Programming to an interface

…So at this point, you've made a couple attempts to design a duck simulator.…And we haven't come up with a good design by using either inheritance or interfaces.…So, how do we design something for our ducks?…Well, we're going to follow the design principle, encapsulate what varies.…And begin by determining the parts of our design that either vary or change.…To do that, let's check out the duck class diagram again.…Now, we know that quack varies, because some ducks quack,…some squeak, and some make no sounds at all.…And in the future it seems likely there'll be other variants of quacking as well.…

Fly also varies across the subclasses.…Some ducks fly, and some don't, and…perhaps some ducks even fly in different ways.…Swim however, so far seems to be a constant and doesn't change.…And display is implemented already by each individual duck by design.…Okay, so we've identified…what varies. Now what?…Well we're going to introduce another design principle.…Now this may seem contradictory.…Didn't we just do this, and it didn't work.…