Here's an older forum post about writing Processing sketches in Eclipse.

http://processing.org/discourse/yabb_beta/YaBB.cgi?board=Integrate;action=display;num=1117133941

Upshot:

• Create a new Java project.
• Add PROCESSING/lib/core.jar to the build path of the project.
• Create a new class that subclasses PApplet.
• Run as Java Applet. You'll have to set width,height of applet separately in the run configuration.
• For testing of new libraries, just add the library project to the build configuration.
• Since you're not running the preprocessor now, the code needs to be "real" java. Some trickiness:
  • "color" is not a valid type. "color()" is still a valid method call though. Replace color as a type decl with a simple int. For Example: color x = color(123,002,203); becomes int x = color(123,002,203);
  • More info: http://processing.org/discourse/yabb/YaBB.cgi?board=Integrate;action=display;num=1106256315;start=48