Hello World! Design Patterns

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Code = Documentation bad — comment inflation:

boolean flag = false; // whether has been done

boolean hasDone = false;

http://en.wikipedia.org/wiki/Self-documenting

1 Computer Languages

1.1 C

main() {
   printf("Hello world!\n");
}

1.2 BASIC

1 print "Hello world!" + CHR(13)
2 end

1.3 HP48 RPL

"Hello"
" "
"world!"
+
+

1.5 php

<?= 'Hello' . ' ' . 'world!' ?>

1.5 J2SE

package com.one0.j2se;

public class HelloWorld {
   public static void main(String[] args) {
      System.out.println("Hello world!");
   } // main
} // HelloWorld

1.6 java.applet.Applet

package com.one0.applet;

import java.applet.Applet;
import java.awt.Graphics;

public class HelloWorld extends Applet {
    public void paint(Graphics g) {
        g.drawString("Hello world!", 100, 75);
    } // paint
} // HelloWorld

1.7 javax.servlet.http.HttpServlet

package com.one0.servlet;

import javax.servlet.*;
import javax.servlet.http.*;

public class HelloWorld extends javax.servlet.HttpServlet {
   private PrintWriter out;

   public doPost(HttpServletRequest req, HttpServletResponse res) {
      doGet(req, res);
   } // doPost

   public doGet(HttpServletRequest req, HttpServletResponse res) {
      res.setContentType("text/plain");
      out = res.getWriter();
      out.println("Hello world!");
      out.flush();
   } // doGet
} // HelloWorld

1.8 JSP

<%= "Hello world!" %>
<% response.flushBuffer() %>

1.9 javax.microedition.midlet.MIDlet

package com.one0.j2me;

import javax.microedition.midlet.*;
import javax.microedition.lcdui.*;

public class HelloWorld extends MIDlet implements CommandListener {
   private Command exitCommand;
   private TextBox tbox;

   public HelloWorld() {
      exitCommand = new Command("Exit", Command.EXIT, 1);
      tbox = new TextBox("Hello world MIDlet", "Hello world!", 25, 0);
      tbox.addCommand(exitCommand);
      tbox.setCommandListener(this);
   } // HelloWorld

   protected void startApp() {
      Display.getDisplay(this).setCurrent(tbox);
   } // startApp

   protected void pauseApp() {
      // nothing
   } // pauseApp

   protected void destroyApp(boolean bool) {
      // nothing
   } // destroyApp

   public void commandAction(Command cmd, Displayable disp) {
      if (cmd == exitCommand) {
         destroyApp(false);
         notifyDestroyed();
      } // if
   } // commandAction

   public static void main(String[] args) {
      System.out.println("Hello world!");
   } // main
} // HelloWorld

1.10 javax.swing.*

package com.one0.swing;

import javax.swing.*;

public class HelloWorld {
   public static void main(String[] args) {
      System.out.println("Hello world!");

      JFrame frame = new JFrame("Hello world!");
      final JLabel label = new JLabel("Hello world!");
      frame.getContentPane().add(label);

      frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
      frame.pack();
      frame.setVisible(true);

      // javax.swing.JOptionPane.showMessageDialog(null, "Hello world!");
   } // main
} // HelloWorld

2 Java

2.1 java.lang.Object

2.2 access modifiers vs. visibility

• public
• protected
• default
• private

visibility vs. scope = lifetime

2.3 Initialization Order

• static initializer
• super.constructor
• constructor
• class variables
• instance variables
• local variables
• dangling references: NullPointerException

2.4 Instance vs. Class Variables

2.5 Inheritance vs. Interfaces

class A extends B

• abstract: A class is abstract if the class or one of its methods is declared to be abstract. It cannot be instantiated but has to be subclassed.
• final: A final class cannot be subclassed.

class B implements IB

2.6 Inheritance vs. Associations

2.7 Multiple Inheritance

class A extends B, C, D

class A implements IB, IC, ID

2.8 Cloneable

2.9 Serializable

2.10 Misc

3 Creational Patterns

3.1 Factory Method

3.1.1 The Factory

package com.one0.FactoryMethod;

public class HelloWorldFactory { // the factory
   public Speaker getSpeaker("") { // the factory method
      return getSpeaker("");
   } // getSpeaker

   public Speaker getSpeaker(String lang) { // another factory method
      if (null == lang)
         lang = "";
      lang = lang.toLowerCase();

      // dispatching creation
      if (lang.equals("en"))
         return new enSpeaker();
      else if (lang.equals("es"))
         return new esSpeaker();
      else if (lang.equals("zh"))
         return new zhSpeaker();
      } else {
         return new enSpeaker(); // default
      } // else
   } // getSpeaker
} // HelloWorldFactory

3.1.2 The Product Interface / Template

package com.one0.FactoryMethod;

public abstract class Speaker { // the interface (abstract template) definition
   public sayHello() {
      // nothing
   } // sayHello
}

3.1.3 The Products

3.1.3.1 Product 1

package com.one0.FactoryMethod;

public class enSpeaker extends Speaker {
   public sayHello() {
      System.out.println("Hello world!");
   } // sayHello
} // enSpeaker

3.1.3.2 Product 2

package com.one0.FactoryMethod;

public class esSpeaker extends Speaker {
   public sayHello() {
      System.out.println("iHola al mundo!");
   } // sayHello
} // esSpeaker

3.1.3.3 Product 3

package com.one0.FactoryMethod;

public class zhSpeaker extends Speaker {
   public sayHello() {
      System.out.println("Ni hao shì jiè!");
   } // sayHello
} // zhSpeaker

3.1.4 Test Driver

package com.one0.FactoryMethod;

public class Test {
   public static void main(String[] args) {
      HelloWorldFactory factory = new HelloWorldFactory();
      Speaker sp = factory.getSpeaker(); // factory can determine what to return
      sp.sayHello(); // no downcast required
   } // main
} // Test

3.2 Abstract Factory

• dimension 1 ("maker"): language: English, Spanish, Chinese
• dimension 2 ("family"): time: morning, noon, evening

3.2.1 The New Product Family

package com.one0.AbstractFactory;

public abstract class LangMaker { // dimension 1: language
   public abstract Speaker forMorning(); // dimension 2: time
   public abstract Speaker forNoon();
   public abstract Speaker forEvening();
} // Maker

3.2.2 Product Makers

3.2.2.1 Product Maker A

package com.one0.AbstractFactory;

public class enMaker extends LangMaker { // dimension 1: sort of a meta factory for English speakers
   public Speaker forMorning() { // dimension 2: time
      return new GenericSpeaker("Good morning world!"); // product A/1
      // alternatively:
      // dedicated class EnMorningSpeaker extends Speaker
   } // forMorning

   public Speaker forNoon() {
      return new GenericSpeaker("Good day world!"); // product A/2
   // forNoon

   public Speaker forEvening() {
      return new GenericSpeaker("Good evening world!"); // product A/3
   } // forEvening
} // enMaker

3.2.2.2 Product Maker B

package com.one0.AbstractFactory;

public class esMaker extends LangMaker { // // dimension 1: sort of a meta factory for Spanish speakers
   public Speaker forMorning() { // dimension 2: time
      return new GenericSpeaker("iBuenos dias al mundo!"); // product B/1
   } // forMorning

   public Speaker forNoon() {
      return new GenericSpeaker("iBuenas tardes al mundo!"); // product B/2
   // forNoon

   public Speaker forEvening() {
      return new GenericSpeaker("iBuenas noches al mundo!"); // product B/3
   } // forEvening
} // esMaker

3.2.2.3 Product Maker C

package com.one0.AbstractFactory;

public class zhMaker extends LangMaker { // dimension 1: sort of a meta factory for Chinese Speakers
   public Speaker forMorning() { // dimension 2: time
      return new GenericSpeaker("Ni zao shì jiè!"); // product C/1
   } // forMorning

   public Speaker forNoon() {
      return new GenericSpeaker("Ni hao shì jiè!"); // product C/2
   // forNoon

   public Speaker forEvening() {
      return new GenericSpeaker("Wan an shì jiè!"); // product C/3
   } // forEvening
} // zhMaker

3.2.3 The Abstract Factory

package com.one0.AbstractFactory;

public class HelloWorldFactory { // the (abstract) factory -- not itself declared abstract
   public final static int MORNING = 0;
   public final static int NOON = 1;
   public final static int EVENING = 2;

   private int time;
   private Maker mk; // dimension 2: time
   private Speaker sp; // dimension 1: for the specific language (at the given time)

   public void setTime(int time) {
      this.time = time;
   } // setTime

   public Speaker getSpeaker("") { // the factory method
      return getSpeaker("");
   } // getSpeaker

   public Speaker getSpeaker(String lang) { // another factory method
      if (null == lang)
         lang = "";
      lang = lang.toLowerCase();

      // dimension 1: dispatching language
      if (lang.equals("en"))
         mk = new enMaker();
      else if (lang.equals("es"))
         mk = new esMaker();
      else if (lang.equals("zh"))
         mk = new zhMaker();
      } else {
         mk = new enMaker(); // default
      } // else

      // dimension 2: dispatching time
      if (NOON == time) {
         return mk.forNoon();
      } else if (EVENING == time) {
         return mk.forEvening();
      } else {
         return mk.forMorning(); // default
      } // else

      /*
      specifically:
      - tries to avoid n x m number of classes ("class explosion") by parameterizing behavior
      - reduces number of classes to n + m = number of language makers + number of times

      generally:
      - reduces 2-dimensional dispatching to a centralized, sequential dispatching sequence
      - could even be more than those 2 dimensions
      */
   } // getSpeaker
} // HelloWorldFactory

3.2.4 Product Interface / Template

package com.one0.AbstractFactory;

public abstract Speaker { // the interface (abstract template) definition
   public sayHello() {
      // nothing
   } // sayHello
}

3.2.5 Generic (Parameterized) Product

package com.one0.AbstractFactory;

public GenericSpeaker extends Speaker {
   private msg = "";

   public Speaker(msg) { // parameterized here to avoid class explosion
      this.msg = msg;
   } // Speaker

   public sayHello() {
      System.out.println(msg);
   } // sayHello
} // GenericSpeaker

3.2.6 Test Driver

package com.one0.AbstractFactory;

public class Test {
   public static void main(String[] args) {
      HelloWorldFactory factory = new HelloWorldFactory();
      factory.setTime(HelloWorldFactory.MORNING); // set the time context once (dimension 2)
      Speaker sp = factory.getSpeaker(); // factory decides about language context (dimension 1)
      sp.sayHello();
   } // main
} // Test

package com.one0.AbstractFactory;

public abstract class TimeMaker { // dimension 1: time
   public abstract Speaker forEn(); // dimension 2: language
   public abstract Speaker forEs();
   public abstract Speaker forZh();
} // TimeMaker

package com.one0.AbstractFactory;

public class MorningMaker extends TimeMaker { // dimension 2: sort of a meta factory for Morning greetings
   public Speaker forEn() { // dimension 1: language
      return new GenericSpeaker("Good morning world!"); // product A/1
      // alternatively:
      // dedicated class EnMorningSpeaker extends Speaker
   } // forEn

   public Speaker forEs() {
      return new GenericSpeaker("iBuenos dias al mundo!"); // product B/1
   // forEs

   public Speaker forZh() {
      return new GenericSpeaker("Ni zhao!"); // product C/1
   } // forZh
} // MorningMaker

3.3 Builder

3.4 Prototype

3.4.1 The Cloneable Template

package com.one0.Prototype;

public class Prototype implements Cloneable {
   // http://java.sun.com/j2se/1.4.2/docs/api/java/lang/Object.html#clone()

   private String msg = "Hello world!";

   public Prototype(String msg) {
      this.msg = msg;
    } // msg

   public void setMsg(String msg) {
      this.msg = msg;
   } // setMsg

   public void sayHello() {
      System.out.println(msg);
   } // sayHello

   protected Object clone() {
      Object o = null;
      try {
         o = super.clone();
      } // try
      catch (Exception e) {
         System.out.println("Error during cloning");
      } // catch
      return o;
   } // clone

   public Prototype pubClone() {
      return (Prototype)clone();
   } // pubClone
} // Prototype

3.4.2 The Client

package com.one0.Prototype;

public class Test {
   public static void main(String[] args) {
      Prototype myP = new Prototype("hello world!");
      myP.sayHello();

      Prototype myP2 = myP.pubClone();
      myP.sayHello();
      myP.setMsg("iHola al mundo!");
      myP.sayHello();

      System.out.println(myP + " vs. " + myP2);
   } // main
} // Test

3.5 Singleton

3.5.1 Singleton Type 1

package com.one0.singleton.type1;

public final class HelloWorld {
   static boolean isCreated = false;

   private HelloWorld { // private - cannot construct from outside
      // nothing
   } // HelloWorld

   public static void sayHello() {
      System.out.println("Hello world!");
   } // sayHello
} // HelloWorld

3.5.2 Singleton Type 2

package com.one0.singleton.type2;

public final class HelloWorld {
   static private boolean isCreated = false;

   public HelloWorld throws Exception {
      if (isCreated) {
         throw new Exception("Only one instance allowed!");
      } // if
      else {
         isCreated = true;
         System.out.println("Hello world!");
   } // HelloWorld

   public static void sayHello() {
      System.out.println("Hello world!");
   } // sayHello
} // HelloWorld

3.5.3 Singleton Type 3

package com.one0.singleton.type3;

public final class HelloWorld {
   static private HelloWorld hw = null;

   private HelloWorld { // private constructor can only be referred from this object
      System.out.println("Hello world!");
   } // HelloWorld

   static public HelloWorld getInstance() { // lazy initialisation
      if (null == hw) {
         hw = new HelloWorld();
      } // if

      return (hw);
   } // HelloWorld

   public static void sayHello() {
      System.out.println("Hello world!");
   } // sayHello

   public void finalize() {
      hw = null;
   } // finalize
} // HelloWorld

3.5.4 n instances

4 Structural Patterns

4.1 Adapter

4.2 Bridge

4.3 Composite

4.4 Decorator

4.5 Façade

4.6 Flyweight

4.7 Decorator

5 Behavioral Patterns

5.1 Chain of Responsibility

5.2 Command

5.3 Interpreter

5.4 Iterator

5.5 Mediator

5.6 Memento

5.7 Observer

5.8 State

5.9 Strategy

5.10 Template Method

5.11 Visitor

6 Misc.

6.1 Model-View-Controller

6.1.1 Model

package com.one0.MVC;

public class Model { // the "data"
	private String text; // the data model
	private View[] v;

	public attach(View v) {
		this.v[v.length++] = v; // rewrite to use collection / iterator
	} // attach

	public String getData() {
		return (text);
	} // getText

	public String addData(String text) {
		this.text = text;
	} // addText

	private notify() {
		for(int i = 0, i < v.length[]; i++)
			v[i].update();
	} // notify
} // Model

6.1.2 View

package com.one0.MVC;

public interface View {
	public Controller initialize(Model m);
	public void display();
} // View

package com.one0.MVC;

public class TextView implements View {
	private Model;

	public Controller initialize(Model m) {
		this.m = m;
		Controller c = null;
		if (true)
			c = new TextController();

		return (c);
	} // initialize

	public void display() {
		System.out.println(m.getData());
	} // display
} // TextView

package com.one0.MVC;

public class GUIView implements View {
	private Model;
	private ...JFrame jf...

	public Controller initialize(Model m) {
		this.m = m;
		Controller c = null;
		if (true)
			c = new GUIController();

		return (c);
	} // initialize

	public void display() {
		jf ... m.getData());
	} // display
} // GUIView

6.1.3 Controller

package com.one0.MVC;

public abstract class Controller {
	private Model m = null;

	public Controller(Model m) {
		this.m = m;
	} // Controller

	public void handleEvent(Object o);
} // Controller

package com.one0.MVC;

public class TextController extends Controller {

	public TextController(Model m) {
		super(m);
	} // GUIController

	public void handleEvent(Object o) {
		m.addData(o.toString());
	} // handleEvent
} // TextController

package com.one0.MVC;

public class GUIController extends Controller {

	public GUI(Model m) {
		super(m);

		...set up GUI...
	} // GUIController

	public void handleEvent(Object o) {
		m.addData(o.toString());
	} // handleEvent
} // GUIController

6.1.4 MVC Application

package com.one0.MVC;

public class MVCApplication {
	public static void main(String[] args) {
		Model m = new Model("Hello world!");

		View vText = new TextView();
		Controller cText = vText.initialize(m);
		cText.handleEvent(args[0]);

		View vGUI = new GUIView();
		Controller cGUI = vText.initialize(m);
		cText.handleEvent(args[0]);

6.1.5 Interaction Diagram

                        --update-->
Controller(s) --> Model              View(s)
       ^                <--getData--   !
       !-------------------handleEvent--

6.2 Publish/Subscribe

6.3 Registry

6.4 EJB Patterns

• More Hello world!
• More Hello world! Design Patterns
• wikipedia: Design Patterns
• Ward Cunningham's pattern repository
• EJB Patterns
• Mobile Design Patterns and Architectures
• "The Design Patterns Java Companion" (free PDF); James W. Cooper; IBM Thomas J. Watson Research Center
• "Head First Design Patterns"; Eric Freeman, Elisabeth Freeman; O'Reilly
• "Design Patterns: Elements of Reusable Object-Oriented Software"; Erich Gamma, Richard Helm, Ralph Johnson, John Vlissides — the "Gang of Four" (GoF); Addison-Wesley
• "Sun-Certified Enterprise Architect for J2EE Technology Study Guide"; Mark Cade, Simon Roberts; Sun Microsystems Press
• related topics on wikipedia and elsewhere:
  Software Development Process | Free Software — free as in freedom, not free as in free beer | GNU | Debian | Richard Stallman | Agile Software Development | XP (Extreme Programming) | Unified Modelling Language | Hello World | Java Programming Language | Real Programmers | Heise / Telepolis | OpenOffice Draw | Visual Paradigm | Octopus | misc