the first thing to note is that the .class file (and the .j file) should end up in the same directory as the .jl file. So, your class file should NOT end upp in the src directory, unless the .jl file was also there.
In your example you seem to run the compiler from the main submission directory and have the coreXXX.jl also in this directory. Then the class file must be also in this directory.
The classpath must include the lib directory (to find Runtime.class) and the current directory (to find coreXXX.class). Thus the command could be
java -cp lib:. coreXXX
In general, I advice you to run the automatic tester; it will find the Runtime.class if it is in the lib directory and it will find your compiled class files if they are in the same directory as the source files.
Best regards
Björn