Classifying expected versus unexpected problems

There are two classes of problems you might hit while testing. The first kind is an expected problem, for instance, an unusual or invalid condition coming in from an external system. Perhaps you send a message and never receive a reply, or the reply contains a blank required field. Maybe you only support 10 simultaneous sessions, but someone tries to connect an 11th. These cases will result in failures, but they are all expected. When your code is running in the real world, people may try to misuse it, and you have to be ready to handle that input. All the tests in this chapter are of this first kind – expected problems. Even if the external system you are talking to is another internal module, you have to be able to handle receiving any kind of rubbish back from it. Input validation is necessary on any interface.

The other class of problem is something invalid happening within code – attempting to use a null pointer...