Compilation of generic class using generic interface failed

As far as I know, the following code should run without any compilation errors

However, when I run this program, I get the following compilation errors

class B<X> {
    interface C {
    }

    interface D<Y> {
    }
}

class Test {
     // compilation fails here
    B<String>.D<String>[] arr = new B<String>.D<String>[10];
}

Please help me understand this behavior

Solution

In a way similar to an internal static class, a nested interface is not associated with an instance of its external class, but only with the class itself All static members are shared among all instances of B regardless of the type parameters of B consider:

class B<T> {
    public static int shared = 0;
}

Variable sharing is the same in B < string >, B < integer >, and B < Object > wait. Attempting to access it on parameterized B will result in a compilation error:

int copy = B<String>.shared; // <<== Does not compile

Therefore, the type parameter of B has no effect on the declaration of arr, so Java wants you to delete it:

B.D<String>[] arr = new B.D[10];