Of course, you are right when you say "Once you create an instance of something [...] code will be created". But what I am pointing here is BEFORE you create an

Of course, you are right when you say "Once you create an instance of something [...] code will be created". But what I am pointing here is BEFORE you create any instance of anything. You define a contract, what your object should look like. With an interface it's checked before runtime and then translates to no code. But using a class, you have code generated for the runtime.