Why can't I create a new method in an anonymous inner class?

Question

If I have the following class:

public class TestObject {
  public String Hooray() {
    return \"Hooray!\";
  }
}

I can obviously instantiate t

Answer 1

You have this:

public class TestObject {
  public String Hooray() {
    return "Hooray!";
  }
}

TestObject a = new TestObject() {
    public String Boo() {
        return "Booooo";
    }
}

System.out.println(a.Boo());

You can't do this. You can create new methods in anonymous inner classes, and, in fact, you are. But you wouldn't be able to call a.Boo() from outside, since a is a TestObject and TestObject has no method named Boo. It's the same reason you can't do this:

public class Base {
    public void something (); 
}

public class Derived extends Base {
    public void another ();
}

Base b = new Derived();
b.another(); // b is a Base, it must be cast to a Derived to call another().

In the above you have to cast b to a Derived to call the new method added to the derived class:

((Derived)b).another();

The reason that you couldn't do this with anonymous inner classes (which are just syntactic shortcuts for deriving new subclasses) is precisely because they are anonymous - there is no type available for you to cast them to.

The reason you can't access another() through type Base, by the way, is pretty simple when you think about it. While Derived is a Base, the compiler has no way of knowing that Base b is holding a Derived as opposed to some other subclass of Base that doesn't have an another() method.

Hope that helps.