匿名扩展类并同时实现接口？

Question

Suppose I have an interface called Interface, and a concrete class called Base, which, to make thing a bit more complicated, has a ctor that requires some arguments.

I'd like to create an anonymous class that would extend Base and implement Interface.

Something like

 Interface get()
 {
     return new Base (1, "one") implements Interace() {};
 }

That looks reasonable to me, but it doesn't work!

(PS: Actually, the Interface and Base are also generic classes :D. But I'll ignore that for now)

Answer 1

No, you can't do that with an anonymous class. You can create a named class within a method if you really want to though:

class Base {
}

interface Interface {
}

public class Test {
    public static void main(String[] args) {
        class Foo extends Base implements Interface {
        };

        Foo x = new Foo();
    }
}

Personally I'd usually pull this out into a static nested class myself, but it's your choice...

Answer 2

This scenario makes little sense to me. Here's why: assume class Base is this:

class Base {
    public void foo();
}

and Interface is this:

interface Interface {
    public void bar();
}

Now you want to create an anonymous class that would be like this:

class MyClass extends Base implements Interface {
}

this would mean that MyClass inherits (or possibly overrides) method bar from Base and must implement method foo from Interface . So far so good, your class has these two methods either way.

Now, think what happens when you are returning an object of type Interface from your method: you only get the functionality that Interface offers, namely method foo . bar is lost (inaccessible). This brings us down to two cases:

1. Extending Base is useless because you do not get to use its methods, since the new object is seen as an Interface .
2. Interface also has a method foo , but that would mean that Base itself should implement Interface , so you can just extend Base directly:

 class Base implements Interface { public void foo(); public void bar(); } class MyClass extends Base { }