Singleton pattern in Java 3 : Singleton Pattern « Design Pattern « Java

1. Java
2. Design Pattern
3. Singleton Pattern

Singleton pattern in Java 3

/*
The Design Patterns Java Companion

Copyright (C) 1998, by James W. Cooper

IBM Thomas J. Watson Research Center

*/
class Printer {
  //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 Printer() 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 SinglePrinter {
  static public void main(String argv[]) {
    Printer pr1, pr2;
    //open one printer--this should always work
    System.out.println("Opening one printer");
    try {
      pr1 = new Printer();
    } catch (SingletonException e) {
      System.out.println(e.getMessage());
    }
    //try to open another printer --should fail
    System.out.println("Opening two printers");
    try {
      pr2 = new Printer();
    } catch (SingletonException e) {
      System.out.println(e.getMessage());
    }
  }
}           
       

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