Exception in thread "main" java.lang.OutOfMemoryError: Java heap space

This can have two reasons:
Your Java application has a memory leak. There are tools like YourKit Java Profiler that help you to identify such leaks.
Your Java application really needs a lot of memory (more than 128 MB by default!). In this case the Java heap size can be increased using the following runtime parameters:

java -Xms -Xmx

Defaults are:

java -Xms32m -Xmx128m

You can set this either in the Java Control Panel or on the command line, depending on the environment you run your application.