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Lecture 16: Singleton Pattern

Computer Science 313 – Advanced Programming Topics. Lecture 16: Singleton Pattern. 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

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Lecture 16: Singleton Pattern

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  1. Computer Science 313 – Advanced Programming Topics Lecture 16:Singleton Pattern

  2. How Can We Code…

  3. 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

  4. 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

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

  6. Singleton Pattern • Ensures classhas 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 • Alluses of classmust go through that instance

  7. Singleton Usage • Singleton is harmful when a global variable • publicstatic 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 Blowhard Jamboree Kill Two Birds With One Stone Design By Committee

  8. 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 methodsin 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

  9. Singleton Pattern Guides Do not use for • 1 instance wanted • Global value • Instead use static field • Suggests poor design • Check instantiation • Use factory method Do use for • 1 instance possible • Global access point • For a global resource • Serves as gatekeeper • Optimize performance • Can be easily removed

  10. For Next Class • Lab #5 on Angel • Implement Decorator 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?

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