mellor-tdd-java

Write one failing test expressing the desired behavior, then write the minimal code to pass it, then refactor both test and production code while staying green. Hardcoding is legal in the green phase; triangulation with a second test forces real generalization.

A stub provides pre-canned return values using when/thenReturn and is the default choice for data flow. A mock verifies interactions using verify() and should be used sparingly for protocol verification, since over-mocking couples tests to call sequences.

Hexagonal architecture places the domain model at the center with ports as interfaces and adapters as implementations. This lets you mock ports and run FIRST unit tests of complete user stories without touching databases or HTTP.

Mellor's synthesis recommends inside-out (Chicago school) for the domain layer where logic is pure, and outside-in (London school) for the adapter layer where user value and wiring matter. Hexagonal architecture draws the boundary between them.

The skill covers only the book's content and does not execute code or integrate with your project tooling. For hands-on implementation, combine it with project-specific tools and your own build environment.