Test-Driven Development (TDD) — Red-Green-Refactor in Java

TDD is red-green-refactor: write a failing test first, write the minimum code to pass it, then refactor with the test as a safety net. The cycle repeats — each test adds one behavior, and the implementation grows incrementally.

The misconception is that TDD is about testing. It's actually a design technique. By writing the test first, you design the API before the implementation — what the method takes, what it returns, what it throws. The test is a specification.

TDD works best for business logic with clear inputs and outputs — calculators, validators, state machines. It's less useful for framework-driven code like controllers and configuration, where the design is dictated by Spring, not by your domain.

For bug fixes, TDD is invaluable: write a test that reproduces the bug, verify it fails, fix it, verify it passes. The test prevents regression permanently.