Singleton

[dardar]

i founr this implementation :
public class Singleton
{
// Private constructor suppresses generation of a (public) default constructor
private Singleton() {}

/**
* SingletonHolder is loaded on the first execution of Singleton.getInstance()
* or the first access to SingletonHolder.INSTANCE, not before.
*/
private static class SingletonHolder
{
private final static Singleton INSTANCE = new Singleton();
}

public static Singleton getInstance()
{
return SingletonHolder.INSTANCE;
}
}

i cant understand how only one Instance of the class is created?
if we call SingletonHolder twice it seems that there will be 2 object created.
what am i missing here?

[JosAH]

i cant understand how only one Instance of the class is created?
if we call SingletonHolder twice it seems that there will be 2 object created.
what am i missing here?

I find that quite an obscure way to implement a singleton; I normally do it like this:

public class Singleton {
   private Singleton instance= new Singleton();
   public Singleton getInstance() { return instance; }
   ...
   private Singleton() { ... }
}

There are a few problems with this class: if several class loaders are used several 'singletons' are created. If the Singleton class isn't used but loaded the singleton is still eagerly created.

kind regard,

Jos

[Tolls]

All the first version is doing is avoiding the eager loading.
However, from my point of view, there's rarely if ever anything else in a Singleton that would result in it being loaded prior to a call to getInstance(), so I don't really see the value in the SingletonHolder style.

Now someone'll come along and point out something...:)