The State Pattern: Modeling Object States Efficiently

1. The State Pattern Joshua Hertz Nik Reiman

2. Roadmap • What is the State Pattern • Modeling States • Setup of the State Pattern • State Diagram • UML Diagram • Oozinoz

3. What is the State Pattern • From Metsker • The intent … is to distribute state specific logic across classes that represent an object’s state. • From the Gang of Four • Allow an object to alter its behavior when its internal state changes. The object will appear to change its class.

4. Modeling States • Can require complex cascading if statements • Have to adjust logic when state changes • Three fundamental parts to this pattern

5. Setup of the State Pattern • Interface which other classes interact with • Class which maintains the current state • Individual classes for each respective state

6. State Diagram

7. UML Diagram

8. Door_2 Class package com.oozinoz.carousel; public class Door_2 extends Observable { public final DoorState CLOSED = new DoorClosed(this); public final DoorState OPENING = new DoorClosed(this); public final DoorState OPEN = new DoorClosed(this); public final DoorState CLOSING = new DoorClosed(this); public final DoorState STAYOPEN = new DoorClosed(this); // private DoorState = CLOSED; // … }

9. Door State Class public abstract class DoorState { protected Door_2 door; public DoorState(Door_2 door) { this.door = door; } public abstract void click(); public void complete(){ } public String status(){ String s = getClass().getName(); return s.substring(s.lastIndexOf (‘.’ + 1); } public void timeout(){ } }

10. package com.oozinoz.carousel public class Door_2 extends Observable{ // … (DoorState variables) public void click(){ state.click(); } public void complete(){ state.complete(); } protected void setState(DoorState state){ this.state = state; setChanged(); notifyObservers(); } public String status(){ return state.status(); } public void timeout(){ state.timeout(); } }

11. package com.oozinoz.carousel; Public class DoorOpen extends DoorState{ public DoorOpen(Door_2 door){ super(door); } public void click(){ door.setState(door.STAYOPEN); } public void timeout(){ door.setState(door.CLOSING); } }

12. Challenge-o-rama! • Write code for DoorClosing.java

13. Challenge-o-rama answer package com.oozinoz.carousel; public class DoorClosing extends DoorState { public DoorClosing(Door_2 door) { super(door); } public void click() { door.setState(door.OPENING); } public.void complete() { door.setState(door.CLOSED); } }

14. Challenge-o-rama II: The Sequel • Complete this diagram to show a design that moves the door states to an interface.

15. Challenge-o-rama II: The Answer