Basic Design Patterns 1. Introduction Gabriel Mañana Ed. 453 Of. 120 Ext. 14080 gjmananag...

Basic Design PatternsBasic Design Patterns

1. Introduction

Gabriel MañanaEd. 453 Of. 120 Ext. 14080

gjmananag @unal.edu.co

1. IntroductionWhat is a Design Pattern?Design Patterns in MVCDescribing Design PatternsCatalog of Design PatternsSolving Design ProblemsSelecting Design PatternsUsing Design Patterns

Designing OO software is hard

Designing reusable OO software is even harder

1. Find pertinent objects

2. Factor objects into classes

3. Define class interfaces

4. Define inheritance hierarchies

5. Establish key relationships

The design should be specific to the problem at hand but also general enough to address future problems and requirements:

avoid redesign

The difference between expert and novice designers is experience:

Recurring patterns of classes and communicating objects.

These patterns solve specific design problems and make OO designs more flexible:

reusable

Each design pattern systematically names, explains and evaluates an important and recurring design in OO systems.

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.”

Essential pattern elements:

1. pattern name

2. problem

3. solution

4. consequences

1. Pattern Name:

Let us design at a higher level of abstraction.

2. Problem:

Explains the problem and its context.

describes when to apply the pattern

3. Solution:

Describes the elements that make up the design, their relationships, responsibilities and collaborations.

4. Consequences:

Are the results and trade-offs of applying the pattern, its impact on a system’s flexibility, extensibility or portability

Level of Abstraction?

Description of communicating objects and classes that are customized to solve a general design problem in a particular context.

The design pattern identifies the participating classes and instances, their roles and collaborations, and the distribution of responsibilities.

It describes when it applies, whether it can be applied in view of other design constraints, and the consequences and trade-offs of its use. (code)

Smalltalk MVCSmalltalk MVC

Model/View/Controller

Triad of classes used to build user interfaces

pattern

Model: application object

View: its screen presentation

Controller: describes the way the UI reacts to user input

MVC: Decouples views and models by establishing a subscribe/notify protocol between them.

(Observer Pattern)

In MVC views can be nested: CompositeView, subclass of View.

(Composite Pattern)

MVC also allows to change the way a view responds to user input without changing its visual presentation.

(Controller Pattern)

View-Controller relationship is an example of the Strategy design pattern.

(Strategy Pattern: an object that represents an algorithm)

MVC main relationships:

Observer

Composite

Strategy

Describing Design PatternsDescribing Design Patterns

Pattern Name

The pattern’s name conveys the essence of the pattern succintly.

IntentWhat does the design pattern do?

What is the rationale and intent?

What particular design issue or

problem does it address?

Also Known As

Other well-known names for the pattern, if any.

Motivation

A scenario that illustrates a design problem and how the class and object structures in the pattern

solve the problem.

ApplicabilityWhat are the situations in which the design pattern can be applied?

What are examples of poor designs that the pattern can address?

StructureA graphical representation of the classes in the pattern using a notation based on the Unified Modeling Language (UML).

Interaction diagrams are also used to illustrate sequences of request and collaborations between objects.

Participants

The classes and/or objects participating in the design pattern and their responsibilities.

Collaborations

How the participants collaborate to carry out their responsibilities.

Consequences What are the trade-offs and results of using the pattern?

What aspect of system structure does it let vary independently?

Implementation What pitfalls, hints or techniques should you be aware of when implementing the pattern?

Are there language-specific issues?

Sample Code

Code fragments that illustrate how you might implement the pattern.

Known UsesExamples of the pattern found in real systems.

At least two examples from different domains.

Related Patterns What design patterns are closely related to this one?

What are the important differences?

With other patterns should this one be used?

Catalog of Design PatternsCatalog of Design Patterns

Abstract Factory

Provide an interface for creating families of related/dependent objects without specifying their

concrete classes.

Adapter

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

Bridge

Decouple an abstraction from its implementation so that the two can vary independently.

Builder

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

Chain of ResponsibilityAvoid 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 request as an object, thereby letting you parameterize clients with different requests, queue or log requests, and support undoable operations.

Composite

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

Decorator

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

Façade

Provide a unified interface to a set of interfaces in a subsystem. Facades define a higher-level interface that makes the subsystem easier to use.

Factory Method

Define an interface for creating an object, but let subclasses decide which class instantiate. Factory Method lets a class defer instantiation to subclasses.

Flyweight

Use sharing to support large number of fine-grained objects efficiently.

Interpreter

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

Iterator

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

MediatorDefine an object that encapsulates how a set of objects interact. Mediators promote loose coupling by keeping objects from referring to each other explicitly, and it lets you vary their interaction independently.

Memento

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

Observer

Define a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically.

Prototype

Specify the kind of objects to create using a prototypical instance, and create new objects by copying this prototype.

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

Singleton

Ensure a class has only one instance, and provide a global point to access to it.

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

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

Template MethodDefine the skeleton of an algorithm in an operation, deferring some steps to subclasses. Template Method lets subclasses redefine certain steps of an algoritm without changing the algorithm's structure.

VisitorRepresent 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.

1. Purpose:

Creational

Structural

Behavioral

2. Scope:

Class (static)

Object (dynamic)

Design Patterns SpaceDesign Patterns SpacePurpose

Creational Structural Behavioral

Scope Class Factory Method Adapter (class) Interpreter

Template Method

Object Abstract Factory

Builder

Prototype

Singleton

Adapter (obj)

Bridge

Composite

Decorator

Façade

Flyweight

Chain of Responsibility

Command

Iterator

Mediator

Memento

Observer

Strategy

Visitor

Solving ProblemsSolving Problems

Finding appropiate objects

Determining object granularity

Specifying object interfaces

Specifying object implementations

Class vs. Interface inheritance

ClassName

operation1()Type operation2()...

instanceVariable1Type instanceVariable2...

AbstractClass

operation1()Type operation2()...

ParentClass

operation()...

Instantiator

SubClass

Instantiatee

AbstractClass

operation()

ConcreteSubClass

operation()implementationpseudocode

Programming to an Interface,not an Implementation

Inheritance vs. Composition

Delegation

Inheritance vs. Parameterized Types

There are two benefits to manipulating objects solely in terms of the interface defined by abstract classes:

1. Clients remain unaware of the specific types of objects they use, as long as the objects adhere to the interface the clients expect.

2. Clients remain unaware of the specific classes that implement these objects. Clients only know about the abstract class(es) defining the interface.

This so greatly reduces implementation dependencies between subsystems that it leads to the following principle of Object Oriented Design:

Program to an interface, not an implementation

Inheritance vs. Composition

There are two common techniques for reusing functionality in object oriented systems:

1. class inheritance

2. object composition

Class Inheritance:

Lets you define the implementation of one class in terms of another’s.

Reuse by subclassing is often referred to as white-box reuse. The term white-box refers to visibility: with inheritance, the internals of parent classes are often visible to subclasses.

Object Composition:

Here, new functionality is obtained by assembling or composing objects to get more complex functionality.

This style of reuse is called black-box reuse, because no internal details of objects are visible.

Inheritance:

Straightforward to use, since it’s supported directly by the programming language

Makes it easier to modify the implementation being used (method overriding)

Inheritance:

You can’t change the implementations inherited from parent classes at run-time, because inheritance is defined at compile-time

Because inheritance exposes a subclass to details of it’s implementation, it’s often said that “inheritance breaks encapsulation”

Object Composition:

Object composition is defined dynamically at run-time through objects acquiring references to other objects (Abstract Factory, Builder, …)

No encapsulation broken (interfaces)

Any object can be replaced at run-time as long as it has the same type

Object Composition:

Favoring object composition over class inheritance helps you keep each class encapsulated and focused on one task.

Your classes and class hierarchies will remain small

A design based on object composition will have more objects

Second principle of Object Oriented Design:

Favor object composition over class

inheritance

Delegation

Delegation is a way of making composition as powerful for reuse as inheritance.

The receiving object delegates operations to its delegate.

This is similar to subclasses deferring operations to parent classes

The receiver passes itself (this) to the delegate, to let the delegated operation refer to the receiver

Window

area()

Rectangle

area()

widthheight

return width*height

return rect->area()

Delegation has a disadvantage it shares with other techniques that make software more flexible through object composition:

dynamic, highly parameterized software is harder to understand than more static software.

Several design patterns use delegation:

Strategy

Visitor

Mediator

Chain of Responsibility

Bridge

Inheritance vs. Paremeterized Types

Another technique for reusing functionality in object oriented systems is trough parameterized types:

This technique lets you define a type without specifying all the other types it uses

The unspecified type is supplied at the point of use

Parameterized types give a third way to compose behavior in object oriented systems:

To parameterize a sorting routine by the operation it uses to compare elements, we could make the comparison

1. An operation implemented by subclasses (Template Method)

2. The responsibility of an object that’s passed to the sorting routine (Strategy)

3. An argument of a C++ template or Java parameterized type

Parameterized Type Example

List<Integer> integerList = new LinkedList<Integer>()

Map<String, Integer> integerMap = new HashMap<String, Integer>()

Generic Class Example

public class C<T1, T2> {

private T1 type1;

private T2 type2;

public C( T1 type1, T2 type2 ) {

this.type1 = type1;

this.type2 = type2;

public T1 getType1() {

return this.type1;

public T2 getType2() {

return this.type2;

public static void main(String args[]) {

C<String, Integer> cStrInt = new C<String, Integer>("one", 1);

C<Integer, Boolean> cIntBool = new C<Integer, Boolean>(1, true);

Generic Interface Example

public interface I<T> {

public T getConnectionPool();

public void releaseConnectionPool(

T connPool );

Generic Method Examplepublic class M {

public static <T extends Comparable> T

minimum( T a, T b )

{ if (a.compareTo( b ) <= 0)

return a;else

return b; }

Designing for ChangeDesigning for Change

The key to maximizing reuse lies in anticipating new requirements and changes, and in designing your systems so that they can evolve accordingly.

Common causes of redesign:

1.Creating an object by specifying a class explicitly

(Abstract Factory)

(Factory Method)

(Prototype)

2.Dependence on specific operations

(Chain of Responsibility)

(Command)

3.Dependence on hardware and software platform

(Abstract Factory)

(Bridge)

4.Dependence on object implementations or representations

(Abstract Factory) (Bridge)

(Memento) (Proxy)

5.Algorithmic dependences

(Builder) (Iterator)

(Strategy) (Template Method)

(Visitor)

6.Tight coupling

(Abstract Factory) (Bridge)

(Chain of Responsibilty) (Command)

(Facade) (Mediator) (Observer)

7.Extending functionality by subclassing

(Bridge) (Chain of Responsibilty)

(Composite) (Decorator)

(Observer) (Strategy)

8. Inability to alter classes conveniently

(Adapter)

(Decorator)

(Visitor)

ToolkitsToolkits

A set of related and reusable classes designed to provide useful, general purpose functionality.

(C++ iostream, STL)(java.lang/java.util)

ToolkitsToolkits

Toolkits emphasize code reuse

The OO equivalent of subroutine libraries

FrameworksFrameworks

A set of cooperating classes that make up a reusable design for a specific class of software.

(C++ MFC)(J2EE)

FrameworksFrameworks

The framework dictates the architecture of the application

Defines the overall structure, its partitioning into classes and objects, the key responsibilities, and the collaboration patterns

Design Difficulty LevelsDesign Difficulty Levels

1.Applications

2.Toolkits (harder)

3.Frameworks (hardest)

Selecting Design PatternsSelecting Design Patterns

Consider how design patterns solve design problems

Scan intent sections

Study how patterns interrelate

Study patterns of like purpose

Selecting Design PatternsSelecting Design Patterns

Examine a cause of redesign

Consider what should be variable in the design

Using Design PatternsUsing Design Patterns

Read the pattern once through

Pay particular attention to the Applicability and Consequences sections to ensure the pattern is right for the problem.

Study the Structure, Participants and Collaboration sectionsMake sure you understand the classes and objects in the pattern and how they relate to one another.

Look at the Sample Code section to see a concrete exampleStudying the code helps you learn how to implement the pattern.

Choose Names for pattern participants that are meaningful in the application contextThe names for particpants in design patterns are usually too abstract to appear in an application.

Define the Classes

Declare their interfaces, establish their inheritance relationships, and define the instance variables that represent data and object references.

Define application specific names for operations in the patternUse the responsibilities and collaborations associated with each operation as a guide.

Implement the operations to carry out the responsibilities and collaborations in the patternThe examples in the sample code section can help.

Basic Design Patterns 1. Introduction Gabriel Mañana Ed. 453 Of. 120 Ext. 14080 gjmananag...

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