Unit Testing

What Is Unit Testing?

Unit testing is the practice of testing the smallest testable parts of a software system, such as individual functions, methods, or classes, to verify that each works correctly in isolation.

A unit is typically a single piece of logic with a clear input and output. Unit tests confirm that this logic behaves as expected under normal conditions, edge cases, and error scenarios.

Why It Matters	Who Performs It	When It Occurs
Detects defects early, when they are cheapest to fix Improves code quality and design Makes refactoring safer Provides fast feedback during development Reduces defects that reach later testing stages	Software Developers Sometimes supported by automated testing as part of CI/CD pipelines	During the development (coding) phase Before integration, system, or user acceptance testing Re-run whenever code changes are made

Typical Unit Testing Techniques

1. White-Box Testing
   • Tests are designed with full knowledge of the internal code structure.
   • Purpose: Ensure all logical paths and conditions are exercised.
   • Techniques include:
     • Statement coverage
     • Branch coverage
     • Path coverage
2. Equivalence Partitioning
   • Input data is divided into valid and invalid groups (partitions), and one value from each group is tested.
   • Purpose: Reduce the number of test cases while maintaining coverage.
3. Boundary Value Analysis
   • Focuses on values at the edges of acceptable input ranges.
   • Purpose: Catch defects that often occur at limits (e.g., minimums and maximums).
4. Error Handling & Exception Testing
   • Verifies how the unit behaves when invalid data, exceptions, or failures occur.
   • Purpose: Ensure graceful handling of errors and predictable system behavior.
5. Mocking and Stubbing
   • External dependencies (databases, APIs, services) are replaced with simulated objects.
   • Purpose: Allow the unit to be tested in isolation, without relying on other components.

Key Takeaways

1. Unit testing ensures individual pieces of code work correctly before they are combined into a complete system.
2. Strong unit testing dramatically reduces downstream defects and creates a stable foundation for all other testing activities.