The answer on SE is still very theoretical. Yes, technically we can model any computation [1] as an FSM, but when does it actually make sense to do so?<p>FSMs are often used when modelling physical devices - think microwaves, elevators, CD-players, something with buttons or sensors, something event-driven, embedded controllers, also network protocols, text parsers, etc. Basically, whenever we must ensure correct behavior having few constraints on sequence or timing of inputs.<p>How can you actually apply it in your everyday programming? Look at your class. Every method call is an event that can cause a transition. Member variables are the state. Theoretically changing any bit of any member variable gives you a different state. In practice you only care about the bits that affect your control flow. Figuring out how variables map to states is the tricky part. Sometimes one variable corresponds to several states (say, when airspeed goes over V1 a plane transitions from <i>can abort</i> to <i>must takeoff</i> state), sometimes several variables define one state (say, when ignition=on and door=closed car is <i>ready to drive</i>), sometimes you'll need to add an extra variable to distinguish between states. Then you draw a table with events on one side and states on another and look at each cell. This is when you have all those precious "oh, crap, what if the user hits <i>play</i> when the song is already playing" moments. And voila - now you know that your class behaves correctly because you've systematically considered every possible scenario. It takes enough effort that you probably don't want to do it for each and every class, but when you do it - it's magic :-)<p>[1] Any computation with finite memory, to be pedantic