what are singleton classes ?

[sunanda kadam]

Please explain it

[Balasubramanian Chandrasekaran]

Originally posted by sunanda kadam:
Please explain it

Singleton is a Design pattern for allowing only one instance of your class.
(i.e.,) You are allowed to create only one Object for your class.

[Laxmikant Ruikar]

Singleton is a design pattern to create a single(or some numbers) instance of class by keeping constructor of class as private.

[BABUN GHOSh]

according to set theory when a set contain only one element is called a singletone set. example A={5}

so single tone class is a class which can able to create only single instance of that class.

 
public class x
{
   private x()
   {
   }
}
 
   

[Guido Sautter]

not to forget about a static factory method, often named getInstance(), which acecesses the private constructor on demand.

public class SingletonExample {
  
  // the only instance
  private static SingletonExample instance = null;
  
  // private constructor
  private SingletonExample() {}
  
  // get the singleton object
  public static SingletonExample getInstance() {
    // produce the singleton object on demand
    if (instance == null) instance = new SingletonExample();
    
    // always return the same instance
    return instance;
  }
}

[Vilmantas Baranauskas]

The better version of singleton template is:

public class SingletonExample {
  
  // the only instance
  private static SingletonExample instance = new SingletonExample();
  
  // private constructor
  private SingletonExample() {}
  
  // get the singleton object
  public static SingletonExample getInstance() {
    // always return the same instance
    return instance;
  }
}

[Rob Spoor]

And just slightly better:

private static final SingletonExample instance = new SingletonExample();

Mind you, singleton objects usually should not be Serializable, and most definitely not Cloneable.

[Guido Sautter]

Then you might just as well make the singleton instance public static final, which will save you the getInstance() method alltogether ...

One good reason for initializing it on demand is resource issues: Don't create the object unless you need it. At least for large singletons.
[ May 03, 2008: Message edited by: Guido Sautter ]

[Rob Spoor]

Then it's ok to not have the field be final, but then you shouldn't initialize it upon declaring. If you initialize a singleton when you declare it, it's most logical to make it final.

But in your code it is not wise to make it final
It does suffer from one possible problem: if multiple threads access the method at the same time, it is possible to create two instances. You'll need some form of synchronization, the simplest being to just synchronize the entire method.

[Mintoo kumar]

Quote

--------------------------------------------------------------------------
And just slightly better:

code:
--------------------------------------------------------------------------------

private static final SingletonExample instance = new SingletonExample();


--------------------------------------------------------------------------------

---------------------------------------------------------------------------

above code can be considered when you have multithreade enviornment.
you need to add some synchronization check in your getinstance method ,which will prevent race condition.
lazy creation of object gives you better performance when you do not require the object frequently.There it gives performance benefit.


Examples of singleton , Runtime class in java is singleton,Printer,there are other several examples of singleton.

________________________
Mintoo
SCJP 1.4
____________________________

[Roger Chung-Wee]

I have never encountered a case where lazy instantiation made sense, so I would always instantiate the class as in Rob's example. This is thread safe and requires no synchronization of any methods.

[Ilja Preuss]

http://faq.javaranch.com/java/SingletonPattern