ANOTHER EASY JAVE QUESTION: What is the cause for the error?

Actually, I think you mean it could be C...C means there's a problem with either MyCl or MyClTester (which is most likely). C does NOT mean "An error in BOTH cl". Why are you 100% it's not a? If you didn't implement MyCl correctly, when you test it with a testing program, you won't get the right output...