Albert Einstein Two things are infinite: the universe & human stupidity; and I'm not sure about the universe.

CSC 313 – Advanced Programming Topics

Open-Closed Principle Classes should be open for extension, but closed to modification  So, what does this mean?

Reality Check Classes should be open for extension…  Other’s code can be updated  Other is important emphasis  Everyone is other to someone  Easily create subclasses to:  Add functionality  Include additional fields  Specialize behavior via overriding

More Reality Check …But closed to modification  Nobody better mess with your code  YOUR code generally being beauty incarnate  Only add errors & soil your perfection  Never let anyone:  Add functionality violating its basic assumptions  Alter the tasks it performs  Anything that might introduce bugs

Strategy Pattern & OCP  Pattern stays true to open-closed principle  Context remains open for extension  Strategies add new functionality  Subclasses provide additional fields  But also remains closed to modification  Strategies must use data provided to them  Can only perform actions for which defined

Observer Pattern & OCP  Observer pattern also principle devotee  Subject remains open for extension  Observers provide ways of handling events  In pull model, extend Subject to provide more information to Observers  But also remains closed to modification  Observers cannot ACT, but only RE-ACT  Subject chooses events to expose to Observers

Warning Remainder of lecture unsafe for: Beginning programmers Unwilling to consider new viewpoints Stubbornly dogmatic Stupid Liberal arts majors

Problem At Hand

Add fields for properties  Add field for each possible property  Does double whip, decaf, soy water make sense?  How to handle taxes for multiple states?  Adding fields is also inefficient  Must modify class for each new property  Creates huge number of rarely used fields  Class remains open for anyone to play with  Violates the open close principle

Create subclasses  Define subclass for each possible situation  Need both MochaWhipCoffee & WhipMochaCoffee?  Volunteers writing all pizza topping combinations?  Does not violate open-close principle  But code duplicated in many places  Who’ll modify classes to update New York tax rate?  Maintaining this code will be major pain in the ass

Decorator Pattern Intent

Decorator Pattern Usage  Flexibly add adjectives to main class’s noun  Decorations provide additional description  Each decoration not always required  Wrappings can be stacked in any order  Add adverbs to enhance main class’s actions  Improve result using additional information  Include additional tasks that must be performed  Invisibly augment main class instances

For Next Lecture  Lab #3 available on web/Angel  Asks you to implement the Observer Pattern  Due before lab on Tuesday  Read pages 95 – 105 in the book  How do we implement the DESIGN PATTERN?  Can anyone actually use this?  What is the main point of this pattern?