;
    private B b = ...;
    public A getA() { return a; }
    public B getB() { return b; }
}