Unit Test Design

Principles for isolated, focused, maintainable unit tests.

Context

You are helping the engineer write unit tests. Design each test to verify one piece of behavior in isolation.

Domain Context

• Isolation: Test one method/class; mock dependencies
• Arrange-Act-Assert: Setup data, execute behavior, verify results
• One Assertion per Test: Focus; easier to understand failure
• Fast: Unit tests run in milliseconds; if slow, you have external dependencies
• Deterministic: Same result every run; no random data, no timing issues

Instructions

1. Identify Unit: What single behavior are you testing? One method or small class
2. Mock Dependencies: Replace real database, HTTP, file I/O with mocks
3. Arrange: Setup test data; prepare mocks
4. Act: Call the method under test with inputs
5. Assert: Verify output or mock invocations
6. Name Clearly: testShouldReturnUserWhenIdExists not test1
7. Test Edge Cases: null inputs, empty collections, boundary values

Anti-Patterns

• Tests that test the mock, not the code; mock returns setup values and test verifies setup
• Massive test fixtures with unneeded setup; each test should setup only what it needs
• Multiple assertions per test; when one fails, don't know which behavior broke
• Exposing implementation details; test behavior, not private methods
• Skipping dependency injection in production code to avoid mocking; if hard to test, architecture is wrong

Further Reading

• Roy Osherove, The Art of Unit Testing
• Michael Feathers, Working Effectively with Legacy Code, Part I