The language designers already thought about that, so these things are enforced by the compiler. So if you define:
interface First {
default void go() {
}
}
interface Second {
default void go() {
}
}
And you implement a class for both interfaces:
static class Impl implements First, Second {
}
you will get a compilation error; and you would need to override go
to not create the ambiguity around it.
But you could be thinking that you can trick the compiler here, by doing:
interface First {
public default void go() {
}
}
static abstract class Second {
abstract void go();
}
static class Impl extends Second implements First {
}
You could think that First::go
already provides an implementation for Second::go
and it should be fine. This is too taken care of, thus this does not compile either.
JLS 9.4.1.3 : 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).
The last point I would bring in, to solidify that multiple inheritance is not allowed even with new additions in java, is that static methods from interfaces are not inherited. static methods are inherited by default:
static class Bug {
static void printIt() {
System.out.println("Bug...");
}
}
static class Spectre extends Bug {
static void test() {
printIt(); // this will work just fine
}
}
But if we change that for an interface (and you can implement multiple interfaces, unlike classes):
interface Bug {
static void printIt() {
System.out.println("Bug...");
}
}
static class Spectre implements Bug {
static void test() {
printIt(); // this will not compile
}
}
Now, this is prohibited by the compiler and JLS
too:
JLS 8.4.8 : A class does not inherit static methods from its superinterfaces.