Computer Science 313 – Advanced Programming Topics

How Can We Code…

Why One Is Better  Can create improvement from uniqueness  Easy universal access to important resources  Error checks and complex locks eliminated  Avoid allocating large number of small objects  Simplifies calculating space-time tradeoffs

One Is The Loneliest Number  Correctness easy with only one object  Trigger for an error is pretty easy to find  Enforcing order of actions much quicker  Problem over which instance dominates is solved

Where To Find Singletons?  Graphic & game engines  System.out  Bars  System.err  Grocery stores  Device drivers  Laundromats  Design patterns

Where To Find Singletons?  Graphic & game engines  System.out  Bars  System.err  Grocery stores  Device drivers  Laundromats  Design patterns

Where To Find Singletons?

Singleton Pattern  Ensures class has exactly 1 instance  Cannot create second instance of this class  Instance constant, but fields can change  Provides single access point to instance  To ease debugging, do not let reference leak  All uses of class must go through that instance

Singleton Usage  Singleton is harmful when a global variable  public static field used for instance  Eliminates ability to track updates & changes  Common anti-pattern is using global variable  Anti-patterns are habits that harm development Big Ball Of Mud Throwaway Code Kill Two Birds With One Stone Design By Committee

Singleton Pattern  System uses exactly 1 instance of the class  Instantiates only 1 instance during entire program  Guarantees that program uses only single instance  Does not restrict methods in any way  Can define and use fields like normal  Creational pattern like the factory patterns  How we (do not) create instances is focus  Usage unimportant and should not be considered

Singleton Pattern Guides D O NOT use for  1 instance wanted  Global value  Instead use static field  Poor design suggested  Instantiation check  Use factory method D O use for  1 instance possible  Global access point  For a global resource  Gatekeeper for actions  Optimize performance  Can be easily removed

For Next Class  Lab #4 on Angel  Implement Singleton pattern & a factory  You can choose the factory to implement  Read pages 179 – 186 in the book  How do we implement these Singletons?  What are the myriad ways to screw them up?  Which system bugs are exposed by this pattern?