Here are some Frequently-Asked-Questions (with answers ;-) and some common mistakes people make when trying to translate and run JR programs. Some of this information applies specifically to UC Davis's CS department's CSIF systems, but it is also likely to apply to other systems.

Here's an example:

jrvm exception: null
java.lang.NullPointerException
        at Main.main(Main.java:24)
        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
        at java.lang.reflect.Method.invoke(Method.java:597)
        at edu.ucdavis.jr.jrx.jrvm.startmain(jrvm.java:333)
        at edu.ucdavis.jr.jrx.jrvm.main(jrvm.java:449)
        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
        at java.lang.reflect.Method.invoke(Method.java:597)
        at edu.ucdavis.jr.jrx.jrvm_wrap.startmain(jrvm_wrap.java:67)
        at edu.ucdavis.jr.jrx.jrvm_wrap.main(jrvm_wrap.java:124)

So, look at line 24 in jrGen/Main.java for the cause of the exception.  Then, locate the corresponding line in Main.jr.