FAQ 10.02 What are the advantages of self-testing objects?

Testing starts earlier, continues longer, requires almost no human intervention, focuses on the most commonly used paths, and adapts as the system evolves.

There is very little human effort required for objects that test themselves other than writing the behavioral self-tests and the testInvariant() member function. The runtime system works a lot harder because it must continually reverify the object's state and its transitions, but there is very little human (payroll-intensive) intervention.

By integrating an object's test harnesses with the object, the self-testing strategy reduces reliance on big-bang testing. In practice, self-testing detects defects earlier than they otherwise would be with traditional, big-bang testing. This is because every use of the class becomes an impromptu test harness. This reduces the cost of finding and repairing defects, and it improves the business efficiency during system integration. The effect is to change the concept of testing from an event to a life style.

By building self-testing objects, developers ensure that their objects continually adapt as the system evolves. Thus, when the objects are used in new and unforeseen ways, the objects continue to verify themselves using the same test code and against the same standards without anyone having to reverse engineer them and build new test harnesses.

Since every use of an object becomes a miniature test harness, the overall effect is to exhaustively test the most commonly used paths through an object. This is quite a different result than that provided by most unit testing approaches, since most unit testing approaches require explicit and often elaborate test harnesses to be built, and these test harnesses typically provide spotty testing at best.

Finally, since self-testing objects check their own results, traditional test harnesses for unit testing are significantly simpler to develop. For example, the test harnesses don't need to check the result of an operation, since the object already checks its own results. This means that the test harnesses don't need to start out with some specific set of tests to be run but instead can generate test scenarios on the fly by passing randomly generated values into randomly selected member functions. This can both reduce the cost of building test harnesses for unit testing and improve their coverage.

The self-testing technique is similar to the quality mandate in manufacturing. The whole is correct because every part is correct and because every combination of parts is tested.