Is it the best practice to extract an interface for every class?

Question

I have seen code where every class has an interface that it implements.

Sometimes there is no common interface for them all.

They are just there and they are

Answer 1

Having the interface and coding to the interface makes it a ton easier to swap out implementations. This also applies with unit testing. If you are testing some code that uses the interface, you can (in theory) use a mock object instead of a concrete object. This allows your test to be more focused and finer grained.

It is more common from what I have seen to switch out implementations for testing (mocks) then in actual production code. And yes it is wroth it for unit testing.