You need to install Java if you don't have it regardless of whether you use Netbeans.
Some comments about installing Java:
You many already have a JDK installed. Check this by typing 'java -version' at the command-line.
You really need JDK version 1.4.2 or better.
I've not tried JDK 1.5 and you might run into compatibility problems. Use at your own risk.
Downloading from Sun's Java website:
Do not download the Netbeans+J2SE bundle because it contains a previous version of Netbeans (3.6).
You want the "J2SE SDK" but the J2EE SDK won't cause problems.
You if like, you can download the documentation for J2SE as well.