Question

Okay, here’s the setup:

EnclosingClass {

    public interface ClassFactory {
        public static SomeClass getInstance(int which);
    }

    private static ClassFactoryImpl {
        @Override
        public static SomeClass getInstance(int which) {
            switch(which) {
            case 1:
                return new SomeClassSubclassA();
            case 2:
                return new SomeClassSubclassB();
            ...
            }
        }
    }
}

I would like to be able to issue statements along the line of:

SomeClass x = EnclosingClass.ClassFactory.getInstance(instanceClassRequest);

Is this possible? If not, how can I access a static nested class through only the interface it implements?

Answer 1

The short answer is “no.” You’ll need to make an instance of your implementation class and put it in a static variable. It’ll look like this:

public class EnclosingClass {
    public interface ClassFactory {
        public SomeClass getInstance(int which);
    }
    public static final ClassFactory CLASS_FACTORY;

    private static class ClassFactoryImpl implements ClassFactory {
        public SomeClass getInstance(int which) { /* ... */ }
    }

    static {
        CLASS_FACTORY = new ClassFactoryImpl();
    }
}

Also note that the method on the instance is no longer static.

Then code that invokes it would look like this:

SomeClass x = EnclosingClass.CLASS_FACTORY.getInstance(2);