How to catch: "cannot find symbol", when function does not always exist

I need an idea. I have to check assignments for students. To the assignments have the same name(Ex2.java),they are in different directories.I have a code that compile and run the assignments.

So in the test class, When I try to test one of the function in the assignments, for example,sum:

if(Ex2.sum(x)){
     grade++;
}

So I get an error because the assignment does not compile yet, but when I run the file that compiles the assignment then runs the test file on the function in the assignment there is no problem.

The problem occurs when one of the assignment does not have a specific function and a call to it will throw an exception. Which stops all other tests. I tried to do try catch but I still get an error.

Any suggestions?