Pattern
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OO Design Generic Design Activities
Find Classes Factor objects form classes Define class interfaces Define Inheritance Hierarchies Find relationships between class
Goals
Avoid Redesign Keep your design open Reuse as much as possible
Reuse classes Reuse solutions "Proven ones"
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Benefits of Design Patterns
Look for Recurring Patterns of classes and communicating
objects. These patterns :
Solve a specific design problem Provide an Elegant, Reusable, and Flexible solutions
Each Design Pattern; Names, Explains, and Evaluates an
important and recurring design problem in OO systems AND Provide an Elegant solution to the problem in its context. Benefits:
Reuse Choose design alternatives "get it right faster"
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Christopher Alexander et al. Oxford Univ. Press 19?
"Each pattern describes a problem which occurs over and
over again in our environment, and then describes the core of the solution for that problem, in such away that you can use this solution a million times over, without ever doing it the same way twice."
Christopher proposed 250 Patterns for Architectural
Design
Each pattern represents a generic reusable solution to a
recurring problem
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Elements of a Design Pattern
Pattern Name
A handle used to describe the design problem, its solution and consequences in one word
Problem
When to apply the pattern Explain the context
Solution
What are the Elements that make up the design, their relationships, responsibilities and collaboration
Consequences
Results and tradeoffs
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Elements of a Design Pattern
Design Patterns are "Descriptions of Communicating
Objects that are customized to solve a general design problem in a particular context". The recurring aspects of designs are called design patterns. A pattern is the outline of a reusable solution to a general problem encountered in a particular context Many of them have been systematically documented for all software developers to use A good pattern should Be as general as possible Contain a solution that has been proven to effectively solve the problem in the indicated context.
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General Example
MVC Pattern
Decouples views and models by establishing subscribe / notify protocol between them Views represent the state of the model Model contains data values Controller mediates between views and model Page 5 for figure
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Observer Pattern
MVC Architecture
The model object holds the information Views objects draw the visible parts of the view (or do something else)
Display number in tables Display a bar chart
Each view has a controller which is an object that processes user interaction
Mouse or keyboard events
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Observer Pattern
Controller insertText
Model
View
notify getText
repaint
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Architectural patterns (architectural style)
MVC Broker COM Common Object Request Broker Architecture (CORBA)
multi-layer pipe and filter (transformational architectural
pattern, transformational architecture)
transaction-processing
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multiplicity patterns
many-to-many many-to-one one-to-many one-to-one
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Describing Design Patterns
Pattern Name Intent what does the DP do? Also known as Motivation scenario shows how the classes involoved
solve the problem
Applicability situations in which the DP can be applied Structure UML/ OMT Participants classes and/or objects and responsibilities Collaboration Consequences Implementations Sample code Know uses Related patterns
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Software Design Patterns
Published by Gamma et al. 23 commonly used design patterns
Creational Patterns
Object creation
Structural Patterns
Static composition of classes
Behavioral Patterns
Dynamic interaction among objects
See Page 8 for a complete list of patterns
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Design Patterns Organization
Criteria 1 Purpose reflects what a pattern does?
Creational Patterns, Deals with Object creation Structural Patterns, deal with the composition of classes and or objects Behavioral Patterns, show the interaction of objects and their responsibilities
Criteria 2 Scope whether a pattern applies to objects or
classes (Compile time vs. Run time)
Class Patterns
Deals with relationships between classes and their subclasses thru Inheritance thus Static or compile time structure
Object Patterns
Deals with object relationships at run time Dynamic run time
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Design Patterns Organization
Creational Factory Method singleton Abstract Factory Builder Prototype Purpose Structural Behavioral Adapter Interpretor Template Methos Composite Visitor Facade Chain Of Responsibility proxy Command Bridge Interpreter Adapter Memento Decorstor Observer Flyweight State Strategy Mediator
Class Object
Scope
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Notation Review
Class Object
Parent Class
AbstractClass
Operation()
An class abstract is a class whose main purpose is to define a common interface for its subclasses and it can't be instantiated
SubClass
Concrete Class
Operation()
Implementation Pseudocode
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OO Design Discussion
Compile Time
Code structure is frozen Classes are structured in fixed inheritance relationships
Run Time
Rapidly changing networks of communicating objects
Very independent from each other
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OO Design Discussion
Class Inheritance
Defines how the object is implemented The class defines the object's internal state and the implementation of its operations Also defines the Object Type
Object Type
Only defines Interfaces the set of requests the object can respond to
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OO Design Discussion
Class Inheritance
Defines object's implementation in terms of another object's implementation Defines new objects in terms of old ones
Interface Inheritance "Subtyping"
Define when an object can be used in place of another (Polymorphism) Shape Example Shape is an abstract class Two DimShape and ThreeDimShapes are 2 abstract classes that inherit from Shape class They define an abstract method draw(); Rectangle class implements TwoDimShapes class and provide implementation to draw a rectangle. At run time you can use shape to draw the intended shape
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OO Design Discussion
Class Inheritance
Define implementation of one class in terms of another Reuse by sub-classing White-box reuse, since the parent class is visible to the subclasses Static compile time since its supported by a programming language
Object Composition
An alternative to class inheritance to solve the encapsulation problem New functionality is obtained by assembling or composing objects to a more complex functionality (Car object is composed of Engine, Transmission, Body and Wheel Objects)
+ve easy to modify the
at run time
implementation "Override"
+ve Dynamic at run time +ve does not break encapsulation -ve more objects are needed and
thus could have performance side effects
-ve can't change implementation -ve breaks encapsulation
Both are techniques for reusing functionality in OO systems
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OO Design Discussion
Delegation
A way of making composition as powerful for reuse as inheritance Two objects are involved in handling a request
Receiving object Delegates operations to its delegate
In inheritance subclasses deferring requests to parent object (key word super() in java)
This key word to refer to the receiving object
In delegation the receiver passes itself to the delegate to let the delegated operation to refer to the receiver
Dynamic
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OO Design Discussion - Delegation
Delegation Example
Window class reuses the behavior of the Rectangle class by keeping a Rectangle instance variable and delegating Rectangle specific behavior
Window
Area()
rectangle
Rectangle
Area()
width height
Return rectangle -> Area(); Return widt...