Learn TDD in Java

Core TDD Cycle

The methodology consists of five steps:

1. Write Test – Consider feature requirements and testable design
2. Test Fails – Confirm the test fails (expected behavior)
3. Write Code – Implement minimal code to pass the test
4. Test Passes – Validate the solution works
5. Refactor – Improve code quality

Key Benefits

• Proactive quality assurance
• Early defect detection
• Focused requirements analysis
• Rapid feedback loops
• Tests as information that document assumptions and decisions

SOLID Principles

Five design principles that enable testability:

• Single Responsibility – Classes serve one purpose
• Open/Closed – Extensible without modification
• Liskov Substitution – Subtypes remain interchangeable
• Interface Segregation – Specific, focused interfaces
• Dependency Inversion – Abstract dependencies through interfaces

Test Doubles

Three types support isolation:

• Stubs – Predetermined responses
• Fakes – Behavioral emulation (e.g., in-memory databases)
• Mocks – Verify method calls and parameters

FIRST Principles

Quality tests should be:

• Fast – Run quickly to encourage frequent execution
• Independent – No dependencies between tests
• Repeatable – Same results every time
• Self-validating – Pass/fail without manual inspection
• Timely – Written at the right time (before production code)

Common Anti-patterns to Avoid

• Singleton usage disrupting test isolation
• Excessive setup requirements indicating tight coupling
• Over-reliance on mocked objects suggesting poor design
• Tests validating only exception absence (weak assertions)
• Flaky tests dependent on external resources or timing

Getting Started

Practice TDD through coding katas—small, focused exercises that let you build muscle memory for the red-green-refactor cycle. Start simple (FizzBuzz, Roman Numerals) and gradually tackle more complex problems.