Designing

Why can't you just listen, write a test case, make it run, listen, write a test case, make it run indefinitely? Because we know it doesn't work that way. You can do that for a while. In a forgiving language you may even be able to do that for a long while. Eventually, though, you get stuck. The only way to make the next test case run is to break another. Or the only way to make the test case run is far more trouble than it is worth. Entropy claims another victim.

The only way to avoid this is to design. Designing is creating a structure that organizes the logic in the system. Good design organizes the logic so that a change in one part of the system doesn't always require a change in another part of the system. Good design ensures that every piece of logic in the system has one and only one home. Good design puts the logic near the data it operates on. Good design allows the extension of the system with changes in only one place.