Summary

In summary, the process of developing tests boils down to considering the various states of your program. In what ways can the program or class be used? What are the valid use cases? What are the invalid cases that could occur for which you could reasonably check and guard against? What values or circumstances cause decisions to be made (boundary cases)? If you can think through the viable scenarios and ensure your program or class behaves properly in those situations, you should have a robust set of test cases. While you may not be able to be 100% sure your program will work 100% correctly no matter what comes at it, you can at least be sure that your program is robust given a reasonable user and common invalid use cases.