CSC321.01 2015F, Class 13: Design Patterns for SaaS =================================================== _Overview_ * Preliminaries. * Admin. * Upcoming Work. * Extra Credit. * Questions. * Design patterns. * The GoF list of patterns. * SOLID. * More code smells. Preliminaries ------------- ### Admin * Welcome back. I hope you had a great Thanksgiving break. * Welcome to our penultimate week of class! * Are any students looking for project work over break and planning to be in town over break? * News. * Testing: &, é ç ### Assignments * Read/view Chapter 12, Practical DevOps. No journal necessary. * No more programming assignments! ### Questions Design patterns --------------- * A design pattern is a blueprint for how to solve certain kinds of problems. * Inspired by a book in the architecture by Christopher Alexander called _A Pattern Language_. * A group of leading object-oriented designers decided to create a set of patterns for object-orented design. Wrote a book: _Design Patterns_. * For better or for worse, patterns have infiltrated computing. There are even "teaching patterns". * Positives * For novice programmers, suggests (a) the problems you are encountering are not new, so there is knowledge you can build upon; (b) here's the core knowledge you need. * For experienced programmers, provide a langauge to talk about solutions. * Negatives * Can be misused - misunderstanding the relationship of the problem at hand to the problem the pattern seems to solve. * Can limit the "creativity" of the profession. * ... Patterns -------- Be ready to give * An overview of the pattern (one or two sentence description)? * How do you achieve it in Java or Ruby? * A situation in which it is useful. * Anything else you think may be helpful. ### Creational #### Factory Method - ZC * Create an object without specifying what the object is. * Example interface FruitTree { Fruit makeFruit(); } // interface FruitTree * We can make lots and lots of fruit by calling the makeFruit method. * Can spawn creatures in a game. interface Creature { } // ... class Alien implements Creature { ... } class Centipede implements Creature { ... } interface CreateFactory { public Creature makeCreature(); } class Game { CreatureFactory factory; public Game(CreatureFactory cf) { this factory = cf; } public void play() { everyTenSeconds factory.makeCreature(); } } // class Game new Game(new CreatureFactory() { Creature makeCreature() { return new Centipede(); } } new Game(new CreatureFactory() { Creature makeCreature() { return new Alien(); } } #### Abstract Factory - AOA * Lets you build a factory for factories. * Example interface GUIFactory { Button createButton(String buttonName); } // interface GUIFactory class WindowFactory implements GUIFactory { Button createButton(String buttonName) { return new windowButton(String buttonName); } // createButton } class iFactory implements GUIFactory { Button createButton(string buttonName) { app.setCustomLayout(...) app.getLayout.addButton() return app.getLayout.getLatestButton(); } // class iFactory #### Singleton - RJ * A class that has only one instance. * You have a global access point to that one instance. * Useful for one thing that you have to coordinate across the system. * Example: You may only have one printer, or one access point to a printer. (Although we often have multiple printers.) * Example: "The Application". * Generally: Some overriding thing (like the application) or some shared value (like the HDD). * Implementation: Use a private constructor with a class method for extracting the value. public class Singleton { public static Singleton theOneAndOnly; public static Singleton get() { if (theOneAndOnly == null) theOneAndOnly = new Singleton(); return theOneAndOnly; } private Singleton() { ...; } } // class Singleton #### Prototype - BC * A memory efficient way of coming up with new classes. Clone the class and then extend it in some way. * Why do this instead of just creating objects in the class? * Specify methods on the fly, rather than in fixed code. * Java example public class Wall { String getContents() { return "Brick"; } } wall1 = new Wall() { String getContents() { if (random() < 0.5) return "You see a smokey mist"; else return "You see misty smoke"; } } wall2 = new Wall(); * Key idea: Once you've made an object (or a copy of an object), you can change its methods (or values). * Alternate version (probably closer to the original). public class Wall { public Wall(String type, int length, int width, int height int x, int y, int z, int loadbearingcapability, ...) { ... } } // class Wall wall1 = new Wall("stone", 10, 10, 5, 100, 200, 300, 50); wall2 = new Wall("brick", 10, 10, 5, 110, 200, 300, 50); wall3 = new Wall("stone", 10, 10, 6, 100, 210, 300, 50); // Another approach wall1 = new Wall("stone", 10, 10, 5, 100, 200, 300, 50); wall2 = wall1.clone(); wall2.setType("brick"); wall2.setX(wall2.x() + 10); wall3 = wall1.clone(); wall3.setX(wall3.x() - 10); // That is, we started with one generic wall and then // customized the parts we needed to customize for each // subsequent wall. #### Builder - AD ### Structural #### Adapter, Bridge - AM * An adapter allows two classes to communicate, even though they have different interfaces. * Bridges are similar; you write them while building your code, while adapters are designed for legacy code. public interface Comparator { /** * Return a negative number if x comes before y * Returns 0 if x equals y * Returns a positive number if x is greater than y */ public int compare(Object x, Object y); } // interface Comparator public class LessThan { public boolean compare(Number x, Number y) { return x < y; } } // class LessThan public static void sort(Integer[] vals, Comparator compare) ... // We need to change the interface of LessThan to match the // interface of comparator. // That's a task for an adapter. public class AdaptedLessThan implements Comparator { this.lt = new LessThan(); public int compare(Object x, Object y) { if (this.lt.compare(x, y)) return -1; else if this.lt.compare(y,x) return 1; else return 0; } } public interface Lightning { } public interface TradiPhone { } public class UpdateiPhone implements Lightning { TradiPhone thingToBeAdapted; ... } * Sam refers to this as "Wrapping" a class. * Good programmers write generic adapters that turn one interface into another. #### Proxy, Façade - ZW * Proxy: Make some things available to client, sometimes caching. * Sam's normal model: Connecting to Web services #### Decorator - SZ #### Composite - DK #### Flyweight - NS ### Behavioral #### Template Method, Strategy - EE * Template Method: Redefine the steps of an algorithm without changing the algorithm. public abstract class SelectionSorter { public int compare(Object val1, Object val2); // abstract public void sort(Object[] values) { ... if (compare(val1,val2) < 0) ... } } public class IncreasingSorter extends SelectionSorter { public int compare(Object val1, Object val2) { return val1 < val2; } } // extends SelectionSorter public class DecreasingSorter extends SelectionSorter { public int compare(Object val1, Object val2) { return val1 > val2; } } // extends SelectionSorter * Strategy: Redefine implementation of the same algorithm public interface Sorter { public void sort(Object[] values); } public class QuickSorter { public void sort(Object[] values) { int pos = partition(values); sort(...); sort(...); } // sort } public class MergeSorter { } #### Observer - EB #### Iterator - LG #### Visitor - DC #### State - RNG #### Chain of Responsibility - MO #### Mediator - AHD #### Interpreter - HD #### Command - TD SOLID Principles ---------------- * ... More code smells ----------------