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I was writing some test code as an example where I had a washing machine class. It has a door state that should be open or closed, and also a running state, either on or off. I want to prevent the running state from changing from 'off' to 'on' when the door is 'open', and also prevent the door from being set to 'open' while the machine is 'on'.

I have this functionality rigged up with a bunch of if statements. It feels inelegant and it could quickly turn into spaghetti code if I want to add another state that puts additional conditions on the changes of other states. I wonder, is a finite state machine a solution for this situation? Would it simplify adding states and defining valid transitions?

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up vote 6 down vote accepted

What you are experiencing is called 'Code Smell' and is a good indicator that you might be doing something wrong. Luckily though, you saw the problem and already came up with a great solution, so there's not much for me to say here. A state machine works perfectly for what you're doing here.

Drawing it out might also help to try and find other states you might not think of (Running, spin cycle, cool down, filling), as well as allowing you to define transition requirements between states (probably don't want to let the door open during the spin cycle, but maybe the filling state allows the door to be opened to add garments).

Obviously you can be as detailed or ambiguous as you want, do as much as fits your needs!

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