Unit Testing Best Practices — Naming, Arrange-Act-Assert, Test Isolation

Good tests follow three principles. Arrange-Act-Assert gives each test a clear structure — setup, execution, verification. Test isolation means each test runs independently with no shared mutable state — @BeforeEach initializes everything fresh. Behavior over implementation means testing what the code does, not how it does it.

Naming matters more than people think. shouldRejectOrderWhenInventoryInsufficient tells you what broke without reading the test. testMethod1 tells you nothing.

The one logical assertion guideline: each test verifies one behavior. Multiple assertThat calls are fine if they verify the same behavior. But discount AND notification AND logging in one test means three behaviors — split into three tests so failures are immediately diagnosable.

Flaky tests almost always come from shared mutable state. Fresh mocks, fresh data, fresh service instances per test — that's isolation.