Bjarne Stroustrup I have always wished that my computer would be as easy to use as my telephone. My wish has come true. I no longer know how to use my telephone.

Computer Science 313 – Advanced Programming Topics

Singleton Pattern  System uses exactly 1 instance of the class  Instantiates only 1 instance of the class  Only lets program use single instance  Does not restrict methods in any way  Can change fields’ values also  Use static method to access instance  Get instance by calling method anywhere in program  Can store a reference to instance in other classes…  … but doing so considered bad style

Singleton Pattern Guides

Identifing a Singleton  College president?  Canisius College president?  CSC313 Professor?  TV Show theme song?

Common Themes in Singleton  Elements always in Singleton class  Static field holds instance  Instance returned by public static method  Singleton class usually includes…  Private constructor limiting instantiation  But class may include  Protected constructor to allow subclassing  Static method also serves as simple factory

How To Create Factory Singleton public class GameFact { private static GameFact me; protected GameFact() { } public static GameFact instance(int userAge){ if (me == null) { if (userAge < 13) me = new AnimatedGameFact(); else me = new RealisticGameFact(); } return me; } }

How Not To Create Singletons public class PrinterSpool { private static PrinterSpool me; private PrinterSpool() { } public static PrinterSpool instance() { if (me == null) { me = new PrinterSpool(); } return me; } }

Synchronization is Hard  Synchronization very expensive  Insures only 1 instance created  Program must remain usable, however  Easy upgrades not helpful if program unused  4 ways to hide costs of synchronization  Each shifts where real costs occur  Minimize costs with a good choice

When Method Rarely Called

When Instantiation is Cheap

For Use on Any JVM

On Modern (1.5+) JVMs

McCloud!  Uniqueness leads to improvements  Provide universal access  Eliminate unneeded checks for errors  Avoid creating large number of small objects  Make space-time tradeoffs with confidence  Can also make correctness easy  Pretty easy to find object triggering error  Ensure order of actions  Prevents problems over which instance in charge

For Next Class  Lab #5 on web/Angel & due week from Tues.  Implements Decorator pattern & a factory  You can choose the factory to implement  Read article linked to from web/Angel page  Describes how inlining works in Java  Why getters & setters are cheap explained  Shows how optimization cascades to generate really, really big improvements