1 Module 3: UML In Action - Design Patterns. 2 Overview Books Design Patterns – Basics Creational Design Patterns Structural Design Patterns

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Module 3: UML In Action - Design

Patterns

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Overview

Books Design Patterns – Basics Creational Design Patterns Structural Design Patterns Behavioral Design Patterns

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Book

Design Patterns : Elements of Reusable Object-Oriented Software (1995) (The-Gang-of-Four Book) The-Gang-of-Four (GoF) - Gamma, Helm, Johnson , Vlissides

Life Challenges

Software Design!!!

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Design Patterns

“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”.

--- Christopher Alexander, 1977

This was in describing patterns in buildings and towns.This was in describing patterns in buildings and towns.In SE, design patterns are in terms of objects and interfaces, not In SE, design patterns are in terms of objects and interfaces, not

walls and doors.walls and doors.

The manner in which a collection of interacting objects collaborate to accomplish a specific task or provide some specific functionality.

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Architecture vs. Design Patterns

High-level framework for structuring an application “client-server based on remote procedure calls” “abstraction layering” “distributed object-oriented system based on CORBA”

Defines the system in terms of computational components & their interactions

Architecture

Design Patterns Lower level than architectures (Sometimes, called micro-architecture) Reusable collaborations that solve subproblems within an application

how can I decouple subsystem X from subsystem Y?

Design patterns support object-oriented reuse at a high level of abstraction Design patterns provide a “framework” that guides and constrains object-oriented implementation

Why Design Patterns?

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4 Essential Elements of Design Patterns

Name: identifies a pattern Problem: describes when to apply the pattern in terms of the

problem and context Solution: describes elements that make up the design, their

relationships, responsibilities, and collaborations Consequences: results and trade-offs of applying the pattern

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Design Patterns in MVC

Model: the application object View: its screen presentation Controller : defines the way the user interface reacts to user

input MVC decouples views and models by establishing a

subscribe/notify protocol between them

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How to Describe Design Patterns more fully

This is critical because the information has to be conveyed to peer developers in order for them to be able to evaluate, select and utilize patterns.

A format for design patterns Pattern Name and Classification Intent Also Known As Motivation Applicability Structure Participants Collaborations Consequences Implementation Sample Code Known Uses Related Patterns

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Organizing Design Patterns

By Purpose (reflects what a pattern does): Creational Patterns Structural Patterns Behavioral Patterns

By Scope: specifies whether the pattern applies primarily to classes or to objects.

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Design Patterns Space

Abstract FactoryBuilderPrototypeSingleton

AdapterBridgeCompositeDecoratorFaçadeFlyweightProxy

Chain of ResponsibilityCommandIteratorMediatorMementoObserverStateStrategyVisitor

Factory Method AdapterInterpreterTemplate

Creational Structural Behavioral

Object

ClassScope

Purpose

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How Design Patterns Solve Design Problems (1)

Finding Appropriate Objects encapsulation, granularity, dependency, flexibility, performance, evolution, reusability, …

Determining Object Granularity Specifying Object Interfaces

Signature Interface Type Dynamic Binding Polymorphism

Specifying Object Implementations

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How Design Patterns Solve Design Problems (2)

Specifying Object Implementations

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Principles of Object-Oriented Design

Programming to an Interface, not an Implementation Design for Reuse

White-box reuse Black-box reuse

Favor object composition over class inheritance Delegation

Inheritance versus Parameterized Types e.g., Templates in C++

Software Design

Big problems in practice Big opportunities in research

Have Fun!!

Why modularization?

Managerial

Changeability

Comprehensibility

What is a “Modularization”?

Making design decisions before the work on independent modules can begin.

Quite different decisions are included for each alternative

In all cases, the intention is to describe all "system level" decisions (i.e. decisions which affect more than one module).

A Canonical Topic

“On the Criteria To Be Used in Decomposing Systems into Modules” David Parnas, 1972

A Canonical Example

Key Word in Context [Parnas72]

“The KWIC index system accepts an ordered set of lines, each line is an ordered set of words, and each word is an ordered set of characters. Any line may be "circularly shifted" by repeatedly removing the first word and appending it at the end of the line. The KWIC index system outputs a listing of all circular shifts of all lines in alphabetical order. “

A Canonical Example

1. Input2. Circular Shift3. Alphabetizing4. Output

Software Architecture

Sense and Sensibility

Crouching Tiger Hidden Dragon

Architecture Software

and Sensibility Sense

Sensibility Sense and

Tiger Hidden Dragon Crouching

Hidden Dragon Crouching Tiger

Dragon Crouching Tiger Hidden

Key Word in Context [Parnas72]

Making your First Decision

How should such a system be designed? Modularized?

What are the factors to consider?

Thinking in the perspective of

Software Design!

What does it mean?

1. Elements

2. Relations

Describe the Design:

1. Plain English

2. Elements and Relations

Main Program/Subroutin

e

Module1: Input“This module reads the data lines from the input

medium and stores them in core for processing by the remaining modules. The characters are packed four to a word, and an otherwise unused character is used to indicate the end of a word. An index is kept to show the starting address of each line. “

Build KWIC Architecture

Input

Characters Line Index

Input Medium System I/O

Data Repr in Memory

Direct Memory Access

Module 2: Circular Shift“This module is called after the input module has

completed its work. It prepares an index which gives the address of the first character of each circular shift, and the original index of the line in the array made up by module 1. It leaves its output in core with words in pairs (original line number, starting address) “


Input


Input Medium

Shifts Index

Circular Shifter

Module 3: Alphabetizing“This module takes as input the arrays

produced by modules 1 and 2. It produces an array in the same format as that produced by module 2. In this case, however, the circular shifts are listed in another order (alphabetically). “


Input


Input Medium

Shifts Index

Circular Shifter Alphabetizer

Sorted Shifts Index

Module 4: Output“Using the arrays produced by module 3 and

module 1, this module produces a nicely formatted output listing all of the circular shifts. In a sophisticated system the actual start of each line will be marked, pointers to further information may be inserted, and the start of the circular shift may actually not be the first word in the line, etc. “


Input


Input Medium

Shifts Index

Circular Shifter Alphabetizer

Sorted Shifts Index

Output

Output Medium

Module 5: Master Control

“This module does little more than control the sequencing among the other four modules. It may also handle error messages, space allocation, etc . “


Is this a good design? What if:

Memory Size change? Input Size change? Input Format change? Alphabetizing Policy change?

Consider the Following Changes:

1. Input Format

Module 1

2. The decision to have all lines stored in core. For large jobs it may prove inconvenient or impractical to keep all of the lines in core at any one time.

All Modules!!!

3. The decision to pack the characters four to a word. In cases where we are working with small amounts of data it may prove undesirable to pack the characters; time will be saved by a character per word layout. In other cases we may pack, but in different formats.

All Modules!!!

4. The decision to make an index for the circular shifts rather than actually store them as such. Again, for a small index or a large core, writing them out may be the preferable approach.

Module 2, 3

5. The decision to alphabetize the list once, rather than either (a) search for each item when needed, or (b) partially alphabetize as is done in Hoare's FIND [2]. In a number of circumstances it would be advantageous to distribute the computation involved in alphabetization over the time required to produce the index. .

Module 3

In Summary

A Canonical Example Describe its Architecture Evaluate its Changeability

Data Abstraction

Elements Objects Interfaces I/O Medium

Relations Method Invocation System I/O

Parnas’s Modularization 2

Module 1: Line Storage Module 2: Input Module 3: Circular Shift Module 4: Alphabetizer Module 5: Output Module 6: Master Control

Our 3-Step Exercise

1. Read module description2. Identify architecture elements and

relations3. Analyze changeability

Module 1: Line Storage

“This module consists of a number of functions or subroutines which provide the means by which the user of the module may call on it. The function call CHAR(r,w,c) will have as value an integer representing the cth character in the rth line, wth word. A call such as SETCHAR(rpv,c,d) will cause the cth character in the wth word of the rth line to be the character represented by d (i.e. CHAR(r,w,c) = d). WORDS(r) returns as value the number of words in line r. …“

Build OO Architecture

1.LineStorage

addW

ord

addLin

e

getW

ord

getLin

e

Public Interface

Modules (Objects)

Module 1: Line Storage

Module 2: Input

“This module reads the original lines from the input media and calls the line storage module to have them stored internally. “


1.LineStorage

addLin

e

addW

ord

getLin

e

getW

ord

2. Input

Input Medium

System I/O

Public Interface

Method Invocation

I/O Medium

Modules (Objects)

Module 2: Input

Module 3: Circular Shifter

“The principal functions provided by this module are analogs of functions provided in module 1. The module creates the impression that we have created a line holder containing not all of the lines but all of the circular shifts of the lines. Thus the function call CSCHAR(I,w,c) provides the value representing the cth character in the wth word of the Ith circular shift. It is specified that (1) if i < j then the shifts of line i precede the shifts of line j, and (2) for each line the first shift is the original line, the second shift is obtained by making a one-word rotation to the first shift, etc. A function CSSETUP is provided which must be called before the other functions have their specified values. “

1. LineStorage

addLin

e

addW

ord

getLin

e

getW

ord

2. Input

Input Medium

3. Circular Shifter

setu

p

getLin

e


Module 3: Circular Shifter

Module 4: Alphabetizer

“This module consists principally of two functions. One, ALPH, must be called before the other will have a defined value. The second, ITH, will serve as an index. ITH(i) will give the index of the circular shift which comes ith in the alphabetical ordering. “


1. LineStorage

addLin

e

addW

ord

getLin

e

getW

ord

2. Input

Input Medium

3. Circular Shifter

setu

p

getLin

e

4. Alphabetizer

Alp

ha

getLin

e

Module 4: Alphabetizer

Module 5: Output

“This module will give the desired printing of set of lines or circular shifts. “


1. LineStorage

addLin

e

addW

ord

getLin

e

getW

ord

2. Input

Input Medium

3. Circular Shifter

setu

p

getLin

e

4.Alphabetizer

Alp

ha

getLin

e

5. Output

Output Medium

Module 5: Output


“Similar in function to the modularization above “ (The first modularization)



Is this a good design? What if:

Memory Size change? Input Size change? Input Format change? Alphabetizing Policy change?

Consider the Following Changes:

1. Input Format change

Input module

2. The decision to have all lines stored in core. For large jobs it may prove inconvenient or impractical to keep all of the lines in core at any one time.

LineStorage Module

3. The decision to pack the characters four to a word. In cases where we are working with small amounts of data it may prove undesirable to pack the characters; time will be saved by a character per word layout. In other cases we may pack, but in different formats.

LineStorage Module

4. The decision to make an index for the circular shifts rather that actually store them as such. Again, for a small index or a large core, writing them out may be the preferable approach.

Circular Shift Module

Changeability Analysis Summary

Changes Affected Modules

1. Input Format Input 2. Not to store all lines in core

Line Storage

3. Not to pack characters

Line Storage

4. Store full shifted lines Circular Shift

5. Different Alphabetizing

Alphabetizer

Changeability Analysis Summary

A much better choice than the main program/subroutine style

under these changes!!

High-level Design Detailed Design

Architecture UML

OO Architecture and UML

Class Diagram: System Statics Sequence Diagram: System Dynamics

OO Architecture UML Class Diagram

Class diagram Describes how modules from the original

architectural solution map onto classes. Depicts all classes from the system we are

implementing. For each class it depicts its public interface, i.e.,

public methods that we may invoke to manipulate instances of that class.

For each class it depicts its instance variables. Shows how classes relate to each other. Class

relations are represented with so-called associations, their type and attributes.


Each module one class. Public interfacethe

bottom sub-box, as a list of method declarations.


Class Relations A directed

association: the originating class has as an instance variable an object of the associated class. For example, the Alphabetizer class has an association with the CircularShift class.

UML Sequence Diagram: System Dynamics

Describe how group of objects collaborate to execute a certain task in a running program.

Describes the time sequence of method invocations on the run-time: Objects are shown as boxes at the top of the

diagram. Time is represented with the so-called object's

lifeline. Method invocation is symbolized through a

horizontal line connecting two vertical object's lifelines.

Conditions for method invocation and iterative method invocations may be symbolized with special signs.

UML Sequence Diagram: System Dynamics

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Design for Change

Creating an object by specifying a class explicitly Dependence on specific operations Dependence on hardware and software platform Dependence on object representations or

implementations Algorithmic dependencies Tight coupling Extending functionality by subclassing Inability to alter classes conveniently

Documents

1 Module 3: UML In Action - Design Patterns. 2 Overview Books Design Patterns – Basics Creational Design Patterns Structural Design Patterns