Today many universities offer courses for learning Java programming. Debugging cannot be avoided in programming. Nevertheless, there are not many chances for students to debugging programs, particularly written by another person. In this paper, for increasing that chance, we propose a function for generating a new kind of questions that are called debugging questions. A debugging question requires students to debug a program mixed with bugs. This function provides three ways of mixing bugs: deletion of reserved words, swapping of variables, and replacement by lists. We implemented this function into a Java programming learning assistant system, called JPLAS. We also discussed the usefulness by evaluating the function.