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intermediate Phase · Testing

Test Doubles

Understand mocks, stubs, fakes, and spies.

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Test Doubles

Types

Type Description
Dummy Passed around, never used
Stub Returns predefined values
Spy Wraps real object, records calls
Mock Pre-programmed with expectations
Fake Working simplified implementation

Key Points

  • Understanding Test Doubles is essential for production systems
  • Always consider scalability and maintainability
  • Test thoroughly before deploying to production
  • Monitor performance and set up alerting

Common Patterns

  1. Validation: Always validate input at the boundary
  2. Error Handling: Use structured error responses
  3. Logging: Log key events for debugging
  4. Testing: Unit, integration, and load tests
  5. Documentation: Keep docs updated with code changes

Testing Strategies

Testing Pyramid

     /  E2E  \
    /----------\
   / Integration \
  /----------------\
 /      Unit        \
/--------------------

Test Types

  • Unit: Individual components
  • Integration: Component interactions
  • E2E: Full user workflows
  • Performance: Load/stress testing

Best Practices

  • Write tests first (TDD)
  • Aim for 80% coverage
  • Test edge cases
  • Keep tests fast and isolated

Key Points

  • Understanding Test Doubles is essential for production systems
  • Always consider scalability and maintainability
  • Test thoroughly before deploying to production
  • Monitor performance and set up alerting

Common Patterns

  1. Validation: Always validate input at the boundary
  2. Error Handling: Use structured error responses
  3. Logging: Log key events for debugging
  4. Testing: Unit, integration, and load tests
  5. Documentation: Keep docs updated with code changes

Practice Problems

0 / 3 solved
Implement Test Doubles

Design and implement a solution for Test Doubles in a backend system. Consider scalability, error handling, and production readiness.

Solution 
// Test Doubles implementation
// Key aspects: validation, error handling, logging, testing

public class TestDoubles {
    // Production-ready implementation
}
Test Doubles Edge Cases

Identify and handle edge cases for Test Doubles. What happens under high load, with invalid input, or during failures?

Solution 
// Edge case handling:
// 1. Null/empty input -> validation
// 2. High load -> rate limiting, queuing
// 3. Failures -> retries, circuit breaker
// 4. Concurrent access -> locks, idempotency
Test Doubles Testing Strategy

Write a testing strategy for Test Doubles. Include unit tests, integration tests, and performance tests.

Solution 
// Test plan:
// - Unit: 80% coverage target
// - Integration: API contracts
// - Performance: latency, throughput
// - Chaos: failure injection

Quiz

1. Stub vs Mock?

Question 1 options

2. Fake is?

Question 2 options

3. What is a common mistake when implementing Test Doubles?

Question 3 options

Flashcards

Question

Stub vs Mock?

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Answer

Stub: return values. Mock: expectations + verify.

Question

Fake?

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Answer

Working simplified implementation

Question

Test Doubles best practices

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Answer

Follow SOLID principles, write clean code, test thoroughly, document decisions, and monitor in production.

Revision Notes

Key Takeaways

  • 1. Dummy: unused, Stub: returns values
  • 2. Mock: expectations + verify
  • 3. Fake: working simplified impl
  • 4. Spy: wraps real object

Interview Tips

  • • Know test double types
  • • Choose the right one

Cheat Sheet

Test Doubles

  • Dummy: unused
  • Stub: returns values
  • Mock: expectations + verify
  • Fake: working simplified impl
  • Spy: wraps real object