I have stumbled upon a strange issue working with default methods.

Consider this situation:

interface A {
    int method();
}

interface B {
    default int method() {
        return 1;
    }
}

class C implements A, B {
    void anotherMethod() { }
}

I would expect class C to be fine as-is: it should implement method, because that's the contract of both A and B, and it actually does, because B provides a default implementation.

However, the compiler does not agree with me, saying:

C is not abstract and does not override abstract method method()

I can totally see the issue in other similar cases, such as this one, but I can't figure out what's wrong with this code.

CodePudding user response：

There may be nothing wrong from a syntax perspective, but the question is: does it make sense to have a default method from interface B, override the abstract method of another unrelated interface A?

Imagine the two interfaces represented two different ways to compute a value: one that allows null values to be returned, and the other does not. If the default method for the first interface returned null, that would violate the contract of the second. And if the compiler allows it to be inherited in class C, then class C is effectively violating one of the implemented interfaces' contract, possibly leading to a bug without the programmer's attention.

Note that if B were a class rather than interface, then there is no issue at compilation, because the issue of "mixed contract" is not present in this case, as there is only one interface.