In this chapter, the following content will be discussed: Design pattern classifications, structural design pattern, adapter pattern, structural design patterns, façade pattern, watch a movie with ease, façade - consequences, individual classes. | Lecture Notes Lecture 29 – DESIGN PATTERNS DESIGN PATTERN: Design patterns make it easier to reuse successful designs and architectures. Expressing proven techniques as design patterns makes them more accessible to developers of new systems. Design patterns help you choose design alternatives that make a system reusable and avoid alternatives that compromise reusability. Design patterns can even improve the documentation and maintenance of existing systems by furnishing an explicit specification of class and object interactions and their underlying intent. Put simply, design patterns help a designer get a design "right" faster. Gang of Four (GOF) Patterns As per the design pattern reference book Design Patterns - Elements of Reusable Object-Oriented Software , there are 23 design patterns which can be classified in three categories: Creational, Structural and Behavioral patterns. Creational Patterns Abstract Factory Creates an instance of several families of classes Builder Separates object construction from its representation Factory Method Creates an instance of several derived classes Prototype A fully initialized instance to be copied or cloned Singleton A class of which only a single instance can exist Structural Patterns Adapter Match interfaces of different classes Bridge Separates an object’s interface from its implementation Composite A tree structure of simple and composite objects Decorator Add responsibilities to objects dynamically Facade A single class that represents an entire subsystem Flyweight A fine-grained instance used for efficient sharing Proxy An object representing another object Behavioral Patterns Chain of Resp. A way of passing a request between a chain of objects Command Encapsulate a command request as an object Interpreter A way to include language elements in a program Iterator Sequentially access the elements of a collection Mediator Defines simplified communication between classes Memento Capture and restore an objects internal state Observer A way of notifying change to a number of classes State Alter an objects behavior when its state changes Strategy Encapsulates an algorithm inside a class Template Method Defer the exact steps of an algorithm to a subclass Visitor Defines a new operation to a class without change STRUCTURAL PATTERNS: These design patterns concern class and object composition. Concept of inheritance is used to compose interfaces and define ways to compose objects to obtain new functionalities. FAÇADE PATTERN: Provide a unified interface to a set of interfaces in a subsystem. Façade defines a higher-level interface that makes the subsystem easier to use. Participants The classes and objects participating in this pattern are: Facade o knows which subsystem classes are responsible for a request. o delegates client requests to appropriate subsystem objects. Subsystem classes o implement subsystem functionality. o handle work assigned by the Facade object. o have no knowledge of the facade and keep no reference to it.
