Question

This is related to final interface in java. Among the discussion there was that the concept of final in relation to interfaces is ambiguous. Would a final interface mean that it can not have subinterfaces? Would it mean that it can not have implementations?

This question is the first: can you write an final interface such that the compiler will prevent you from implementing it?

Answer 1

As I will show, it is possible to implement the interface above using a proxy. The more meaningful question is, why would you try to make a unimplementable interface? Even as a philosophical point, it seems rather shallow.

import java.lang.reflect.Proxy;
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;

class NoFinal
{
    public static void main(String[] a) throws Throwable
    {
        FinalInterface o = (FinalInterface) Proxy.newProxyInstance(FinalInterface.class.getClassLoader(), new Class[]{FinalInterface.class}, new InvocationHandler()
        {
            public Object invoke(Object proxy, Method method, Object[] args)
            {
                System.out.println(method);
                return null;
            }
        });
        Method[] methods = FinalInterface.class.getDeclaredMethods();
        methods[0].invoke(o, new Object[]{null});
        methods[1].invoke(o, new Object[]{null});
    }
}

This won’t give an error at compile or run-time, and it shows you can make a real instance of this interface with both methods callable.