import java.util.Hashtable;

     /*********************************************************************
     * An example of the Singleton design pattern using a registry.
     * <P>
     * @author
     *   <A HREF="http://www.alumni.caltech.edu/~croft">David W. Croft</A>
     * @version
     *   1998-04-07
     *********************************************************************/

     public class  RegistrySingleton {
     //////////////////////////////////////////////////////////////////////
     //////////////////////////////////////////////////////////////////////

     private static final Hashtable  registry = new Hashtable ( );

     private final String  name;

     public static RegistrySingleton  instance ( String  name )
     //////////////////////////////////////////////////////////////////////
     {
       return ( RegistrySingleton ) registry.get ( name );
     }

     protected  RegistrySingleton ( String  name )
     //////////////////////////////////////////////////////////////////////
     {
       this.name = name;
       if ( !register ( name, this ) )
       {
         throw new IllegalArgumentException (
           name + " already in the registry" );
       }
     }

     /*********************************************************************
     * While all RegistrySingleton objects are usually registered
     * automatically when the constructor is called, it may be necessary
     * to register an instance that was revived from persistent storage.
     *********************************************************************/
     protected static synchronized boolean  register (
       String  name, RegistrySingleton  registrySingleton )
     //////////////////////////////////////////////////////////////////////
     {
       if ( registry.get ( name ) != null ) return false;
       registry.put ( name, registrySingleton );
       return true;
     }

     //////////////////////////////////////////////////////////////////////
     //////////////////////////////////////////////////////////////////////
     }