New Adventures in Software by Dan Dyer

Version 0.4.0 of the Watchmaker Framework for Evolutionary Computation is now available for download. This release adds support for interactive evolutionary algorithms. An example applet, based on Richard Dawkin’s famous Biomorph program, demonstrates how the framework can be used with an interactive selection strategy. Using the applet you can direct the evolution of recursive pictures that resemble biological entities such as plants and insects.

The new release also adds a blazingly fast random number generator (a Java port of Tony Pasqualoni’s cellular automaton RNG). This RNG out-performs even the Mersenne Twister. By offering this RNG as an option, the framework provides potential for improved evolutionary performance.

• Change Log
• API Documentation
• Introductory Tutorial
• Travelling Salesman Example Applet
• Biomorphs Example Applet

Having been exposed to pure functional programming in Miranda at university, Haskell is a programming language that I like a lot. I’ve dabbled with it on and off using GHC but I’ve never written a really substantial program in it. Certainly nothing to rival the Java behemoths that I develop day-to-day. Programs written (well) in Haskell are concise, expressive and have an elegance that is hard to match in more general-purpose languages such as C and Java.

Despite the appeals of strongly-typed, side-effect-free programming, Haskell (and non-strict, pure functional languages in general), have long been regarded as primarily the domain of academics. Haskell’s design owes much to the proprietary Miranda system but, unencumbered by commercial constraints, it has rapidly supplanted its predecessor. Even the University of Kent, previously the principal champion of Miranda, has long since switched to Haskell for teaching purposes. Haskell is now a part of undergraduate computer science courses around the world but, despite this, has little visibility in the world of commercial software development.

However, all that may be about to change. This week saw the announcement of the book “Real World Haskell”, to be published by O’Reilly. If the authors deliver on the proposed outline, it promises to provide a real boost for Haskell as a viable language for mainstream software development. The book will cover topics that are often ignored by existing Haskell guides but that are essential for solving real world problems. I/O, databases, GUI development, concurrency and unit testing are just some of the items to be addressed.

At present there is nothing much to see, but I’ll be monitoring their progress closely over the coming months in eager anticipation. Hopefully, by the time the book is out, GHC will have broken free from its LGPL shackles to remove another barrier to widespread Haskell adoption.

Double dispatch is a neat trick for getting around the problem highlighted by Anders Bengtsson in his blog yesterday. The idea is similar to the visitor pattern and is particularly useful for implementing the command pattern. The Java Performance Tuning site has an article describing its use in Java.

Applying the concept to Anders’ example, we would introduce an intermediate call to the Person object that merely calls back to the invoking object to provide the necessary type information that would otherwise be unknown. This will probably be clearer with some example code.

Anders’ example had a Person base class with two sub-classes, Worker and Capitalist. There is also another class that defines overloaded methods called doStuff that operate on each of the concrete types. The problem is how do you invoke the correct doStuff method if you do not know the concrete type of a Person object at compile time?

The solution is to add an abstract method to the Person class. We’ll call this method dispatch, for want of a better name. Assuming that the class that contains the doStuff methods is called Executor, this method takes a single parameter – an instance of Executor:

public abstract void dispatch(Executor executor);

Both Worker and Capitalist would implement this method identically and simply invoke the doStuff method on the Executor, like so:

public void dispatch(Executor executor)
{
    executor.doStuff(this);
}

By implementing this in each of the concrete sub-classes rather than the abstract base class, we are providing the exact runtime type of the object to the executor and the appropriate overloaded method is invoked. This is arguably cleaner than a long chain of instanceof checks.

For the command pattern this approach has the advantage of decoupling the command objects from the behaviour that they invoke. This makes it possible to provide different “Executor” implementations to the same command objects.

You know the situation, you’ve got a NoClassDefFoundError but you’re not sure which of 20+ JAR files you need to add to fix it. This is a question that has cropped up a few times on the Java newsgroups. There are a few solutions to this problem. A colleague of mine has a 60-line Java utility that he’s pleased with, but then terseness is not Java’s strong point. Here’s the equivalent shell script:

#!/bin/sh
find "$1" -name "*.jar" -exec sh -c 'jar -tf {}|grep -H --label {} '$2'' ;

(NOTE: The command may be wrapped when it is displayed on this page, but everything from “find…” should be on a single line).

Save this as findclass.sh (or whatever), put it on your path and make it executable. Problem solved.

The first parameter is the directory to search recursively and the second parameter is a regular expression (typically just a simple class name) to search for. The script relies on the -t option to the jar command (which lists the contents) and greps each table of contents, labelling any matches with the path of the JAR file in which it was found.

For shell-impaired Windows users, Cygwin (possibly with Poderosa) is the answer.

I’ve moved from Blogger to my own site so that I can have more control (via WordPress). I’ve managed to import all of the old articles but seem to have lost the comments in the process. I’ve also updated my JavaBlogs entry, which has had the unfortunate side-effect of spamming the front page with old articles. Sorry about that…