The Nature of the Machine

To get a better feel for the object as machine metaphor, I think it's helpful to look at the relationship between objects and state machines. Among the most useful insights that originally helped me get a feel for object design was that many objects act like state machines. You can think of all objects as machines. The kind of machine mutable service-oriented objects most resemble, mathematically speaking at least, is the state machine.

A state machine is defined by:

• A set of states, one of which is identified as the initial state
• A set of messages the state machine can receive
• A set of actions the state machine can take
• The actions taken and next state that results from receiving each message in each state

When a state machine is in some way built and put into motion, it starts out its lifetime in its initial state. At any time during its life it has a current state. The outside world interacts with the state machine by sending it messages. When the state machine receives a message, it performs actions, including potentially changing state.

Similarly, when a service-oriented object is instantiated, it begins its lifetime in some initial state, established by the object's constructor. At any time during its life, it has a current state. The outside world interacts with the object by invoking its methods. When a method is invoked, the object performs actions, including potentially changing state, and then returns.