Restrict generic types in java based on implementation of generic interface

Question

So I have got 2 generic interfaces.

First interface is implemented like this.

public interface First<E>
{
   void method(E e)
}

public class FirstImpl implements First<String>
{
   void method(String s) { System.out.println(s); }
}

public class FirstImpl2 implements First<Double>
{
    void method(Double d) { System.out.println(d); }
}

I need the second interface's (second interface is shown below) generic type to allow only the classes that are used when implementing the first interface, in our case String and Double. Is there any clean way to do this, something like

public interface Second <E, ? extends First<E>>
{
    void method(E e);
}

public class SecondImpl <E> implements Second <E, ? extends First<E>>
{
    void method(E e) { System.out.println(e); }
}

, so the in Second's generic E would fit only String and Double and all classes that are used to implement First<E>?

Answer 1

Nope. You can not restrict the generic type of the Second in that sense. You can still provide an another type information independently. Say,

class XYZ implements First<Bar> { ... }

an another class may provide an another type information for the Second, like

class ZYX implements Second<Foo, SomeOtherType<Foo>> { ... } 

assuming SomeOtherType implements/extends whatever from type First. If you want to bind those two interfaces on their generic type, you can use inheritance between the implementations:

  interface First<T> {}
  interface Second<T> {}
  class Foo<E extends T> implements First<T> {}
  class Bar<E extends T> extends Foo<E> implements Second<E> {}

Now, the type E, is associated with the type T, via E extends T.