Date: Sun, 17 Nov 2002 21:13:36 -0500 (EST)
From: Li Liao <lliao@ren.eecis.udel.edu>
Subject: Re: casting 



>
> I'm having problem understanding how casting is supposed to work.
> Is it almost the same as creating a new object?Then how do we initialize
> the fields in this case?To default values as well?
> It would be vary helpful to have a detailed procedure description
> of how casting is done.

About Downcasting:

Normally, when you assign an object of a subclass to a variable (or
parameter or field) that is expecting an object of a superclass, you lose
the ability the access any new fields or methods of the object.
For example, consider two classes:
  class A {
    int m1() = ...;
  }

  class B extends A {
    int m2() = ...;
  }

Now, suppose we have two variables
  A a;
  B b;

At some point in the program, we could say
  a = b;

We could then call a.m1(), but we could no longer call a.m2(). In this
situation, we know that the actual object in variable a supports the m2
method. The way to regain access to the m2 method is to do a downcast, i.e,
  ([B]a).m2()

This performs a run-time check that the actual object has type B (or a
subtype of B). If this check fails, then the program halts with a
BadDowncast exception. The downcast is also considered to succeed
if the actual value is null.

Hope this is helpful.

Li