State Pattern

Hi friends,
Can anyone explain the State Pattern.
Thanx,
Vidya

Well, the best place to start would be the Gang of Four book ("Design Patterns: Elements of Reusable Object-Oriented Design") which was the first description of the pattern.
Another try at explaining it can be found here: http://hometown.aol.com/kgb1001001/Companion/Companion.htm
and another is here:
http://www.enteract.com/~bradapp/javapats.html#Behavioral
or you can get Mark Grand's book, or Alan Shalloway's book , or Jim Cooper's book
or you can go look at the earlier thread comparing State and Strategy in this forum.
Having said all of that, is there a particular question or misunderstanding ABOUT the State pattern that you have?
Kyle
------------------
Kyle Brown,
Author of Enterprise Java (tm) Programming with IBM Websphere
See my homepage at http://members.aol.com/kgb1001001 for other WebSphere information.
[This message has been edited by Kyle Brown (edited October 06, 2001).]
[This message has been edited by Kyle Brown (edited October 06, 2001).]
[This message has been edited by Kyle Brown (edited October 06, 2001).]
[This message has been edited by Kyle Brown (edited October 06, 2001).]

Kyle:
Thanks for mentioning my book, but it doesn't cover the state pattern. Your other recommendations are good.
------------------
Alan Shalloway,
Look for Jim Trott and my book: Design Patterns Explained
Visit our site Net Objectives.
Visit our Design Patterns Explained Community of Practice
Check out our CDROM based audio training in XML

Mea Culpa, Alan. I could have swore from your discussion on another thread that you said that it had -- oh well. Sounds like you've got a "volume II- more patterns experiences" to come, then

Personally, I can't claim knowledge from your book directly yet, I'm waiting for Amazon to ship my copy

Kyle
------------------
Kyle Brown,
Author of Enterprise Java (tm) Programming with IBM Websphere
See my homepage at http://members.aol.com/kgb1001001 for other WebSphere information.

Thanx Kyle for the info...
Vidya

One more link: http://c2.com/cgi-bin/wiki?StatePattern

The State pattern is defined in GOF as
"Allow an object to alter its behavior when its internal state changes. The object will appear to change its class"
This is clearly explained in the example given by the code below
taken from thinking in Patterns
//: c04:statemachine:StateMachineDemo.java
// From 'Thinking in Patterns (with Java)' by Bruce Eckel
// www.BruceEckel.com. See copyright notice in CopyRight.txt.
// Demonstrates StateMachine pattern
// and Template method.
package c04.statemachine;
import java.util.*;
import com.bruceeckel.test.*;
// Adeyemi Adegbile Comment added: State represented as a
// interface
interface State {
void run();
}
abstract class StateMachine {
protected State currentState;
abstract protected boolean changeState();
// Template method:
protected final void runAll() {
while(changeState()) // Customizable
currentState.run();
}
}
// A different subclass for each state:
// Adeyemi Adegbile Comment added: ConcreteState subclasses
// according to GOF
class Wash implements State {
public void run() {
System.out.println("Washing");
}
}
// Adeyemi Adegbile Comment added: ConcreteState subclasses
// according to GOF
class Spin implements State {
public void run() {
System.out.println("Spinning");
}
}
// Adeyemi Adegbile Comment added: ConcreteState subclasses
// according to GOF
class Rinse implements State {
public void run() {
System.out.println("Rinsing");
}
}
class Washer extends StateMachine {
private int i = 0;
// The state table:
private State states[] = {
new Wash(), new Spin(),
new Rinse(), new Spin(),
};
public Washer() { runAll(); }
public boolean changeState() {
if(i < states.length) {
// Change the state by setting the
// surrogate reference to a new object:
currentState = states[i++];
return true;
} else
return false;
}
}
public class StateMachineDemo extends UnitTest {
Washer w = new Washer();
public void test() {
// The constructor does the work.
// This just makes sure it will complete
// without throwing an exception.
}
public static void main(String args[]) {
new StateMachineDemo().test();
}
}
This example clearly demonstracte the state pattern.
According to GOF the pattern, the participants are
Context, this will be the class to set the the current state
State this is the abstract class wash which go through different state.
ConcreteState subclasses : One of this class will be initantiated as run time depending on the status of the class.
State objects are often Singletons (127).GOF.
Adeyemi Adegbile

Originally posted by Iyengar Srividya76:

Hi friends,
Can anyone explain the State Pattern.
Thanx,
Vidya

Hi..
Thanx for yr explanation...
Can u explain just the statement....
"Allow an object to alter its behavior when its internal state changes. The object will appear to change its class"
Thanx,
Vidya

What that statement means is that if you send a message to the context object (the one holding the state) while it holds one state object (State "A") then it will respond to the message in one way. If you send the same message at a later time while the context holds a different state object (State "B") it will respond to the message in a different way.
As the GOF book clearly shows, this is because the messages will be delegated to the state object.
Kyle
------------------
Kyle Brown,
Author of Enterprise Java (tm) Programming with IBM Websphere
See my homepage at http://members.aol.com/kgb1001001 for other WebSphere information.

Thanx Kyle...