1. Eagleson’s Law: Any code of your own that you haven't looked at for 6+ months might as well have been written by someone else.

2. Computer Science 313 – Advanced Programming Topics

3. How About that Recursion Flat profile of 0.67 secs (32 total ticks): main Interpreted + native Method 3.1% 1 + 0 Recursive02.binomialExpansion Compiled + native Method 96.9% 26 + 5 Recursive02.binomialExpansion Flat profile of 2.61 secs (145 total ticks): main Interpreted + native Method 0.7% 1 + 0 java.util.Vector.elementAt Compiled + native Method 85.5% 118 + 6 Recursive02.binomialExpansion 13.8% 20 + 0 java.util.Vector.addElement

4. What Happened? Why?

5. Simple Explanation  Java’s implementation of Stack sucks

6. Real Explanation  Java’s Stack slower than system stack  Well, duh, but more important is why  Stack class must work in lots of situations  Includes checks to grow on each call to push  Must balance size and speed demands  Decent performance often; Great performance never  System stack is highly specialized  Explodes size to gain speed  Benefits from knowing it has only one use

7. One is Special

8. There Can Be Only One!  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

9. Common Situation  Graphic & game engines  System.out  System.err  AbstractFactory instances  FactoryMethod instances  Device drivers

10. Singleton Pattern  Ensures class has exactly 1 instance  Always use same instance of class  Fields can change, but instance should not  Provides single access point to instance  All uses of class should go through this  Methods should not let this reference leak

11. Singleton Anti-Pattern  Process or model harmful to overall process  Alcohol Fueled Development  Big Ball Of Mud  Blowhard Jamboree  Design By Committee  Kill Two Birds With One Stone  Singleton is harmful when a global variable  Also known as public static field  Can be updated & changed in many places

12. 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 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?