[DGD] object types: examples from the kernel lib

[Greg Lewis]

On Mon, Jul 12, 2004 at 11:55:24AM +0200, Par Winzell wrote:
> In e.g. Java, an object that tries to call a function on an object will 
> simply not compile unless the object is of a type undeniably known to 
> implement F. It's not merely impossible to call a non-existent function, 
> it is impossible for the code to even try.

This isn't strictly true.  There are a number of ways to get around this
(i.e. to produce compilable code that will cause an exception at runtime).

Off the top of my head:

1. Pass the object to a function which takes a parameter of type Object
   (or any other common base class) and casts it to a specific type before
   calling the method.  This will produce a ClassCastException at runtime
   if you pass an object not of the correct type.

2. Use the reflection framework to get a Method object from one class and
   simply use its invoke() method with an instance of a class of a 
   different type.  This will produce an IllegalArgumentException at
   runtime.

Admittedly both of these require that there is _some_ class which
implements F.  This is, however, weaker than the assertion that the
object's class itself must implement F for the code to be compilable.

But back to our regularly scheduled DGD programming :).

-- 
Greg Lewis                          Email   : glewis at eyesbeyond.com
Eyes Beyond                         Web     : http://www.eyesbeyond.com
Information Technology              FreeBSD : glewis at FreeBSD.org
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