Lecture 9 Design Patterns CSCI – 3350 Software Engineering II Fall 2014 Bill Pine

CSCI 3350Lecture Lecture Overview Background from architecture Basic design patterns Standard format Example patterns

CSCI 3350Lecture Background Recall the Chess Master Analogy The Software Design Master –Must know, understand, and apply the deisgn patterns There are hundreds of these patterns The more frequently occurring patterns have been cataloged We will examine a subset of these

CSCI 3350Lecture Introduction Motivation: Promote reuse at design level An o-o system is an assembly of classes Want to leverage previous efforts New systems contain functionality not present in old –Else why build a new one? Existing classes will likely be used in different ways than originally designed

CSCI 3350Lecture Introduction (continued) At least some of needed functionality will have been previously developed –Why re-invent the wheel? –Developing new implementations to familiar problems Is a waste of time and money while under development Serves as an injector of faults –And therefore a further waste of time and money Design patterns are an attempt to provide a body of knowledge to commonly recurring problems –In a standard format

CSCI 3350Lecture Introduction (continued) The standard reference for object-oriented design patterns is the book –Title: Design Patterns: Elements of Reusable Object-Oriented Software –Authors: Erich Gamma, Richard Helm, Ralph Johnson, and John Vlissides –The authors are referred to as the “Gang of Four” in object-oriented design literature

CSCI 3350Lecture Definition Quote from Christopher Alexander “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.”

CSCI 3350Lecture Definition (continued) What is the origin of design patterns? –Conventional architecture –Proposed by Christopher Alexander –The architectural patterns described towns and buildings A pattern is a bit of insight that conveys the essence of a proven solution to a commonly recurring problem

CSCI 3350Lecture Definition (continued) An analogy from Jim Coplien “I could tell you how to make a dress by specifying the route of a pair of scissors through a piece of cloth, in terms of angles and length of cut. Or, I could give you a pattern. By reading the specification, you would have no idea of what was being built. The pattern foreshadows the product; it is the rule for making the thing, but it is also, in many respects, the thing itself.”

CSCI 3350Lecture Basic Design Patterns GoF describes 23 of the more commonly recurring patterns The patterns are classified into three categories –Creational Concerned with creating object Object instantiation is deferred to a subclass –Structural Concerned with the composition of classes or objects –Behavioral How classes or object interact or distribute responsibility

CSCI 3350Lecture Design Pattern Classification Purpose CreationalStructuralBehavioral Class Scope Factory MethodAdapter(class)Interpreter Template Method Object Scope Abstract Factory Builder Prototype Singleton* Adapter* Bridge* Composite* Decorator Façade* Flyweight Proxy* Chain of Responsibility Command Iterator* Mediator Memento Observer* State Strategy Visitor

CSCI 3350Lecture Standard Format 13 section standard format –Pattern name and classification –Scope Class –Deals with relationships between classes and their subclasses –Established through inheritance and are static –Fixed at compile time Object –Deals with relationships between objects –Set or changed at runtime and are therefore dynamic

Standard Format (cont) –Category Creational Structural Behavioral CSCI 3350Lecture

CSCI 3350Lecture Standard Format (continued) –Intent What does the pattern do? What problem is solved? –Also known as (optional) Alternative names for the pattern –Motivation A scenario that illustrates –The problem –How the pattern solves the problem

CSCI 3350Lecture Standard Format (continued) –Applicability To what situation can the pattern be applied? A diagram of the classes involved –Uses OMT (Object Modeling Technique) not UML –Participants Classes or objects involved –Collaborations How the participants interact to carry out their responsibilities

CSCI 3350Lecture Standard Format (continued) –Consequences Trade-offs and results of using the pattern –Implementation Pitfalls, hints, and techniques Language-specific issues –Sample code Code snippets to illustrate the pattern –Known uses Examples of the pattern found in “real” systems

CSCI 3350Lecture Standard Format (continued) –Related Patterns Closely related patterns and differences among patterns When studying a design pattern, I find it helpful to begin with –Intent –Applicability –Known uses –Motivation

CSCI 3350Lecture Singleton (Object – Creational) Intent –To ensure that a class has only 1 instance –Provide a single point of access to the instance Applicability –Use the Singleton pattern when: There must be exactly 1 instance of a class That instance must be accessible to all from 1 point

CSCI 3350Lecture Singleton (continued) Known uses –Smalltalk only examples supplied by Gamma Motivation –To avoid conflicts, it is critical that some classes have only 1 instance A file system that is a class within an operating system

CSCI 3350Lecture Class Exercise Produce a list of classes that might be used to create a graph of the type shown –Your design should be highly modular Each aspect of the graph must be modeled as a separate class –The actual “drawing” will be achieved by calling a low level class Plot, whose specification is supplied

CSCI 3350Lecture Façade (Object-Structural) Intent –Provide a unified interface to a set of interfaces –Define a higher-level interface that make the underlying functionality easier to use Applicability –You need to provide a simple interface to a complex subsystem Many users don’t need the flexibility of the subsystem All that flexibility is difficult to manage Provide a default view of the subsystem

CSCI 3350Lecture Façade (continued) Known uses –Compilers Suppose you want to compile a single line Don’t need the hassle of calling the scanner, parser, parse tree generator, optimizer, code generator Provide a simple interface with defaults Motivation –Structure a system into a subsystem to manage complexity –Shield the client from complex interfaces

CSCI 3350Lecture Observer (Object-Behavioral) Intent –Define a one-to-many dependency between objects so all dependents of an object are notified when the primary object changes state Applicability –When an object’s state changes, with this change causing changes in other objects, but the changing object doesn’t know how many dependant objects there are

CSCI 3350Lecture Observer (continued) Known uses –Model / View / Controller Controller gets user inputs and sends message to model Model performs its calculations View displays the model’s state, when notified by subscribe/notify protocol Motivation –Create a loose coupling between the objects involved

CSCI 3350Lecture Proxy (Object-Structural) Intent –To provide a placeholder for another object Applicability –When you need a more versatile reference than a pointer –Client sends messages to the proxy –Proxy provides additional services Security Data validation –Can be used to delay server request until really needed

CSCI 3350Lecture Proxy (continued) Known uses –Instead of inserting a complex graphic into a document, insert a proxy –The proxy will load the real graphic when needed Motivation –When you need to enhance a server –Postpone an activity until it is needed

CSCI 3350Lecture Composite (Object-Structural) Intent –Decompose objects into tree structures that represent a part/whole hierarchy Applicability –When you need to represent whole/part relationship –When you want to treat objects and composition of objects equivalently

CSCI 3350Lecture Composite (continued) Known uses –File systems consist of Directories Files But directories can hold files and other directories –Composite drawings Motivation –Provide a uniform treatment of objects and composites of objects

CSCI 3350Lecture Adapter (Object-Structural) Intent –Convert the interface of a class into another interface more convenient for the client Applicability –When you want to use an existing class, but the interface doesn’t match the one you need –Need to use several classes, each with different interfaces

CSCI 3350Lecture Adapter (continued) Known uses –Suppose you have a Windows application that uses the Windows file system –You need to run the application under UNIX –Write an adapter that accepts Windows file system calls and in turn makes the appropriate UNIX file system calls Motivation –You have existing classes that provide the services you need, but not the interface

CSCI 3350Lecture Bridge (Object-Structural) Intent –Decouple an abstraction from its implementation to allow the two to vary independently Want to provide the capability to run an application on multiple platforms Applicability –When you want changes in the implementation to not affect their clients –When you want to hide the implementation from a client C++ (unfortunately) refers to this as a proxy class

CSCI 3350Lecture Bridge (continued) Known uses –Isolate graphic clients from the hardware –UNIX –Isolate clients using windowing from platform specifics Motivation –Isolate client abstractions from their implementation –Difference between Bridge and Adapter? The Adapter is used for existing client code The Bridge is an integral part of the initial design

CSCI 3350Lecture Iterator (Object-Behavioral) Intent –Provide a means of accessing the elements of an aggregate structure, sequentially, without exposing the underlying structure Applicability –To provide access without knowledge of the internals of the structure –To support multiple access to the object –To provide a means of traversal that is uniform across all aggregate structures

CSCI 3350Lecture Iterator (continued) Known uses –Java For each loop –Standard Template Library (C++) Iterator class Motivation –Isolate the structure from the means of traversing

CSCI 3350Lecture Summary This has been only a brief introduction to design patterns Recall directive from the opening analogy –These designs contain patterns that must be Understood Memorized Applied repeatedly At best you have only begun step 1