Pragmatic Unit Testing in Java 8 with JUnit

Adding Another Increment

For each failing test, seek to add only the code needed to pass the test—to add the smallest possible increment. The mentality: build code exactly to the “specifications” that the tests represent. If the tests all pass, you know you could potentially ship the code—the tests document what the system does, no more, no less. You avoid the potential waste of speculative development.

More practically (in terms of following the TDD cycle) writing the smallest amount of code means that in most cases we can write another test that will first fail. Writing more code than needed means you could find yourself writing lots of tests that pass immediately. That might seem like a good thing, but it takes you right back to the old way ...