In object oriented programming, I have read that you should program to an interface not an implementation but do they mean literal interfaces (no shared code at all)?
Is it okay to program to an abstract base class that would have been an interface except that there were variables in this "interface" that all sub-classes were expected to have? Replicating a variable across sub-classes would have been an inconvenience because if I changed the name of one of the variables in one of the sub-classes I would have to change the name of that variable in all of the sub-classes.
In following the principle of "program to an interface not an implementation", is this okay or would you create another interface on top of the abstract base class and program to that interface?