Re: Force loading of derived classes?

From:

"A. W. Dunstan" <no@spam.thanks>

Newsgroups:

comp.lang.java.programmer

Date:

Thu, 27 Jul 2006 11:29:23 -0400

Message-ID:

<ybednV8VYIVIRlXZnZ2dnUVZ_o-dnZ2d@speakeasy.net>

AndrewMcDonagh wrote:

A. W. Dunstan wrote:

I'm developing an application where the user selects an algorithm named
in a drop-down list, optionally sets parameters, then presses "go". We
then do stuff with the results of running the algorithm, but that has
no bearing on my question.

Each algorithm is embodied in a class, where each of these classes is
derived from an abstract base class. At present, the GUI drop-down
list knows about all available algorithms at compile time, which seems
ugly.

I thought that if the base class could keep a static map of <algorithm
name, exemplar> then the GUI could iterate over the names to build the
drop-down. Hard-coding that information in the base class wouldn't be
much better than hard-coding it in the GUI, but if the map is static
and is built as the classes are loaded then I wouldn't have that
worry.

This would have worked, were it not for the fact that the GUI gets
constructed before the algorithm classes are even loaded, leaving me
with a null map. I've got a static getter method in the base class
where I can check for a null map, but until the derived classes are at
least loaded it does me no good.

Any suggestions on how to force class loading when you don't know in
advance what those classes are? I know they'll be public and derived
from my base class, but not much else.

Or is there a better way of solving this problem?

Thanks.

Its not class loading you need, its Reflection.

With Reflection you can create an instance of a class using the String
name of the class.

e.g:

  try {
    Class cls = Class.forName("com.myorg.mypackage");

    MyAbstractClass abc = cls.newInstance();

    myDropDown.add(abc);

  } catch (Throwable e) {
//...
  }

The package and Class names can be loaded from a resource file at runtime.

This is essentially how those Inversion of Control frameworks work.
(Google 'java Spring').

Both this and Chris Uppal's suggestion would work, but it seems to me that
they're both variations on what I've got now. Somewhere, there's still a
hardcoded list of which classes to use. Granted, using reflection and a
resource file means I don't have to recompile anything but I still have to
remember a) that I have to put an entry in a resource file, and b) where IS
that file, anyway? The way my memory works, anything that happened before
breakfast is ancient history :-}

If I knew that all classes would be loaded before any code was executed I'd
be all set, but the existence of the ability to do reflection makes that
impossible.

I tried forcing a load of the base class, but that does nothing to the
derived classes. I suppose if I put my class files into a jar file I could
do a wildcard search for com.myorg.mypackage.*, drop anything with a $ in
the name & force a load of what was left. I'd still have to get the name
& path to the jar file, but that might not be insurmountable. Hmmmm...

I guess I'm stuck with hardcoding something somewhere.

Thanks!

--
Al Dunstan, Software Engineer
OptiMetrics, Inc.
3115 Professional Drive
Ann Arbor, MI 48104-5131