Java: How write a cast that specifies both a superclass and an interface?

Question

I have something like this going on in my Java program:

void f(Object o) {
    g(o);
}

<T extends MySuperClass & MyInterface> void g(T x) {
    ...;
}

How can I cast o so that this works? There seems to be no way to specify both a base class and an interface in variable declarations without using generics. I don't think generics will work here, because o is being created dynamically using reflection, so its actual class is not known at compile time.

(Yeah, I know this is a weird thing to want to do. But I actually do need functionality from both the superclass and the interface. I guess I could do all of the type checking at runtime with instanceof, but that just seems so Java 1.4...)

Answer 1

Could it be something like

<T extends MySuperClass implements MyInterface>

I'm just guessing here...

Answer 2

You can create a new inner class within the class containing f() calling g()

static abstract class MyCastingClass extends MySuperClass implements MyInterface {}

Then you can cast:

 g((MyCastingClass)o);

EDIT:

This doesn't appear to work though.

It allows your code to compile, and has no warnings, but at runtime, you'll get a ClassCastException unless your object is actually an instance of MyCastingClass

Answer 3

It seems to be there is no way to invoke a "raw" generic method. But you can create an object of raw type (the following conversion is, obviously, unsafe):

void f(Object o) {
    Caster<?> c = new Caster(); 
    g(c.cast(o)); 
}

class Caster<T extends MySuperClass & MyInterface> {
    public T cast(Object o) {
        return (T) o;
    }
}

Answer 4

You can use reflection to invoke g, similar to the below, but with more checks on picking the correct merhod, and handling exceptions:

Object dest_obj_containing_g = this; // Replace with your object.
Method[] methods = dest_obj_containing_g.getClass().getDeclaredMethods();
for (Method m : methods) {
  if (m.getName().equals("g")) {
    m.invoke(dest_obj_containing_g,o);
    break;
  }
}