Interviewer asks: "What's the value of dependency injection?" [duplicate]

Question

Possible Duplicate:
What is dependency injection?

I respond: "You don't need to use the new operator."

Interviewer's response: "<SIGH>. That's it?"

That's all I could come up with.

What would have been a more correct response if that is not correct?

Answer 1

It decouples a component from its external dependencies (e.g. other libraries, databases, etc.) allowing you to easily change them - even at runtime.

This can (for example) be useful in automated testing as you can inject mock objects via the public API.

Answer 2

Dependency injection decouples classes from the services they depend on, allowing you to register a set of services once and use them throughout your codebase.

This allows you to switch implementations non-intrusively, especially if the implementations are unit-tested.

It also allows you to put different service implementations with the same classes for different use-cases, such as web vs. GUI vs. testing.