Question

I’ve used the State pattern to implement a simple finite state machine. Looking at the description given on Wikipedia, and more specifically at the suggested Java implementation, I wondered why classes implementing the State interface (i.e. the various states) are not Singletons?

In the suggested implementation a new State is created whenever a transition occurs. However, one object is sufficient to represent each state. So, why wasting time creating a new instance every time a transition occurs?

Answer 1

Because each state can store instance variables?

Take a look at the Wikipedia example you reference:

class StateB implements State { 
    private int count=0; 
    public void writeName(StateContext stateContext, String name) { 
        System.out.println(name.toUpperCase()); 
        if(++count>1) { 
            stateContext.setState(new StateA()); 
        }
    }
}

Can you see how it stores a count of the number of times it has been entered?

Now, in a FSM you probably want each state to be idempotent (subsequent calls give the same feedback) but the State pattern is more general. One target use as described on the wikipedia page is:

A clean way for an object to
partially change its type at runtime

As most objects probably use their local variables when performing actions, you would want the “changed type” version to use local variables as well.