How can a public method return a private type?

Question

Why is it possible to return a private nested class from a public method in a public class? Shouldn't the compiler complain about the return type's visibility being less than the method?

public final class Outer {
    private static final class Configurator {
        private Configurator() {
        }
    }

    public static Configurator configure() {
        return new Configurator();
    }
}

Answer 1

You can call such a method from outside the class, but only if you are happy to throw away the result.

public class TestClass {
  public static void main(String[] args) throws Exception {
    Outer.configure(); // this is valid
  }
}

or if you are happy to refer to the result as an Object:

public class TestClass {
  public static void main(String[] args) throws Exception {
    Object o = Outer.configure(); // this is valid
  }
}

The compiler allows this because it doesn't break any rules of Java. Object is simply the only publicly available superclass of your private class.

I doubt there are many practical uses for this pattern. If you wish to return an opaque object, that's better done by passing back a public class with no public methods, since you can at least type-check it when passed back into classes that need to use it.