1. Test-Driven Development

2. Three Rules
Don’t write any production code until you have written a failing unit test.
Don’t write more of a unit test than is sufficient to fail or fail to compile
Don’t write any more production code than is sufficient to pass the failing test.

3. Advantages
All program functions are testable and have tests that verify their operation.
We can tell when we break something.
We see the function from the caller’s point of view.
The function will be “conveniently callable.”
Because the function is callable, we have decoupled it from other system software – e.g., the user interface.
Test provides documentation in the form of a working example.