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Merci Head First Design Pattern et Wikipedia

singleton.java

public class Singleton
{
  private volatile static Singleton instance = null;
 
  private Singleton ()
  {
  }
 
  public static Singleton getInstance ()
  {
    Singleton result = instance;
    if (result == null)
    {
      synchronized (Singleton.class)
      {
        result = instance;
        if (result == null) {
          instance = result = new Singleton ();
        }
      }
    }
    return result;
  }
}