Default interface method for abstract superclass

Question

Lets say I have the following structure:

abstract class A {
     abstract boolean foo();
}

interface B {
     default boolean foo() { return doBlah(); }
}

class C extends A implements B {
    //function foo
}

Java will now complain that class C must implement abstract method foo from A. I can work around this problem relatively easy by redefining the function in C and simply calling B.super.foo();.

however I do not understand why the default function from interface B does not forfill this requirement on its own, and I would like to have a better understanding of the underlying mechanics of java.

Answer 1

Default methods in Interfaces are used to prevent breaking programs that depend on your Interface when you add a method to your Interface.

In the case you describe, it is unclear if the default method in the interface (which by design must have been added later on) actually fulfills the contract as originally envisioned by the Abstract Class.

In this case it is safer for Java to complain.

Above text is my interpretation of the paragraph in 9.4.1.3 of the JLS SE8 spec, and I quote:

Similarly, when an abstract and a default method with matching signatures are inherited, we produce an error. In this case, it would be possible to give priority to one or the other - perhaps we would assume that the default method provides a reasonable implementation for the abstract method, too. But this is risky, since other than the coincidental name and signature, we have no reason to believe that the default method behaves consistently with the abstract method's contract - the default method may not have even existed when the subinterface was originally developed. It is safer in this situation to ask the user to actively assert that the default implementation is appropriate (via an overriding declaration).