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Unit Testing

Write fast, isolated tests for individual components.

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Unit Testing

What Is Unit Testing?

Testing individual units (methods/classes) in isolation.

JUnit 5

@SpringBootTest
class ProductServiceTest {

    @MockBean
    private ProductRepository repository;

    @Autowired
    private ProductService service;

    @Test
    void shouldReturnProductWhenExists() {
        Product product = new Product(1L, "Laptop", new BigDecimal("999.99"));
        when(repository.findById(1L)).thenReturn(Optional.of(product));

        Product result = service.getById(1L);

        assertThat(result.getName()).isEqualTo("Laptop");
        verify(repository).findById(1L);
    }
}

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 Unit Testing 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

0/3solved
Implement Unit Testing

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

Solution
// Unit Testing implementation
// Key aspects: validation, error handling, logging, testing

public class UnitTesting {
    // Production-ready implementation
}
Unit Testing Edge Cases

Identify and handle edge cases for Unit Testing. 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
Unit Testing Testing Strategy

Write a testing strategy for Unit Testing. 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. Unit testing tests?

Question 1 options

2. @MockBean does?

Question 2 options

3. What is the primary purpose of Unit Testing?

Question 3 options

4. What is a common mistake when implementing Unit Testing?

Question 4 options

Flashcards

Question

Unit testing tests?

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Answer

Individual units in isolation

Question

@MockBean?

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Answer

Creates mock, replaces real bean in context

Question

What is Unit Testing?

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Answer

Unit Testing is a key concept in backend development.

Question

When to use Unit Testing?

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Answer

Use Unit Testing when building production systems that require reliability, scalability, and maintainability.

Question

Unit Testing 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.Unit testing: test in isolation
  • 2.JUnit 5 for test framework
  • 3.MockBean for mocking Spring beans
  • 4.AssertJ/Hamcrest for assertions

Interview Tips

  • •Write unit tests
  • •Know testing frameworks

Cheat Sheet

Unit Testing

  • Test in isolation
  • JUnit 5: @Test, @BeforeEach, @AfterEach
  • @MockBean: mock in Spring context
  • Assertions: assertThat, assertEquals