Java block
By default the code in an RGG file is interpreted as a java block. Additionally, these blocks are included in { } brackets.
The java block allow java syntax up to java 1.6. (this is independent from the java version that is used to run GroIMP). Therefore, it is possible to use features such as file reading and writing, mathematical calculations, abstraction/inheritance or library functions. If you are not experienced with java it might be useful for you to look into available online tutorials.(For example https://www.w3schools.com/java/default.asp)
Default additions
Every new RGG file also imports by default a set of library functions that are used to interact with the graph or the GroIMP platform. This functions can be explored in the function explorer in the software. You can find it on the main menu under 'Help/Function Browser'.
WARNING This is not the case in .xl files 
Not included
XL includes many syntax and features of Java. But not all. Essentially the language includes all Java 1.6.
Not included: <type>, lambda, …