If you are developing Java on a Mac, chances are pretty high that you are using Eclipse for development. In addition you probably (or at least you better) have lots of RAM too, however, Eclipse is a bit conservative when allocating RAM. If you have tons of plugins installed, or the plugins are quite heavy (Android SDK, I am looking at you), then you can easily get in low memory situations.
The way to change the max amount of memory reserved for eclipse is somewhat buried and not trivial to find (unless you ask Google). The way to do it is to go into the eclipse folder, right click on Eclipse.app and then select “Show Package Contents”. Inside look for the eclipse.ini file (if you are running windows you can skip this step).
Here is my version of the eclipse.ini
The sections in bold are the ones that I have changed. Then just save the file and relaunch Eclipse. That should give you a bit more RAM and you should hopefully get better performance.