THE DESIGN PATTERNS JAVA COMPANION 1. Creational Patterns 17 The Factory Pattern 18 How a Facto

				class Spooler 
				{
				   //this is a prototype for a printer-spooler class
				   //such that only one instance can ever exist
				   static boolean instance_flag=false; //true if 1 instance
				
				   public Spooler() throws SingletonException
				   {
				      if (instance_flag) 
				         throw new SingletonException("Only one printer allowed");    
				      else
				         instance_flag=true;     //set flag for 1 instance
				      System.out.println("printer opened");
				   }
				   //-------------------------------------------
				   public void finalize()
				   {
				      instance_flag = false;
				   }
				}                                                        
				//===============================================
				class SingletonException extends RuntimeException
				{      
				   //new exception type for singleton classes
				   public SingletonException()
				   {
				      super();
				   }
				//-----------------------------------------------
				   public SingletonException(String s)
				   {
				      super(s);
				   }
				}
				//===============================================
				public class singleSpooler
				{
				   static public void main(String argv[])
				   {
				      Spooler pr1, pr2;
				      //open one printer--this should always work
				      System.out.println("Opening one spooler");
				      try{
				      pr1 = new Spooler();
				      }
				      catch (SingletonException e)
				      {System.out.println(e.getMessage());}
				      //try to open another printer --should fail
				      System.out.println("Opening two spoolers");
				      try{
				      pr2 = new Spooler();
				      }
				      catch (SingletonException e)
				      {System.out.println(e.getMessage());}
				   }
				}