Java。 如何上投类型使用反射？

Question

I have a two classes UpObj and DownObj

public class UpObj {

    public UpObj() {
        System.out.println("Load UpObj ");
    }

}

public class DownObj extends UpObj {

    public DownObj() {
        System.out.println("Load DownObj ");
    }

}

---

public class Caller {

    UpObj obj;

    public Caller(UpObj obj) {
        this.obj = obj;
        System.out.println("!!!");
    }

}

---

public class GNUMakeFile {

    /**
     * @param args
     */
    public static void main(String[] args) {
        DownObj iView = new DownObj();
        Class<?> iViewClass = iView.getClass();
        Class<?> clazz;
        try {
            clazz = Class.forName("bla.bla.bla.Caller");
            Constructor<?> ctor = clazz.getDeclaredConstructor(iViewClass);
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        } catch (SecurityException e) {
            e.printStackTrace();
        } catch (NoSuchMethodException e) {
            e.printStackTrace();
        }

    }

}

So, I want upcast my child type DownObj to parent UpObj in Caller constructor. I think this is possible with help generics. Something like this . Anybody know how exactly this use.

Thanks.

Answer 1

There is no standard way doing it. Try to google for some solutions, eg http://www.xinotes.org/notes/note/1329/ or How to get parameter types using reflection? . As I remember the Spring and some other libraries have similar util functions to do it. Do you use the Spring?

Answer 2

You shouldn't really be using Class.getConstructor() with the runtime type of what you want to pass to it – the method works off the formal parameters.

If you need to search for a constructor that matches some objects that you have, you'll have to loop over getConstructors() , inspect the formal parameters of each constructor, and check whether you found a suitable signature using Class.isAssignableFrom() . I don't think there's a convenient way to have the reflection API do overload resolution for you.

(Alternatively, rethink your approach so messing with reflection is no longer necessary.)

Answer 3

I didn't really understand the problem - upcasting is normally not needed.
but it seams like the problem is to get the upper class of the object,
if use the getSuperclass method:

    Constructor<?> ctor = clazz.getDeclaredConstructor(iViewClass.getSuperclass());

Documentation: getSuperclass()