Ways by which singleton can be created

Singleton classes can be created in 3 ways

Case 1:

package org.vijayan.sample;

public class Singleton {

 /**
 * Advanced Initialization
 */
  public static final Singleton SINGLETON=new Singleton();
     private Singleton(){
  }

  public static void main(String[] args) {
     for(int i=0;i<4;i++){
       System.out.println(Singleton.SINGLETON);
     }
  }
}

In this case the statically created object is made final and publicly exposed to everyone.

Case 2:

package org.vijayan.sample;

public class Singleton {

/**
* Advanced Initialization
*/
private static Singleton singleton=new Singleton();
   private Singleton(){
}
public static Singleton getInstance(){
   return singleton;
}
public static void main(String[] args) {
  for(int i=0;i<4;i++){
    System.out.println(Singleton.getInstance());
  }
}
}

Case 3:

package org.vijayan.sample;

public class Singleton {

/**
* Lazy Initialization
*/
private static Singleton singleton=null;
  private Singleton(){
}

public static Singleton getInstance(){
  if(singleton==null){
    singleton=new Singleton();
  }
  return singleton;
}

public static void main(String[] args) {
  for(int i=0;i<4;i++){
    System.out.println(Singleton.getInstance());
  }
 }
}

In this third case is preferable because it creates the object on-demand. But it can create issue when it is used in multi threaded environment.

So the code can be changed to case 4

Case 4:

package org.vijayan.sample;

public class Singleton {

/**
* Lazy Initialization
*/
private static Singleton singleton=null;
  private Singleton(){
}
/**
*
* Will take care multi threaded environment but it expensive
*/
public static synchronized Singleton getInstance(){
  if(singleton==null){
    singleton=new Singleton();
  }
  return singleton;
}
public static void main(String[] args) {
  for(int i=0;i<4;i++){
    System.out.println(Singleton.getInstance());
  }
}
}

Though it takes care multi threaded this will become a costly method. to make it a light-weight we can change the code to case 5

Case 5:

package org.vijayan.sample;

public class Singleton {

/**
* Lazy Initialization
*/
private static Singleton singleton=null;
  private Singleton(){
}
/**
*
* Will take care multithreaded environment also less expensive
*/
public static Singleton getInstance(){
  if(singleton==null){
    synchronized (Singleton.class) {
      if(singleton==null){
        singleton=new Singleton();
      }
    }
   }
   return singleton;
}

public static void main(String[] args) {
  for(int i=0;i<4;i++){
    System.out.println(Singleton.getInstance());
  }
}

}

This method is an efficient but complex one, this is also called as double-null checking method. more info on this can be found in http://en.wikipedia.org/wiki/Double-checked_locking

Output:

org.vijayan.sample.Singleton@10b62c9
org.vijayan.sample.Singleton@10b62c9
org.vijayan.sample.Singleton@10b62c9
org.vijayan.sample.Singleton@10b62c9

Please note all 4 times it returns the same object reference.