Some time ago I was looking for an alternatiive to heavy and slow Eclipse and NetBeans, for Java development, and I found a few Java IDEs (written on Java) but most of them where a little slow. But after a few hours (or days..., I dont remember already) I found one excellent aplication called tIDE. It is writtem on Java, but fast enough to compete with such monsters as netBeans. It has most of the features that commersial IDEs have. It is open source, and it weights only 2Mb. If you are Java developer - you should check this  IDE. Here is what the author say about his programme:

"tIDE is a full featured opensource Java integrated development environment (IDE), distributed under the GNU General Public Licence (GPL).

More precisely: tIDE is a small, powerful, quick, efficient, easy and open framework to create 100% Java applications.

Try tide, you'll be astonished by its simplicity and rapidity. No installation is required. A single click on the JNLP and the IDE is on your screen ! An import tool let you import the main settings of any existing Eclipse, NetBeans or Schmortopf project."

Check this tool here: http://tide.olympe-network.com/