Is it possible to create an anonymous class while using reflection?

I would like to be able to implement a method at runtime that is called before an object runs the initializers. This will allow me to set fields that are used during initialization.

Here is an example:

class A {
  public A() {
    initialize();
  }
  public void initialize() { }
}

class B extends A {
  public String message;
  {
    System.out.println(message);
  }
}

public class MainClass {
  public static void main(final String[] args) throws Exception {

    Class<A> aClass = (Class<A>)Class.forName(args[0]);
    // what's next in order to something like this even though
    // I don't know what subclass of A was passed in as an
    // argument above 
    A a = aClass.newInstance()
    {
      public void initialize() {
        this.message = args[1];
      }
    };
  }
}

I will probably end up using aspects, but I would like to know if there is a pure Java way.