What is the dependency inversion principle and why is it important?

Question

What is the dependency inversion principle and why is it important?

Answer 1

Good answers and good examples are already given by others here.

The reason DIP is important is because it ensures the OO-principle "loosely coupled design".

The objects in your software should NOT get into a hierarchy where some objects are the top-level ones, dependent on low-level objects. Changes in low-level objects will then ripple-through to your top-level objects which makes the software very fragile for change.

You want your 'top-level' objects to be very stable and not fragile for change, therefore you need to invert the dependencies.