CSC 313 – Advanced Programming Topics

Strategy Pattern Usage public class RubberDuck extends Duck { FlightBehavior flyBehavior; QuackBehavior quackBehavior; public RubberDuck() { quackBehavior = new Squeak(); flyBehavior = new FlyNoWay(); }

Strategy Pattern Usage public class RubberDuck extends Duck { FlightBehavior flyBehavior; QuackBehavior quackBehavior; public RubberDuck() { quackBehavior = new Squeak(); flyBehavior = new FlyNoWay(); } Squeak FlyNoWay RubberDuck

Decorator Pattern Usage Pizza pie = new DeepDish(); pie = new Garlic(pie); pie = new Onion(pie); pie

Zen & the Art of Programming Identify and isolate what will change from what stays the same Favor composition over inheritance Classes should be open for extension, but closed to modification Program to a concept, not a class

Zen & the Art of Programming Identify and isolate what will change from what stays the same Favor composition over inheritance Classes should be open for extension, but closed to modification Program to a concept, not a class

Zen & the Art of Programming Identify and isolate what will change from what stays the same Favor composition over inheritance Classes should be open for extension, but closed to modification Program to a concept, not a class

Zen & the Art of Programming Identify and isolate what will change from what stays the same Favor composition over inheritance Classes should be open for extension, but closed to modification Program to a concept, not a class

Zen & the Art of Programming Identify and isolate what will change from what stays the same Favor composition over inheritance Classes should be open for extension, but closed to modification Program to a concept, not a class

Programming to a Concept

Decorator Pattern Problem Need DoubleGarlicOnionDeepDish class for this? Pizza pie = new Garlic(DeepDish()); pie = new Onion(Garlic(pie));

Strategy Pattern Usage

Relations Between Patterns  Design patterns can be used alone  Intent & purpose differs for each pattern  “Official” patterns in at least 3 industrial projects  Often travel together in code  Many strong relationships between patterns  Combination stronger than using by itself

Improving Constructor  May want to limit creating objects  Control instantiation and when or how it occurs  May want to enforce limit on number instantiated  Boolean instances silly, for example  Wastes time & memory, only have 2: true & false  Constructors limited in their actions, however  Before constructor starts, instance already allocated  Exception only option to disrupt allocation process

Smarter Move public class Bool { static Bool TRUE = new Bool(true); static Bool FALSE = new Bool(false); private boolean value; private Bool(boolean b) { value = b; } static Bool fromBoolean(boolean b) { if (b) return TRUE; else return FALSE; }

Cache Data To Save Time public class Int { static Map map; int value; private Int(int i) { value = i; } public Int fromString(String s) { Int retVal = map.get(s); if (retVal == null) { retVal = new Int(Integer.parseInt(s)); map.put(s, retVal); } return retVal; }

Smart Instantiation

Better Instantiation Factory Methods  Methods like these called Factory Methods  Method creates objects just like it is a factory  Factory methods provide additional features  Meaningful name can be given to method  Using set of methods, creation options clarified  Readable code easy & no jumping through hoops

Factory Method Example public class Int { static Map map; int value; private Int(int i) { value = i; } public Int fromString(String s) { Int retVal = map.get(s); if (retVal == null) { retVal = new Int(Integer.parseInt(s)); map.put(s, retVal); } return retVal; }

Simple Factory Pattern  Also known as Static Factory Pattern  (Only if method static & not instance based)  Pattern uses single factory method as base  Multiple types instantiated in factory method new  Contains specific, hard-coded “ new ” commands  Other classes use method to skip instantiations  Changing or adding types is much easier  Only need to modify factory method  Client code may not know other types exist!

Static Factory Method public class StaticFactory { static Pizza createPizza(String type) { if (type.equals(“cheese”)) { return new CheesePizza(); } else if (type.equals(“fish”)) { return new AnchovyPizza(); } else { throw Exception(“No Pizza for you!”); } } Pizza pie = StaticFactory.createPizza(“fish”);

Simple Factory Method public class SimpleFactory { Pizza createPizza(String type) { if (type.equals(“cheese”)) { return new CheesePizza(); } else if (type.equals(“fish”)) { return new AnchovyPizza(); } else { throw Exception(“No Pizza for you!”); } } SimpleFactory simple =... Pizza pie = simple.createPizza(“fish”);

Creating Simple Factory  Factory method instantiates many types  protected  protected constructors prevents unlimited alloc.  Within method, parameter(s) specify what to allocate  Pluses & minuses to static factory method use  Factory method much easier to find and call  Client tied to factory class via hard-coded call  Keeping factory method non-static means  More generic client code calling factory method  Exactly which method called not clear

For Next Lecture  Lab #3 available on Angel  Asks you to implement Decorator Pattern but  Have time today, but may want help profiling  In your book, read pages  How do we write these factories?  Is there a good way to do this?  What design pattern could be used in these cases?