CSC/ECE 517 Fall 2011/ch7 7f bn

Wiki Chapter: CSC/ECE 517 Fall 2011/ch7 7f bn

7f. Pattern classification and importance. A common classification of the GoF patterns is into "creational," "structural," and "behavioral categories, and, orthogonally, into "class" and "object" patterns. But the GoF patterns are only a few of the patterns that have been identified in programs. Look up other ways of classifying patterns.

Introduction

Design patterns are general reusable solutions which can be used to tackle recurring problems in software engineering. They have been developed empirically as a result of programmers facing similar challenges frequently. Design patterns act as guides and not the actual solution to the problem at hand. Design patterns typically show relationships between classes or objects. Design patterns are tried and tested solutions and thus can speed up the development process by providing tested, proven development paradigms. A robust design involves having a broad view of the problem at hand rather than coding to patch a problem. This foresight helps predict, avoid and in the worst case tackle problems. This helps procrastinating fire-fighting efforts until implementation phase. Reusing design patterns helps to prevent subtle issues that can cause major problems and improves code readability for coders and architects familiar with the patterns. They give the best practices and are in general applied to Object-oriented design patterns and so are mostly not applicable to functional programming techniques.

In addition, patterns allow developers to communicate using well-known, well understood names for software interactions. Common design patterns can be improved over time, making them more robust than ad-hoc designs. Design patterns reside in the domain of modules and interconnections. At a higher level there are architectural patterns that are larger in scope, usually describing an overall pattern followed by an entire system.

Class and Object Patterns

Depending on the scope design patterns are broadly classified into 2 of these categories:

Class patterns:

Class patterns specify the relationship between classes and their subclasses. These patterns tend to use inheritance to establish relationships. Unlike object patterns and object relationships, class patterns generally fix the relationship at compile time. They are less flexible and dynamic and less suited to polymorphic approaches.

Object Patterns:

Object patterns, the more common of the two, specify the relationships between objects. In general, the purpose of an object pattern is to allow the instances of different classes to be used in the same place in a pattern. Object patterns avoid fixing the class that accomplishes a given task at compile time. Instead the actual class of the object can be chosen at runtime. Object patterns mostly use object composition to establish relationships between objects.

Gang Of Four Patterns

These 23 GoF patterns are generally considered the foundation for all other patterns. They are categorized in three groups: Creational, Structural, and Behavioral.

Creational Patterns

In software engineering, creational design patterns are design patterns that deal with object creation mechanisms, trying to create objects in a manner suitable to the situation. The basic form of object creation could result in design problems or added complexity to the design. Creational design patterns solve this problem by somehow controlling this object creation. Creational design patterns are further categorized into Object-creational patterns and Class-creational patterns. Where, Object-creational patterns deal with Object creation and Class-creational deal with Class-instantiation.

Abstract Factory

Creates an instance of several families of classes. Provide an interface for creating families of related or dependent objects without specifying their concrete classes. It is a class that exists to create instances of another class.

Typically, if one wants to construct instances of a class, where the class is selected at run time

• Create one AbstractFactory class for each existing class (or group of related classes) you wish to create.
• Have a polymorphic "create instance" method on each AbstractFactory class, conforming to a common method signature, used to create instances of the corresponding class.
• Store and pass around instances of the AbstractFactory class to control selection of the class to create.

Suppose an abstract class wants to hide its sub class name and its instantiation. If we request one static method of the class that return its sub class object.

abstract class AA
{
  static AA getInstance()
  {
    return new A();
  }
}

class A extends AA
{

}

class Client
{
AA aa=AA.getInstance();
aa.method();
}

Builder

Separates object construction from its representation. Separate the construction of a complex object from its representation so that the same construction processes can create different representations so that

• An object with strict properties (e.g. immutable, or say, maxTemperature >= minTemperature) can be configured in less strict steps.
• Avoid hard-to-remember/understand chatty constructors with many arguments, and
• Where possible allow the caller to reuse steps for creating similar instances.

Factory Method

Creates an instance of several derived classes. Define an interface for creating an object, but let subclasses decide which class to instantiate. Factory Method lets a class defer instantiation to subclasses.

Prototype

A fully initialized instance to be copied or cloned. Specify the kinds of objects to create using a prototypical instance, and create new objects by copying this prototype.

Singleton

A class of which only a single instance can exist. Ensure a class only has one instance, and provide a global point of access to it.

Structural Patterns

Adapter

Match interfaces of different classes.Convert the interface of a class into another interface clients expect. Adapter lets classes work together that couldn’t otherwise because of incompatible interfaces.

Bridge

Separates an object’s interface from its implementation. Decouple an abstraction from its implementation so that the two can vary independently.

Composite

A tree structure of simple and composite objects. Compose objects into tree structures to represent part-whole hierarchies. Composite lets clients treat individual objects and compositions of objects uniformly.

Decorator

Add responsibilities to objects dynamically. Attach additional responsibilities to an object dynamically. Decorators provide a flexible alternative to subclassing for extending functionality.

Facade

A single class that represents an entire subsystem. Provide a unified interface to a set of interfaces in a system. Facade defines a higher-level interface that makes the subsystem easier to use.

Flyweight

A fine-grained instance used for efficient sharing. Use sharing to support large numbers of fine-grained objects efficiently. A flyweight is a shared object that can be used in multiple contexts simultaneously. The flyweight acts as an independent object in each context — it’s indistinguishable from an instance of the object that’s not shared.

Proxy

An object representing another object. Provide a surrogate or placeholder for another object to control access to it.

Behavioral Patterns

Chain of Resp

A way of passing a request between a chain of objects. Avoid coupling the sender of a request to its receiver by giving more than one object a chance to handle the request. Chain the receiving objects and pass the request along the chain until an object handles it.

Command

Encapsulate a command request as an object. Encapsulate a request as an object, thereby letting you parameterize clients with different requests, queue or log requests, and support undoable operations.

Interpreter

A way to include language elements in a program. Given a language, define a representation for its grammar along with an interpreter that uses the representation to interpret sentences in the language.

Iterator

Sequentially access the elements of a collection. Provide a way to access the elements of an aggregate object sequentially without exposing its underlying representation.

Mediator

Defines simplified communication between classes. Define an object that encapsulates how a set of objects interact. Mediator promotes loose coupling by keeping objects from referring to each other explicitly, and it lets you vary their interaction independently.

Memento

Capture and restore an object's internal state. Without violating encapsulation, capture and externalize an object’s internal state so that the object can be restored to this state later.

Observer

A way of notifying change to a number of classes. Define a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically.

State

Alter an object's behavior when its state changes. Allow an object to alter its behavior when its internal state changes. The object will appear to change its class.

Strategy

Encapsulates an algorithm inside a class. Define a family of algorithms, encapsulate each one, and make them interchangeable. Strategy lets the algorithm vary independently from clients that use it.

Template

Defer the exact steps of an algorithm to a subclass. Define the skeleton of an algorithm in an operation, deferring some steps to subclasses. Template Method lets subclasses redefine certain steps of an algorithm without changing the algorithm’s structure.

Visitor

Defines a new operation to a class without change. Represent an operation to be performed on the elements of an object structure. Visitor lets you define a new operation without changing the classes of the elements on which it operates.

Resources

• • Software Engineering Design Patterns
  • Class and Object Patterns
  • Patterns
  • Design Patterns
  • Gang of Four
  • Binders Pattern