State Pattern
Author: Deron Eriksson
Description: This Java tutorial describes the state pattern, a behavioral object pattern.
Tutorial created using: Windows Vista || JDK 1.6.0_11 || Eclipse JEE Ganymede SR1 (Eclipse 3.4.1)

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(Continued from page 1)

The Demo class demonstrates the state pattern. First, it creates a Person object with a HappyState object. We display the results of sayHello() and sayGoodbyte() when the person object is in the happy state. Next, we change the person object's state with a SadState object. We display the results of sayHello() and sayGoodbyte(), and we see that in the sad state, the person object's behavior is different.

package com.cakes;

public class Demo {

	public static void main(String[] args) {

		Person person = new Person(new HappyState());
		System.out.println("Hello in happy state: " + person.sayHello());
		System.out.println("Goodbye in happy state: " + person.sayGoodbye());

		person.setEmotionalState(new SadState());
		System.out.println("Hello in sad state: " + person.sayHello());
		System.out.println("Goodbye in sad state: " + person.sayGoodbye());



The console output of executing Demo is shown here.

Console Output

Hello in happy state: Hello, friend!
Goodbye in happy state: Bye, friend!
Hello in sad state: Hello. Sniff, sniff.
Goodbye in sad state: Bye. Sniff, sniff.

Note that we don't necessarily need to have the Context (ie, Person) implement the EmotionalState interface. The behavioral changes could have been internal to the Context rather than exposing EmotionalState's methods to the outside. However, having the Context class implement the State interface allows us to directly access the different behaviors that result from the different states of the Context.

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