1. State pattern – A logical ‘Finite State Machine’
State - Design Pattern
State pattern – A logical ‘Finite State Machine’

2. Introduction State pattern Intent
For an abstraction that executes a finite state machine
Intent
To allow an object to alter its behavior when its internal state changes. The object appears to have changed its class

3. Details Implementing objects states using explicit classes Examples
TCP_Connection
Established, Listen, Closed
Car_PowerBand
In Powerband, Below PowerBand and Above PowerBand
Implementing the state concept
Constant identifiers and Conditional logic
Define different State class for each logical state

4. Implementing the State pattern
From the object collaboration angle
Context object and the State object
Context object
An object going through the different states
Context maintains an object reference to the state
State
All different states are ‘concrete classes’ inheriting from an ‘abstract state base class’
State classes implement the ‘handle()’ method which localizes actions to be taken on an event in the FSM

5. Details Working the State Pattern Benefits of the State pattern
Client configures Context with initial state
States take care of state transition and state specific actions to be performed
Benefits of the State pattern
Localization of State specific behavior
Partition behavior for different states

6. The Car Example

7. Writing a generic State class
abstract class State {     static State initialState;     static final State state1 = new State1();     static final State state2 = new State2();     protected State() {       if (initialState == null) initialState = this;     }     abstract void stateExit(StateOwner owner);     abstract void stateEnter(StateOwner owner);   }   

8. Writing a generic State class
class State1 extends State {     void stateExit(StateOwner owner) {     }     void stateEnter(StateOwner owner) {     }   }   class State2 extends State {     void stateExit(StateOwner owner) {     }     void stateEnter(StateOwner owner) {     }   }

9. Conclusion State pattern
For an abstraction that executes a finite state machine
Relies heavily on the OO concept of dynamic binding / Abstract base classes