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Design Patterns

Design Patterns. CMPS 2143. Design Patterns. Consider previous solutions to problems similar to any new problem must have some characteristics in common tweak the solution Very few really new problems Whole bunch of patterns are documented that you can use. What is a design pattern?.

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Design Patterns

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  1. Design Patterns CMPS 2143

  2. Design Patterns • Consider previous solutions to problems similar to any new problem • must have some characteristics in common • tweak the solution • Very few really new problems • Whole bunch of patterns are documented that you can use

  3. What is a design pattern? • Each pattern describes a problem which occurs over and over again in our environment, and then describes the core of the solution to that problem, in such a way that you can use this solution a million times over, without ever doing it the same way twice. Christopher Alexander • Aid in discussing structures and relationships at a higher level of abstraction (viewed as a community of interacting agents)

  4. Two Reasons to use design patterns • Speeds process of finding a solution • don’t reinvent the wheel • Naming patterns gives programmers a common vocabulary or language to discuss design alternatives.

  5. What patterns aren’t. • Patterns are a generalized concept, an idea formed in succinct manner, yet expressible in many different ways. They aren’t: • an exact solution • the basis of a code catalog

  6. Controlling Information Flow • Many design patterns deal with how information flows between a client/server or consumer/producer boundary. client info server

  7. Easy Example: Adapter Pattern • An object in your community (client) needs a service and requires a specific interface • Another object provides the service, but does not support the desired interface • CREATE AN ADAPTER! • speaks the language of the client, but doesn’t do the work itself, it gets the service to do it

  8. Coded example • Client wants a DataBox but wants the methods in Collection class MyCollection implements Collection{ public booleanisEmpty() {return data.count()== 0;} public int size () {return data.count();} public void addElement (Object e) {data.add(e);} public booleancontainsElement (Object e) { return data.find(e) != null;} public Object findElement (Object e) { return data.find(e);} private DataBox data = new DataBox(); }

  9. Describing Patterns • Special format to describe patterns • Use a narrative with these parts • name • synopsis • forces (or requirements of pattern) • solution (or rather the essence of a solution) • counterforces (reasons not to use the pattern) • related patterns

  10. Describing the Adapter Pattern • name: Adapter • synopsis: Used to connect a client who needs a service described using one interface with a provider who implements the desired functionality but uses a different interface • solution : implements the interface as specified by the client, but rather than performing the given operations, passes the work on to the service provider • counterforces: adds one layer of indirection between the client and provider and also introduces another class

  11. Singleton Pattern • Not all patterns deal with client/server interactions • In Singleton pattern the developer of a class wants to ensure there is never more than one instance of the class

  12. Coded Example of Singleton Pattern class SingletonClass { public: static SingletonClass * oneAndOnly () {return theOne;} //other non-constructor methods here private: //constructor Singleton () { //theOne is initialized here } //the member data static SingletonClass * theOne; }

  13. Decorator Pattern • Decorator pattern deals with how new functionality can be attached to existing objects • inheritance is one way to do this • inheritance is static and heavyweigth • inheritance does not permit values to change their behavior dynamically during execution • A decorator wraps around an existing object and satisfies the same requirements • delegates much of responsibility to original • occasionally adds new functionality

  14. Decorator Pattern

  15. Coded Example of Decorator from Java class BufferedInputStream extends InputStream { public BuffererInputStream (InputStream s) {data =s;} //other methods here private InputStream data; } • InputStream reads bytes from input device, such as file. • BufferedInputStream is a subclass of InputStream, adding the ability to buffer input • can reset to an earlier point and can reread values • NOTE: BufferedInputStreamis-an Input Stream AND has-anInputStream

  16. Observer Pattern • Addresses problem of how two objects can stay synchronized without having direct knowledge of each other

  17. Observer Pattern • The model – part that is doing the actual work • The view – portion of the program charged with displaying the results • Need an intermediary • ObserverManager communicates with both the model and the observers • ObserverManager maintains list of models and they notify it when they change • Observers register with ObserverManager • Also known as Publish-Suscribe and ModelViewController

  18. Model-View-Controller

  19. Study • pg. 478: 1, 3, 5, 15

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