How does dependency injection improve testability?

• A. Remove interfaces
• B. Provide dependencies from outside; decouples implementation; enables mocking/stubbing in tests
• C. Use global variables
• D. Hard-code dependencies

Answer: (B)

Providing dependencies from outside the component lets you decide, at test time, which collaborators to use. This decouples the component from concrete implementations, so you can swap in lightweight mocks or stubs instead of real collaborators. With this setup, tests can focus on the unit’s behavior and verify interactions (such as which methods were called and with which arguments) without triggering real side effects like sending emails or touching a database.

For example, a service that relies on an email sender can be given an EmailSender interface and different implementations. In production, you wire up the real email sender; in tests, you pass a mock that records calls and returns controlled results. This makes tests faster, deterministic, and easier to reason about because they don’t depend on external systems.

If dependencies are created inside the component or tied to global state, tests become harder to control and more fragile, since you can’t substitute test doubles or isolate the unit from real collaborators. Hard-coding dependencies similarly locks you to specific implementations, hurting testability and flexibility.