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Unit, integration, component, UI, E2E, and contract tests

Teams should define these terms in repository documentation; names vary. Scope, environment, dependencies, and purpose matter more than labels.

Example: checkout​

  • Unit: PriceCalculator handles tax/discount/rounding boundaries.
  • Component: CheckoutViewModel maps cart/submit outcomes to UI state.
  • Integration: repository talks to Room + MockWebServer and reconciles outbox.
  • UI component: Compose checkout renders loading/error/price and emits submit intent.
  • Application UI: cart → checkout navigation with fake backend.
  • Contract: Android request/response fixtures conform to API/OpenAPI/provider behavior.
  • End-to-end: one sandbox payment journey across app/backend/provider.

Contract testing​

An Android consumer depends on method, path, headers, auth, JSON field type/nullability/defaults, enums, error format, pagination, idempotency, and compatibility rules. Test those assumptions independently of a brittle full environment.

@Test
fun `unknown optional field and enum are handled safely`() {
    val json = """{
      "id":"O-1",
      "status":"FUTURE_STATUS",
      "newServerField":{"ignored":true}
    }"""
    val dto = tolerantJson.decodeFromString<OrderDto>(json)
    assertEquals(OrderStatus.Unknown("FUTURE_STATUS"), dto.toDomain().status)
}

Provider/consumer contracts do not replace integration tests: gateway/auth/database/runtime configuration may still fail.

Test boundaries, not private methods​

Assert observable outputs/state/effects at a stable public boundary. A test that verifies private call ordering often breaks during safe refactoring while missing the business invariant.

Bad:

verify repository.get() called exactly once before analytics.track()

Better:

given duplicate submit events, one idempotent order exists,
UI shows confirmed state, and a permitted analytics outcome is emitted

E2E discipline​

Keep E2E journeys few and valuable. Own test accounts/data/environment reset, external provider stubs/sandboxes, timeouts, observability correlation, and artifact capture. Avoid assertions on every text pixel; verify the critical business result.

Negative and recovery paths​

For each happy path add the highest-risk alternatives: timeout after server commit, malformed/unknown response, permission revoked, process recreation, disk full, duplicate event, stale account, offline/online, and retry cancellation.

Sources​

Exercise​

Take one existing E2E test and split its policy checks into unit/component tests, wire/schema checks into integration/contracts, leaving only the cross-system journey in E2E.