Some characteristics of safe applications follow:

• Each module exposes a minimum set of types, functions, and variables.
• Each function is called with arguments such that the respective types implement the expected behavior of the function (the L in SOLID; POLA).
• The return value of a function is clear and documented (POLA).
• Missing data is handled properly (POLA).
• Variables are limited to the smallest scope.
• Exceptions are caught and handled accordingly.

Safety is one of the most important objectives here. An erroneous application can cause major disasters. It can even cost a company millions of dollars. In 2010, Toyota recalled over 400,000 of its Prius hybrid cars due to a software defect with the Anti-lock Braking System (ABS). In 1996, the Ariane 5 rocket launched by the European Space Agency exploded just 40 seconds after launch. Of course, these are only a few more extreme examples. By utilizing best practices, we can avoid getting into these kinds of embarrassing and costly incidents.

Now, we understand the importance of software design principles and software quality objectives.