dependency injection - Abstractions should not depend upon details. Details should depend upon abstractions?

Question

In past couple of days, I have read about quite a lot about dependency injection/inversion of control/inversion of dependency. I think that, now my understanding of the concept is much better. But I still don't get the following from wikipedia:

A. High-level modules should not depend on low-level modules. Both should depend on abstractions. B. Abstractions should not depend upon details. Details should depend upon abstractions.

I understand the part of High-level modules should not depend on low-level modules. But, I am confused about abstractions and details.Can someone please simplify them for me. Thanks.

Answer 1

It means that if the details change they should not affect the abstraction. The abstraction is the way clients view an object. Exactly what goes on inside the object is not important. Lets take a car for example, the pedals and steering wheel and gear lever are abstractions of what happens inside the engine. They do not depend on the details though because if someone changes my old engine for a new one I should still be able to drive the car without knowing that the engine changed.

The details on the other hand MUST conform to what the abstraction says. I would not want to implement an engine that suddenly causes the brakes to double the speed of the car. I can re-implement brakes any way I want as long as externally they behave the same way.