nullables

Nullables: Testing Without Mocks

STARTER_CHARACTER = ⭕️

Nullables are production code with an off switch: classes with external I/O anywhere in their dependencies offer create() (real) and createNull() (I/O disabled, everything else runs normally). Tests are narrow (focused on one class), sociable (dependencies run for real), and state-based (assert outputs and state, never method calls). Don't use mocking libraries or DI frameworks — Nullables make them unnecessary.

The cut

Stub at the lowest point — the third-party edge — never your own code:

OrderService  →  PaymentClient  →  HttpClient  →  third-party lib
  app code       high-level        low-level       ✂ stubbed when nulled
                 wrapper           wrapper

• PaymentClient is a high-level wrapper: it abstracts one service and speaks domain language.
• HttpClient is a low-level wrapper: it abstracts one technology and is generic and highly reusable.
• The low-level wrapper holds the fork: create() wires the real library (node http, RestTemplate); createNull() wires an embedded stub — your code, returning canned data, doing no I/O.
• Everything left of the cut is your code and runs for real in tests.

With mocks, you only mock code you own; with Nullables, you only stub code you don't own. Only the bottom layer has a stub — one per technology. Everything above runs real in tests, so a bug anywhere in your code turns tests red. Mocking your own classes breaks that chain: mocked code never runs, and its bugs hide behind green tests.

An invented internal seam is the same break in disguise: cutting at rows() → List<Row> instead of the driver puts your mapping loop below the seam, where nulled tests never run it. The test: any parsing, mapping, or normalization you wrote must sit above the cut. When the third-party API is a chain of objects, mirror it — one stub class can play the whole chain (see the low-level wrapper files).

Two channels, plus events

Every class that talks to infrastructure anywhere in its dependencies offers the same two factory methods: