1. Chapter 8, Design Patterns Singleton

2. A Pattern Taxonomy Pattern Behavioral Creational Structural Pattern
Composite
Decorator
Adapter
Bridge
Façade
Proxy
Iterator
Visitor
Command
Observer
Template
Strategy
Singleton
Abstract Factory
Builder
Factory
Prototype

3. The Singleton Pattern - 1
This is perhaps the easiest design patter
The goal is to insure that only one instance of an object is possible
Give some applications where you think the singleton design pattern would be useful?

4. The Singleton Pattern - 2
Implementing singletons in Java
Make the method getInstance public and static
getInstance calls the constructor for the singleton pattern, which is not public, it is protected
In this way, only one instance can be created

5. The Spooler as a Singleton
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();

6. The Exception and the Spooler
public class SingletonException extends RuntimeException {
//new exception type for singleton classes
public SingletonException() {
super(); }
public SingletonException(String s) {
super(s);
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");