Lets see how that is done here.

1. First lets create a OneListener class that should be able to listen to ActionEvents and also be able to add Commands to itself. Please refer to my previous post on Command,Singleton if you would like to see more about this patterns and there usage.

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package com.shaafshah.jmenus;
 
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.util.ArrayList;
 
import javax.swing.AbstractButton;
 
// Implements the ActionListener and is a Singleton also.
 
public class OneListener implements ActionListener{
 
	private static OneListener oneListener = null;
 
	// Holds the list of all commands registered to this listener
	private ArrayList<Command> commandList = null;
 
	// A private constructor
	private OneListener(){
		commandList = new ArrayList<Command>();
	}
 
	// Ensuring one instance.
	public static OneListener getInstance(){
		if(oneListener != null)	
			return oneListener;
		else return oneListener = new OneListener();
	}
 
	// Add the command and add myself as the listener
	public void addCommand(Command command){
			commandList.add(command);
		    ((AbstractButton)command).addActionListener(this);
	}
 
 
	// All Events hit here.
	@Override
	public void actionPerformed(ActionEvent e) {
		((Command)e.getSource()).execute();
	}
 
}

In the above code, the addCommand method adds the command Object and adds a listener to it.
Now how is that possible.
Basically because I am categorizing my UI objects as Commands with in the system having some UI. And I am also assuming that these commands are Currently AbstractButton i.e. JMenuItem, JButton. Lets have a look at the Command Interface and its Implementation.

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public interface Command {
	public void execute();	
}

And the implementation, note that the Command is an interface where as the class is still extending a UI object.

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import javax.swing.JMenuItem;
 
public class TestCmd extends JMenuItem implements Command{
 
	public TestCmd() {
		super("Test");
		OneListener.getInstance().addCommand(this);
	}
 
	@Override
	public void execute() {
		System.out.println("HelloWorld");
	}
}

Personally don’t like calling the OneListener in the constructor of the class but just for the sake of simplicity of this post I have left it that way. There are many different ways to omit it.

So the TestCmd is a JMenuItem but is also a Command and thats what the OneListener understands.
As this commads Listener is also OneListener all ActionEvents are thrown there and from there only the Command.execute is called on.

So now you dont have to worry about what listeners are you in. The only thing you know is that when execute is called you need to do your stuff.

You can download the code from Here.

Hope this helps.

So today I am talking about the Abstract Factory pattern. Its not an “abstract” class or object that you call a pattern. But its a Factory of facotries and that is what exactly makes it so much wordingly abstract. Having “abstract” classes is there but just some other side of the coin.

When should I use an Abstract Factory:

• Independence of how products are created, composed or represented
• Should be configurable with one of the multiple families or products
• You need enforcable constraints for the products used as a group
• You need to reveal only the interfaces of products and not thier implementation as part of a bigger picture.

So lets begin with the fun.

This is how I plan to implement it:
Has A:
Product has a Specification
Factory has a Product
FactoryManager has FactoryConstants
FactoryManager has ComputerFactory

Is A:
BFactory is a ComputerFactory
AFactory is a ComputerFactory

Not shown.
ProductA is a Product
ProductB is a Product

Diagram:

AbstractFactory

Creating a simple factory that returns products.

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public abstract class ComputerFactory {
 
 public abstract String getName();
 
 public abstract Product[] getProducts();
 
 public abstract Product getProduct(int ProductID);
 
}

Implementation of the ComputerFactory

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public class AFactory extends ComputerFactory {
 
public String getName(){
return "A";
}
 
public Product[] getProducts(){
return null;
}
 
public Product getProduct(int productID){
switch(productID){
case 1:
return new ProductA();
 
case 2:
return new ProductB();
 
default:
throw new IllegalArgumentException("Sorry you hit the wrong factory, we closed down in 1600 BC");
}
}
}

A register base for factories. Refer to the main method for use later in this post.

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public interface FactoryConstants {
 
 public int A = 1;
 public int B = 2;
 
}

The main Entrant class. the Factory Manager that will give the ComputerFactory resultant. Its assumed to be a Singleton as it registers as a Creator in the system (assumption).

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public class FactoryManager{
 
 private static FactoryManager factoryManager = null;
 
 private FactoryManager(){
 
 }
 
 public static FactoryManager getInstance(){
  if(factoryManager != null){
   return factoryManager;
  }
  else return factoryManager = new FactoryManager();
 }
 
 public ComputerFactory getFactory(int factory) throws IllegalArgumentException{
 
  switch(factory){
   case FactoryConstants.A:
   return new IBMFactory();
 
   case FactoryConstants.B:
   return new SUNFactory();
 
   default:
   throw new IllegalArgumentException("Sorry you hit the wrong factory, we closed down in 1600 BC");
  } 
 }
}

A main method to test the AbstractFactory

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 public static void main(String args[]){
 
  System.out.println(FactoryManager.getInstance().getFactory(FactoryConstants.A).getName());
  System.out.println(FactoryManager.getInstance().getFactory(FactoryConstants.B).getName());
  System.out.println(FactoryManager.getInstance().getFactory(3).getName());
 
 }

You can find the complete code listing here:
AbstractFactory source

This post should help you to get your hands right on the Singleton Pattern and find the kind there off.

Like any other pattern Singleton also has an objective behind it. What is that?

Motivation:
A Singleton ensures that a class has only one instance, and provides a global point of access to that class.

Benefits
The very simple benefits of a singleton can be:

* Controlled access
* Permits a variable number of instances
* Reduced name space

When to use:
There must be exactly once instance of a class

How to use: Walk Through

1. Create a class

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public class SimplySingleton{}

2. Declare a member variable. This variable will be used for keeping the singleton instance.

It has to be private so that it is not accessible from anywhere else. It has to be static so that it holds only one instance in all entirety.

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private static SimplySingleton simplySingleton = null;

3. Declare a private constructor.

Creating a private constructor would mean no one else can instantiate this class.

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private SimplySingleton(){}

4. So now everything seems private how do we access it. Create a global access point.

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public static SimplySingleton getInstance(){}

How would I access it from outside SimplySingleton.getInstance();

This method should return a SimplySingleton instance.

So here comes the logic to create the one and only instance.

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// 4a. is the variable null?
if(simplySingleton != null)
return simplySingleton;
// 4b. if not assign it an instance.
else return simplySingleton = new SimplySingleton();

Following is the complete code listing for writing a Singleton.

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// Declaring the class
public class SimplySingleton {
 
// 1. a private and a static member variable
 
private static SimplySingleton simplySingleton = null;
 
// 2. a private constructor
 
private SimplySingleton(){}
 
// 3. a global access point
 
public static SimplySingleton getInstance(){
// 4a. is the variable null?
if(simplySingleton != null)
return simplySingleton;
// 4b. if not assign it an instance.
else return simplySingleton = new SimplySingleton();
}
 
}

Following are some good resources for in depth peek into the Singleton Pattern.

More Resources:

http://en.wikipedia.org/wiki/Singleton_pattern

http://radio.weblogs.com/0122027/stories/2003/10/20/implementingTheSingletonPatternInJava.html

http://www.oodesign.com/singleton-pattern.html

http://www.oaklib.org/docs/oak/singleton.html