Why are stubs and mocks used in software testing?

Stubs and mocks are used in testing to isolate the component under test from external dependencies.

• Stubs provide fixed responses to calls, allowing to verify system behavior under certain conditions.
• Mocks not only provide responses but also verify which methods were called with what parameters.

This helps to:

• Test components independently of external services, databases, network calls.
• Create controlled scenarios to test error handling and boundary cases.
• Speed up test execution by avoiding real-time costs of external operations.

Example in Go using an interface and a stub:

// Interface for external service
type DataFetcher interface {
    FetchData() (string, error)
}

// Stub for testing
type StubFetcher struct{}

func (s StubFetcher) FetchData() (string, error) {
    return "test data", nil
}

// In a test, you can use StubFetcher instead of a real service