Test Driven Development

Writing test cases before writing the code doesn’t take any more effort than writing test cases after the code; it simply resequences the test-case-writing activity.
When you write test cases first, you detect defects earlier and you can correct them more easily.
Writing test cases first forces you to think at least a little bit about the requirements and design before writing code, which tends to produce better code.
Writing test cases first exposes requirements problems sooner, before the code is written, because it’s hard to write a test case for a poor requirement.
If you save your test cases, which you should do, you can still test last, in addition to testing first.