carfield.com.hk

Main.java

2004-03-24T16:00:00Z

package examples.observer.java;

/* -*- Mode: Java; tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 4 -*-
 *
 * This file is part of the design patterns project at UBC
 *
 * The contents of this file are subject to the Mozilla Public License
 * Version 1.1 (the "License"); you may not use this file except in
 * compliance with the License. You may obtain a copy of the License at
 * either http://www.mozilla.org/MPL/ or http://aspectj.org/MPL/.
 *
 * Software distributed under the License is distributed on an "AS IS" basis,
 * WITHOUT WARRANTY OF ANY KIND, either express or implied. See the License
 * for the specific language governing rights and limitations under the
 * License.
 *
 * The Original Code is ca.ubc.cs.spl.patterns.
 *
 * Contributor(s): 
 */

import java.awt.Color; 

/**
 * Implements the driver for the observer design pattern example.
 *
 * Intent: Define a one-to-many dependency between objects so that when one
 * object changes state, all its dependents are notified and updated 
 * automatically
 *
 * Participatng objects are <code>Point</code> p and <code>Screen</code>
 * s1, s2, s3, s4, and s5.
 *
 * Three different kinds of observing relationships are realized: <UL>
 * <LI> <code>Screen</code> s1 and s2 observe color changes of <code>Point
 * </code> p.
 * <LI> <code>Screen</code> s3 and s4 observe coordinate changes of <code>
 * Point</code> p.
 * <LI> <code>Screen</code> s5 observes the <code>display(String)</code> 
 * methods of <code>Screen</code> s2 and s4.
 * </UL>
 *
 * Every time an event of interest occurs, the observing <code>Screen</code>
 * prints and appropriate message to stdout. 
 *
 * This is the pure Java version. 
 *
 * The example illustrates that it is hard to
 * cleanly modularize the different observing relationships. The following
 * implementation issues have to be considered for the Java version: 
 * <UL>
 * <LI> Observer and Subject can only be interfaces (as opposed to abstract
 * classes) if we do not want to restrict inhertance and thus code
 * reuse of existing classes completely.
 * <LI> As interfaces, we cannot attach default implementations for methods
 * like <code>attach()</code>, <code>notify()</code>, etc. Note that
 * these two problems only apply because Java does not offer multiple
 * inheritance. 
 * <LI> Some implementation constraints are made implicit and are thus not
 * enforced: I.e., each Subject needs a field to store its observers
 * <LI> The classes that become subject and observer in the pattern context
 * need to be modified. In particular, subjects need to store the 
 * mapping, implement the appropriate procedures. Observers need to
 * implement <code>update()</code>
 * <LI> If a particular class takes part in more than one observing
 * relationship (as in this example), it is difficult to have both
 * notify/update mechanisms go through the same interface and yet 
 * separate them cleanly. 
 * </UL>
 *
 * @author Jan Hannemann
 * @author Gregor Kiczales
 * @version 1.0, 05/13/02
 *
 * @see Point
 * @see Screen
 * @see Observer
 * @see Subject
 */
 
public class Main { 
 
 /**
 * Implements the driver for the observer example. It creates five 
 * <code>Screen</code> objects and one <code>Point</code> object
 * and sets the appropriate observing relationships (see above). 
 * After the setup, the color of the point is changed, then it's 
 * x-coordinate. 
 * The following results should be expected: <OL>
 * <LI> The color change should trigger s1 and s2 to each print an 
 * appropriate message.
 * <LI> s2's message should trigger it's observer s5 to print
 * a message.
 * <LI> The coordinate change should trigger s3 and s4.
 * <LI> s4's message should trigger it's observer s5 again.
 */ 

 public static void main(String argv[]) {
	
 	Point p = new Point(5, 5, Color.blue);
 	
 	System.out.println("Creating Screen s1,s2,s3,s4,s5 and Point p");
 	
 	Screen s1 = new Screen("s1");
 	Screen s2 = new Screen("s2");
 	
 	Screen s3 = new Screen("s3");
 	Screen s4 = new Screen("s4");
 	
 	Screen s5 = new Screen("s5");

 System.out.println("Creating observing relationships:"); 	
 System.out.println("- s1 and s2 observe color changes to p"); 	
 System.out.println("- s3 and s4 observe coordinate changes to p");
 System.out.println("- s5 observes s2's and s4's display() method");
 
 	p.attach(s1); 
 	p.attach(s2);
 	
 	p.attach(s3); 
 	p.attach(s4);
 	
 s2.attach(s5);
 s4.attach(s5);
 
 System.out.println("Changing p's color:"); 	

 	p.setColor(Color.red);

 System.out.println("Changing p's x-coordinate:"); 	

 	p.setX(4); 
 	
 System.out.println("done."); 	
 }
}

Observer.java

2004-03-24T16:00:00Z

Point.java

2004-03-24T16:00:00Z

Screen.java

2004-03-24T16:00:00Z

Subject.java

2004-03-24T16:00:00Z

files.lst

2004-03-24T16:00:00Z