Singleton

A singleton in Java is a class for which only one instance can be created provides a global point of access this instance. The singleton pattern describe how this can be archived.
Singletons are useful to provide a unique source of data or functionality to other Java Objects. For example you may use a singleton to access your data model from within your application or to define logger which the rest of the application can use.

Example 1
This is the preferred style of implementing singletons. It uses a simple enumeration. It has no special needs for serialization, and is immune to clever attacks.

/** Preferred style for singletons. */public enum SantaClaus {
  INSTANCE;
  
  /**Add some behavior to the object. */
  public void distributePresents(){
    //elided    
  }
  
  /** Demonstrate use of SantaClaus. */
  public static void main(String... aArgs){
    SantaClaus fatGuy = SantaClaus.INSTANCE;
    fatGuy.distributePresents();
    
    //doesn't compile :
    //SantaClaus fatGuy = new SantaClaus();
  }
} 

Example 2 Here is an alternate style. If you decide that the class should no longer be a singleton, you may simply change the implementation of getInstance.

public final class Universe {

  public static Universe getInstance() {
     return fINSTANCE;
  }

  // PRIVATE //

  /**
  * Single instance created upon class loading.
  */
  private static final Universe fINSTANCE =  new Universe();

  /**
  * Private constructor prevents construction outside this class.
  */
  private Universe() {
    //..elided
  }
} 

Example 3 If the above style of singleton is to be Serializable as well, then you must add a readResolve method.

import java.io.*;

public final class EasterBunny implements Serializable {

  public static EasterBunny getInstance() {
     return fINSTANCE;
  }

  // PRIVATE //

  /**
  * Single instance created upon class loading.
  */
  private static final EasterBunny fINSTANCE =  new EasterBunny();

  /**
  * Private constructor prevents construction outside this class.
  */
  private EasterBunny() {
    //..elided
  }

  /**
  * If the singleton implements Serializable, then this
  * method must be supplied.
  */
  private Object readResolve() throws ObjectStreamException {
    return fINSTANCE;
  }
}