public 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 one instance

    public Spooler() throws SingletonException
    {
        if (instance_flag)
            throw new SingletonException("Only one printer allowed");
        else
            instance_flag=true;     //set flag for one instance
        System.out.println("printer opened");
    }
}                                                        

