The Java4K competition is a game programmer competition. The goal of the competition is to create the best game possible in Java. So what's the catch? There is a 4KB limit! That means maximum of 4096 bytes! You can read the competition rules for more details on this.
But why 4KB, and not 8KB or 6KB? The answer to that question is simple. 4KB is the smallest unit of file possible on most operating systems, if you create a file that contains only one character (or one byte) it takes up 4KB on the disk. Along with that fact, mere 4KB seem to be just the right size, anything more and it would matter if you had a good artist doing your graphics or if you had 10x more time than the other developers. 4KB seems to be limitating enough, in other words it's a perfect fit for 1 person to work on, it wouldn't be practical for a large team, consisting of programmers and artists, to work on a 4KB game, it makes it more fair.
Due to these limitations programmers are given certain boundaries to work within, which results in higher productivity. While game programmer churn out only a few games, if any, over a period of years, some programmers manage to churn out many 4K games during the 3 months. The 4K contest is really a contest in making a prototype game with added challenges, which makes it fun!
If you're interested in creating a game and submitting it into the Java4K competition then we recommend you visit the Java 4K Wiki Page, which should help you understand the fundamentals, and the JavaGaming.org forums, where most of the 4K developers hang out at. You'll also need to create an account on this site to submit your game.
This all started back in 2002 when Lloyd Lee (aka mlk) hosted the first contest (2002-2003), which was hosted at JGO (JavaGaming.org) forums and also Sun Microsystem Java forums. A year later the contest was held again, but this time Matt Campbell (aka woogley) managing it. He put up the site javaunlimited.net, where the contest was hosted three times in a row; 2004-2005, 2005-2006 and 2006-2007. Once it became clear that Matt was not going to manage the 2007-2008 competition, then Java4K.com website was kick-started by Arni Arent (aka appel) and Joakim Johnsson (aka jojoh) in December 2007. Arni Arent has managed follow up contests.