There are some instances when we need only single object of class. For example MDI form is always has single object. For this we need to restrict class to create multiple objects. For this we can use private constructor. Class having private constructor cannot instantiate. In other word, we cannot create object of class.

The declaration of empty constructor is prevents the declaration of default constructor. If you are not giving any access modifier of with the constructor its by default private.

Now how you stop creating more than one object of class. With the help of private constructor you can achieve this functionality.

    public class SingletonClass
    {
        int i;
        static SingletonClass objSingletonClass;
        private SingletonClass()
        {
            i = 10;
        }

        public static SingletonClass getInstance()
        {
            if (objSingletonClass == null)
            {
                objSingletonClass = new SingletonClass();
            }
            return objSingletonClass;
        }

        public int getValue()
        {
            return i;
        }
    }

In this code, I am creating private constructor of class. Now problem is that how I create instance of class if I had created private constructor. To solve this problem I had create static method getInstance(). In this method I am returning instance of static class object. The pattern I am using for creating object is singleton.

Singleton pattern ensures a class has only single instance, and provide a global point of access to it.

For download of code please fo to below link

http://www.dotnetpal.com/pattern/singleton.htm

What could be more Agile than a Simpler Singleton requiring far less code that any other Singleton Pattern you may have heard about ?

Role

One way solution for achieving reusability of the Singleton Pattern

Implementation

using System;

namespace SingletonPatternGenericCode
{
    public class Singleton<T> where T : class, new()
    {
        private Singleton()
        {
        }

        public static T UniqueInstance
        {
            get { return SingletonCreator.Instance; }
        }

        #region Nested type: SingletonCreator

        private class SingletonCreator
        {
            internal static readonly T Instance = new T();

            static SingletonCreator()
            {
            }
        }

        #endregion
    }

    internal class Test1
    {
    }

    internal class Test2
    {
    }

    public class Client
    {
        public static void Main()
        {
            Test1 t1A = Singleton<Test1>.UniqueInstance;
            Test2 t2A = Singleton<Test2>.UniqueInstance;
            Test1 t1B = Singleton<Test1>.UniqueInstance;

            if (!Equals(t1A, t2A))
            {
                Console.WriteLine("t1a and t2a objects are different");
            }

            if (Equals(t1A, t1B))
            {
                Console.WriteLine("t1a and t2a objects are the same instance");
            }
        }
    }
}

Output:

Bibliography

• C# Design Patterns

Role

The purpose of the Singleton pattern is to endure that there is only one instance of a class, and that there is a global access point to that object.

Design

Implementation

• Singleton -> Facade

namespace FacadePattern
{
    public interface IFacade
    {
        void OperationFirst();
        void OperationSecond();
        void OperationThird();
    }

    public interface ISystem
    {
        void A();
        void B();
    }
}

using System;

namespace FacadePattern
{
    internal class SystemOne : ISystem
    {
        #region ISystem Members

        public void A()
        {
            Console.WriteLine("Operation A in SystemOne !");
        }

        public void B()
        {
            Console.WriteLine("Operation B in SystemOne !");
        }

        #endregion
    }

    internal class SystemTwo : ISystem
    {
        #region ISystem Members

        public void A()
        {
            Console.WriteLine("Operation A in SystemTwo !");
        }

        public void B()
        {
            Console.WriteLine("Operation B in SystemTwo !");
        }

        #endregion
    }
}

namespace FacadePattern
{
    public class Facade : IFacade
    {
        private readonly SystemOne _systemOne;
        private readonly SystemTwo _systemTwo;

        private Facade()
        {
            this._systemOne = new SystemOne();
            this._systemTwo = new SystemTwo();
        }

        public static Facade Instance
        {
            get { return SingletonCreator.Instance; }
        }

        #region IFacade Members

        public void OperationFirst()
        {
            this._systemOne.A();
            this._systemTwo.B();
        }

        public void OperationSecond()
        {
            this._systemTwo.A();
        }

        public void OperationThird()
        {
            this._systemOne.B();
        }

        #endregion

        #region Nested type: SingletonCreator

        private class SingletonCreator
        {
            static SingletonCreator() {}
            internal static readonly Facade Instance = new Facade();
        }

        #endregion
    }
}

using System;

namespace FacadePattern
{
    public class TestFacadePattern
    {
        public static void Main(string[] args)
        {
            Console.WriteLine("Operation first of facade singleton: ");
            Facade.Instance.OperationFirst();
            Console.WriteLine("Operation second of facade singleton: ");
            Facade.Instance.OperationSecond();
            Console.WriteLine("Operation third of facade singleton: ");
            Facade.Instance.OperationThird();
        }
    }
}

OUTPUT:

Use when

• You need to ensure there is only one instance of a class.
• Controlled access to that instance is essential.
• You might need more than one instance at a later stage.
• The control should be localized in the instantiated class, not in some other mechanism.

Bibliography

• Erich Gamma et al. Design Patterns
• C# Design Patterns

Aufgrund einer Diskussion um das Singleton Pattern im Java-Forum (siehe hier: Umfrage zu Singletons und hier: Denkfehler Singleton) habe ich mich mal mit Code Injection beschäftigt.
Daraus entstand dieser Blog Eintrag von mir im Java-Forum: Code Injection.

Dann habe ich gestern ein weiteres Hardwareteil für meine Dreambox erhalten. Und zwar den Siemens Gigaset WLAN Repeater. Diesen kann man auf einen Ethernet Adapter umstellen, sodass meine Dreambox 100 dann einen WLAN Zugriff bekommt.

Leider ist die Einstellung über die GUI recht verwirrend. Die Umstellung auf den Modus: Ethernet Adapter ist zwar recht einfach, aber wie man sich zu einem WLAN verbindet dann nicht mehr.
Man stellt das ganze so ein, als würde man ein WLAN betreiben. Also SSID vergeben und dann die Verschlüsselung auswählen sowie das Passwort. Dann verbindet sich das Siemens Teil automatisch mit diesem WLAN (sieht man auch, da dann das WLAN Licht leuchtet).

Ich war am Anfang verwirrt, da mein anderer Ethernet Adapter Linksys WET-11 über die Weboberfläche das WLAN einfach auszuwählen war. Leider kann der nur WEP und deswegen habe ich mir das Siemens Teil gekauft.
Habe es dann am Abend mit dem Macbook probiert und es hat problemlos funktioniert.

Let’s pause developing our simple serial focused web crawler and discuss about the singleton pattern. You will find this post very useful for our little crawler.
So, if you haven’t heard about design patterns, I recommend you to read a little on Wikipedia:
http://en.wikipedia.org/wiki/Design_pattern_(computer_science).

The singleton pattern (more on Wikipedia) is a very simple and useful pattern.

There are many cases when we need an instance from a class (an object) but we kind of need it to be the same instance. If it doesn’t seem obvious, just think about the display driver (or any kind of driver) or the Graphics object.

In many cases, the singleton pattern can be replaced by using an utility class: a class with public static methods.

But, in some circumstances, we need the same object which means an instance

Let’s take a look at the following example.

package ro.teo.singletonexample;

public class MyIncrement {
   private int i;
   public MyIncrement(){
      i=0;
   }
   public int nextVal(){
      i++;
      return i;
   }
}

package ro.teo.singletonexample;

public class SomeInc {
   public void doStuff(MyIncrement mi){
      System.out.println("I do some serious stuff ...");
      int v=mi.nextVal();
      System.out.println(v);
   }
}

package ro.teo.singletonexample;

public class SEMain {
  public static void main(String[] args){
    MyIncrement mi1=new MyIncrement();
    MyIncrement mi2=new MyIncrement();

    SomeInc si1=new SomeInc();
    SomeInc si2=new SomeInc();

    for (int i=0;i<10;i++){
      mi1.nextVal();
    }
    for (int i=0;i<4;i++){
      mi2.nextVal();
    }

    si1.doStuff(mi1);
    si2.doStuff(mi2);
  }
}

Let’s see the output:

I do some serious stuff ...
11
I do some serious stuff ...
5

As you can see, the result of using 2 different instances is obvious.
Of course, we could just use the same mi1 instance in this example. But what if we had more methods? Or more objects? Or more threads?

Now, let’s make MyIncrement singleton:

package ro.teo.singletonexample;

public class MyIncrement {
  private static class MyIncrementHolder{
    private final static MyIncrement
              INSTANCE=new MyIncrement();
  }

  public static MyIncrement getInstance(){
    return MyIncrementHolder.INSTANCE;
  }

  private int i;
  private MyIncrement(){
    i=0;
  }
  public int nextVal(){
    i++;
    return i;
  }
}

package ro.teo.singletonexample;

public class SEMain {
  public static void main(String[] args){
    MyIncrement mi1=MyIncrement.getInstance();
    MyIncrement mi2=MyIncrement.getInstance();

    SomeInc si1=new SomeInc();
    SomeInc si2=new SomeInc();

    for (int i=0;i<10;i++){
      mi1.nextVal();
    }
    for (int i=0;i<4;i++){
      mi2.nextVal();
    }

    si1.doStuff(mi1);
    si2.doStuff(mi2);
  }
}

Let’s see the output in this case:

I do some serious stuff ...
15
I do some serious stuff ...
16

As you can see the sollution of Bill Pugh works fine.

We are going to use the pattern for the NewUrlsQueue for our focused web spider.

In software engineering, a design pattern is a general reusable solution to a commonly occurring problem in software design. The advantage of knowing and using these patterns is save time as well as give developers a common language in software design. Here I discuss the implementation of some common design patterns that are easily adapted to PHP.

1. Strategy Pattern:

The strategy pattern is typically used when your programmer’s algorithm
should be interchangeable with different variations of the algorithm. For
example, if you have code that creates an image, under certain circumstances, you might want to create JPEGs and under other circumstances, you might want to create GIF files.
The strategy pattern is usually implemented by declaring an abstract
base class with an algorithm method, which is then implemented by inheriting concrete classes. At some point in the code, it is decided what concrete strategy is relevant; it would then be instantiated and used wherever relevant.
Our example shows how a download server can use a different file selection
strategy according to the web client accessing it. When creating the
HTML with the download links, it will create download links to either .tar.gz
files or .zip files according to the browser’s OS identification. Of course, this
means that files need to be available in both formats on the server. For simplicity’s sake, assume that if the word “Win” exists in $_SERVER["HTTP_
USER_AGENT"], we are dealing with a Windows system and want to create .zip
links; otherwise, we are dealing with systems that prefer .tar.gz.
In this example, we would have two strategies: the .tar.gz strategy and
the .zip strategy, which is reflected as the following strategy hierarchy (see
Figure 1).

Figure 1: Strategy hierarchy.

The following code snippet should give you an idea of how to use such a
strategy pattern:
abstract class FileNamingStrategy {
abstract function createLinkName($filename);
}
class ZipFileNamingStrategy extends FileNamingStrategy {
function createLinkName($filename)
{
return “http://downloads.foo.bar/$filename.zip”;
}
}
class TarGzFileNamingStrategy extends FileNamingStrategy {
function createLinkName($filename)
{
return “http://downloads.foo.bar/$filename.tar.gz”;
}
}
if (strstr($_SERVER["HTTP_USER_AGENT"], “Win”)) {
$fileNamingObj = new ZipFileNamingStrategy();
} else {
$fileNamingObj = new TarGzFileNamingStrategy();
}
$calc_filename = $fileNamingObj->createLinkName(“Calc101″);
$stat_filename = $fileNamingObj->createLinkName(“Stat2000″);
print <<<EOF
<h1>The following is a list of great downloads<</h1>
<br>
<a href=”$calc_filename”>A great calculator</a><br>
<a href=”$stat_filename”>The best statistics application</a><br>
<br>
EOF;
Accessing this script from a Windows system gives you the following
HTML output:
<h1>The following is a list of great downloads<</h1>
<br>
<a href=”http://downloads.foo.bar/Calc101.zip”>A great calculator<
/a><br>
<a href=”http://downloads.foo.bar/Stat2000.zip”>The best statistics
application</a><br>
<br>

Tip: The strategy pattern is often used with the factory pattern, which is
described later in this section. The factory pattern selects the correct strategy.

2. Singleton Pattern:
The singleton pattern is probably one of the best-known design patterns.
You have probably encountered many situations where you have an object that
handles some centralized operation in your application, such as a logger
object. In such cases, it is usually preferred for only one such application-wide
instance to exist and for all application code to have the ability to access it.
Specifically, in a logger object, you would want every place in the application
that wants to print something to the log to have access to it, and let the centralized
logging mechanism handle the filtering of log messages according to
log level settings. For this kind of situation, the singleton pattern exists.
Making your class a singleton class is usually done by implementing a
static class method getInstance(), which returns the only single instance of
the class. The first time you call this method, it creates an instance, saves it in
a private static variable, and returns you the instance. The subsequent
times, it just returns you a handle to the already created instance.
Here’s an example:
class Logger {
static function getInstance()
{
if (self::$instance == NULL) {
self::$instance = new Logger();
}
return self::$instance;
}
private function __construct()
{
}
private function __clone()
{
}
function Log($str)
{
// Take care of logging
}
static private $instance = NULL;
}
Logger::getInstance()->Log(“Checkpoint”);

The essence of this pattern is Logger::getInstance(), which gives you
access to the logging object from anywhere in your application, whether it is
from a function, a method, or the global scope.
In this example, the constructor and clone methods are defined as private.
This is done so that a developer can’t mistakenly create a second
instance of the Logger class using the new or clone operators; therefore, getInstance() is the only way to access the singleton class instance.

3. Factory Pattern:

Polymorphism and the use of base class is really the center of OOP. However,
at some stage, a concrete instance of the base class’s subclasses must be created.
This is usually done using the factory pattern. A Factory class has a
static method that receives some input and, according to that input, it decides
what class instance to create (usually a subclass).
Say that on your web site, different kinds of users can log in. Some are
guests, some are regular customers, and others are administrators. In a common
scenario, you would have a base class User and have three subclasses:
GuestUser, CustomerUser, and AdminUser. Likely User and its subclasses would
contain methods to retrieve information about the user (for example, permissions
on what they can access on the web site and their personal preferences).
The best way for you to write your web application is to use the base class
User as much as possible, so that the code would be generic and that it would
be easy to add additional kinds of users when the need arises.
The following example shows a possible implementation for the four User
classes, and the UserFactory class that is used to create the correct user object
according to the username:
abstract class User {
function __construct($name)
{
$this->name = $name;
}
function getName()
{
return $this->name;
}
// Permission methods
function hasReadPermission()
{
return true;
}
function hasModifyPermission()
{
return false;

}
function hasDeletePermission()
{
return false;
}
// Customization methods
function wantsFlashInterface()
{
return true;
}
protected $name = NULL;
}
class GuestUser extends User {
}
class CustomerUser extends User {
function hasModifyPermission()
{
return true;
}
}
class AdminUser extends User {
function hasModifyPermission()
{
return true;
}
function hasDeletePermission()
{
return true;
}
function wantsFlashInterface()
{
return false;
}
}
class UserFactory {
private static $users = array(“Andi”=>”admin”, “Stig”=>”guest”,
“Derick”=>”customer”);
static function Create($name)
{
if (!isset(self::$users[$name])) {
// Error out because the user doesn’t exist
}
switch (self::$users[$name]) {

case “guest”: return new GuestUser($name);
case “customer”: return new CustomerUser($name);
case “admin”: return new AdminUser($name);
default: // Error out because the user kind doesn’t exist
}
}
}
function boolToStr($b)
{
if ($b == true) {
return “Yes\n”;
} else {
return “No\n”;
}
}
function displayPermissions(User $obj)
{
print $obj->getName() . “’s permissions:\n”;
print “Read: ” . boolToStr($obj->hasReadPermission());
print “Modify: ” . boolToStr($obj->hasModifyPermission());
print “Delete: ” . boolToStr($obj->hasDeletePermission());
}
function displayRequirements(User $obj)
{
if ($obj->wantsFlashInterface()) {
print $obj->getName() . ” requires Flash\n”;
}
}
$logins = array(“Andi”, “Stig”, “Derick”);
foreach($logins as $login) {
displayPermissions(UserFactory::Create($login));
displayRequirements(UserFactory::Create($login));
}
Running this code outputs
Andi’s permissions:
Read: Yes
Modify: Yes
Delete: Yes
Stig’s permissions:
Read: Yes
Modify: No
Delete: No
Stig requires Flash
Derick’s permissions:
Read: Yes
Modify: Yes
Delete: No
Derick requires Flash
This code snippet is a classic example of a factory pattern. You have a class
hierarchy (in this case, the User hierarchy), which your code such as displayPermissions() treats identically. The only place where treatment of the classes differ is in the factory itself, which constructs these instances. In this example, the factory checks what kind of user the username belongs to and creates its class accordingly. In real life, instead of saving the user to user-kind mapping in a static array, you would probably save it in a database or a configuration file.

Tip: Besides Create(), you will often find other names used for the factory
method, such as factory(), factoryMethod(), or createInstance().

4. Observer Pattern:

PHP applications, usually manipulate data. In many cases, changes to one
piece of data can affect many different parts of your application’s code. For
example, the price of each product item displayed on an e-commerce site in the
customer’s local currency is affected by the current exchange rate. Now,
assume that each product item is represented by a PHP object that most likely
originates from a database; the exchange rate itself is most probably being
taken from a different source and is not part of the item’s database entry. Let’s
also assume that each such object has a display() method that outputs the
HTML relevant to this product.
The observer pattern allows for objects to register on certain events
and/or data, and when such an event or change in data occurs, it is automatically
notified. In this way, you could develop the product item to be an observer
on the currency exchange rate, and before printing out the list of items, you
could trigger an event that updates all the registered objects with the correct
rate. Doing so gives the objects a chance to update themselves and take the
new data into account in their display() method.
Usually, the observer pattern is implemented using an interface called
Observer, which the class that is interested in acting as an observer must
implement.
For example:

interface Observer {
function notify($obj);
}
An object that wants to be “observable” usually has a register method
that allows the Observer object to register itself. For example, the following
might be our exchange rate class:

class ExchangeRate {
static private $instance = NULL;
private $observers = array();
private $exchange_rate;
private function ExchangeRate() {
}
static public function getInstance() {
if (self::$instance == NULL) {
self::$instance = new ExchangeRate();
}
return self::$instance;
}
public function getExchangeRate() {
return $this->$exchange_rate;
}
public function setExchangeRate($new_rate) {
$this->$exchange_rate = $new_rate;
$this->notifyObservers();
}
public function registerObserver($obj) {
$this->observers[] = $obj;
}
function notifyObservers() {
foreach($this->observers as $obj) {
$obj->notify($this);
}
}
}
class ProductItem implements Observer {
public function __construct() {
ExchangeRate::getInstance()->registerObserver($this);
}
public function notify($obj) {
if ($obj instanceof ExchangeRate) {
// Update exchange rate data
print “Received update!\n”;
}
}
}
$product1 = new ProductItem();
$product2 = new ProductItem();
ExchangeRate::getInstance()->setExchangeRate(4.5);
This code prints
Received update!
Received update!

Although the example isn’t complete (the ProductItem class doesn’t do
anything useful), when the last line executes (the setExchangeRate() method), both $product1 and $product2 are notified via their notify() methods with the new exchange rate value, allowing them to recalculate their cost.
This pattern can be used in many cases; specifically in web development,
it can be used to create an infrastructure of objects representing data that
might be affected by cookies, GET, POST, and other input variables.

Thanks to the advances of PHP 5, using common OO methodologies, such as
design patterns, has now become more of a reality than with past PHP versions. For more you can visit http://www.cetus-links.org/.

]]> http://susanneurich.wordpress.com/2008/08/01/static-classes-statt-singleton/ Fri, 01 Aug 2008 18:08:38 +0000 Susann http://susanneurich.wordpress.com/2008/08/01/static-classes-statt-singleton/

Unter bestimmten Umständen, kann ab c# 2.0 statt des singleton patterns eine “static class” verwendet werden

Definition:
“The Singleton Pattern ensures a class has only one instance, and provides a global point of access to it.”

Anmerkungen dazu:

Das Beispiel im Head First ist der Boiler in der Schokoladenfabrik.

Ein interessanter Artikel zur richtigen Verwendung vom Singleton gibt bei Steve Yegge zu lesen