I love this quote by the Buddha. Within the software world, it translates perfectly into the healthy habit of never trusting code just because someone smart wrote it or because it's been working fine for a long a time. If it has not been tested, code is not to be trusted.

Why are tests so important? Well, for one, they give you predictability. Or, at least, they help you achieve high predictability. Unfortunately, there is always some bug that sneaks into the code. But we definitely want our code to be as predictable as possible. What we don't want is to have a surprise, in other words, our code behaving in an unpredictable way. Would you be happy to know that the software that checks on the sensors of the plane that is taking you on your holidays sometimes goes crazy? No, probably not.

Therefore, we need to test our code; we need to check that its behavior is correct, that it works as expected when it deals with edge cases, that it doesn't hang when the components it's talking to are broken or unreachable, that the performances are well within the acceptable range, and so on.

This chapter is all about that—making sure that your code is prepared to face the scary outside world, that it's fast enough, and that it can deal with unexpected or exceptional conditions.

In this chapter, we're going to explore the following topics:

• Testing (several aspects of it, including a brief introduction to test-driven development)
• Exception handling
• Profiling and performances

Let's start by understanding what testing is.