Which design principles support testability in software design?

• A. Use high coupling and global state to speed up tests.
• B. Avoid interfaces to reduce complexity.
• C. Expand monoliths to include many responsibilities to improve coverage.
• D. Favor low coupling, clear interfaces, dependency injection, and small cohesive units; SOLID principles aid testability.

Answer: (D)

Testability in software design benefits from modular, loosely coupled components with clear interfaces and controllable dependencies. When components have low coupling, a change in one part is unlikely to ripple through the system, making tests more reliable and easier to write. Clear interfaces define precise contracts for what a component does, enabling targeted unit tests and the use of mocks or stubs to simulate other parts of the system. Dependency injection lets tests supply fake or controlled dependencies, keeping test environments predictable and isolating the unit under test. Small cohesive units, aligned with the single responsibility idea, are easier to understand and test because each unit has a narrow focus. The SOLID principles reinforce these ideas—single responsibility reduces the surface area to test; dependency inversion and programming to interfaces make substituting test doubles straightforward; keeping interfaces small and focused avoids unnecessary dependencies, and substitutability helps ensure tests validate behavior rather than wiring.

In contrast, high coupling and global state make tests fragile because changes can trigger widespread effects and require elaborate setup. Skipping interfaces removes the ability to mock or stub dependencies, complicating isolation. Expanding a monolith to take on many responsibilities creates large, intertwined code paths that are hard to exercise in isolation and prone to flaky tests.