Unit Testing Strategies

# Unit Testing Strategies

## 1. The AAA Pattern
Every unit test should follow the **AAA** structure to maintain readability and separation of concerns.
- **Arrange**: Initialize the SUT, mock dependencies (DOCs), and configure state.
- **Act**: Execute the specific method or function under test.
- **Assert**: Verify the actual outcome against expected behavior.

## 2. Test Doubles and Mocking
Effective isolation is critical for unit testing. Use the following strategies:
- **Stubs**: Provide canned answers to calls made during the test.
- **Mocks**: Verify interactions (e.g., ensuring a repository method was called exactly once).
- **Fakes**: Simplified implementations (e.g., an in-memory database).

## 3. Boundary Value Analysis
Tests should focus on edge cases rather than just the happy path:
- **Equivalence Partitioning**: Group inputs into valid and invalid sets.
- **Boundary Analysis**: Test values at the edges of ranges (e.g., -1, 0, 1 for a non-negative constraint).

## 4. Test-Driven Development (TDD) Lifecycle
- **Red**: Write a failing test for a new requirement.
- **Green**: Write the minimum amount of code to pass the test.
- **Refactor**: Clean up the code while ensuring tests remain green.

## 5. Best Practices
- **Isolation**: Tests should not rely on external networks or databases.
- **Speed**: Unit tests must execute in milliseconds.
- **Determinism**: Tests must yield the same result every time.
- **Naming Conventions**: Use descriptive names like `Should_ThrowException_When_InputIsNull`.