IEG3080 Software Engineering

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IEG3080 Software Engineering

• Instructor Prof. Michael Chang
• About this course
• Object-oriented programming
• C
• Design Patterns
• Software Methodologies
• Reference text
• Design patterns by Gamma et al (Addison Wesley)
• The Bible in OO, must read it if you are
  interested in OO. Examples written in C, but
  not hard to follow
• www.wikipedia.org

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• 4 Assignment 10
• 1 Project 30
• Final Exam (open notes, one A4 paper) 60
• Homepage http//course.ie.cuhk.edu.hk/ieg3080
• How to download Visual Studio .NET
• Get the form from
• www.ie.cuhk.edu.hk/fileadmin/download/student/docu
  ment/msdnaa.pdf
• Ignore the CD part, dont need my signature, sign
  it and return it to tutors

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• No plagiarism
• Policy of zero tolerance
• Penalties include failing the course and
  receiving demerits.
• http//www.cuhk.edu.hk/policy/academichonesty/
• Please sign the No plagiarism declaration form
  and attach it together with your
  assignment/project
• The form can be downloaded from section 9 in
  above site

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• Interface
• Implementation

5
What is software engineering?

• How to build
• large-scale software
• by a large team of people
• quickly
• and
• inexpensively?

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Problems

• Software is expensive
• Delivery is always late
• Software is unreliable
• Software is difficult to maintain
• the code is written by other people
• Software is difficult to adapt to future change
• Difficult to manage a large team

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Problems

• Technical problems
• To build complex software reliably, quickly,
  cheaply, easy to maintain, and can be changed
  flexibly in future
• OO, design patterns, etc
• Management problems
• How to manage software project
• Software methodology

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Management solution

• Effective communication
• How to manage a team
• People has different skills
• Turnover
• Communicate in a large team
• Quality of the software
• How to manage the customers
• How to collect the requirements
• Are we building the system that the customers
  want?

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Management solution

• Software methodologies
• Best practices in software development
• Traditional methods
• Rational Unified Process
• Extreme Programming (XP)
• CASE tools
• UML - schematic diagram for software
• NUnit for regression testing of software

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Technical solution

• How to build complex software reliably, quickly,
  cheaply, easy to maintain, and can be changed
  flexibly in future
• Solution
• Reusable software
• Key to reusable software
• Many solutions, but the most important concept is
  the interface
• In particular, the separation of interface with
  the implementation

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Levels of reuse

• Source code reuse
• But vendors dont give the source code away
• Binary code reuse
• But how to modify existing binary code?
• Interface reuse
• Key to flexible software
• Design reuse
• Design patterns, solutions to common programming
  problems

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Software is complex

• How to deal with complex system?
• Divide and conquer
• Divide a complex system into sub-systems, and a
  sub-system into objects
• Each component can be developed independently
  from each other
• How to divide a system into parts
• Key issue - the interface between parts

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????
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To divide a system into parts

• A decomposed cow

Body
Head
Read Leg
Front Leg
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Is software easy to change?

• Good software will be used for many years
• Must be able to extend/modify the software in
  order to cater for future needs
• Is software easy to change?
• Look easy, but in fact difficult !!
• Why?
• Too many dependencies between different modules
• ???????

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Solution?

• Reduce dependency between parts
• Can change a component freely without affecting
  the rest of the system
• Interface is the key to flexible and extensible
  software
• Related concepts
• Loose coupling
• Modularity
• Encapsulation
• Information hiding

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Interface so that parts can be changed
independently

• USB interface

PC
Camera
USB Interface
Disk
Phone
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To connect to a new camera
New Camera
PC
USB Interface
Disk
Phone
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Interface and Implementation

• Interface is a specification, describing how
  components should be joint
• Implementation is the real stuff (the code)
• Interface is MORE IMPORTANT than implementation
• Why?
• Interface, once fixed, is hard to change
• e.g. USB
• Implementation can be easily changed
• e.g. mobile phone, camera, printer

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What is an object?

• Object is a software component that you can reuse
  easily
• Each object has its own localized functions and
  data
• Object function data
• Why do we want localized functions and data?
• So that we can change a component easily

New Camera (data)
PC (data)
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Separation of interface and implementation

• Interface and implementation are two different
  things

New Camera (data)
PC (data)
Implementation
Interface (USB)
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What is a USB interface?

• Just a specification on paper
• Abstract concept, not concrete product
• Interface, not implementation

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What is the use of header file in C?
Interface
implementation
.h file int foo(int x)
.c file int foo(int x) . . .
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The reason of using header file

• Separation of interface and implementation

.h file
Rest of the system Can only see the header file,
so that the implementation can be changed freely
.c file
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The advantage of the separation

• .c file (the code) can be changed without
  affecting the rest of the system
• .h file (the header) cannot be changed!!

.h file
Rest of the system Can only see the header file,
so that the code can be changed freely
New .c file
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The essential idea of OO

• Base class provides the interface (function
  declaration)
• The subclass provides the function implementation
• The subclass inherits the interface of the base
  class

Interface class
Rest of the system Can only see the base class
(interface), so that the subclass (implementation)
can be changed freely
Implementation
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Inheritance

• Inheritance is a key concept in OOP. What does it
  mean?
• If class B inherits class A, then class B has
  everything in A
• Class interface implementation
• An easy way to reuse other peoples interface and
  implementation
• UML notation

A
B
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Interface inheritance versus implementation
inheritance

• Class A (the base class) may have interface and
  implementation
• Class B (the sub-class) inherits both the
  interface and implementation in A
• Interface inheritance
• Implementation inheritance
• Interface inheritance is much more important than
  implementation inheritance
• The main purpose of the base class is to provide
  an interface, so that the sub-class
  (implementation) can be changed freely in future

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Reusable software

• Inheritance reuse
• By inheritance
• Implementation reuse
• By delegation
• Reuse other peoples code by using other peoples
  object to do the work for you
• Inheritance and delegation are the two key
  concepts in OO

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Common misconception about OO

• Wrong concept
• The main purpose of inheritance is to inherit the
  codes in the base class
• This concept is wrong
• It confuses the concept of interface inheritance
  with implementation inheritance
• Implementation can be reused by delegation rather
  than by inheritance
• Just creates an object and use it (like getting a
  code library and use it)

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Inheritance

• Example
• Interface
• USB specification (just a piece of paper, totally
  abstract)
• Implementation
• MP3 player
• Digital camera
• Mobile phone

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Inheritance and implementation
Rest of the System
USB
Interface
Mobile Phone
Implementation
MP3
Digital Camera
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Interface and implementation

• Interface promotes the following related design
  concepts
• Modularity
• Divide a system into modules
• Encapsulation
• Hide the complexity of module, expose only the
  interface
• Abstraction
• Intellectual simplification, e.g. USB is an
  abstraction of devices (existing or yet to be
  invented)
• Reduce the amount of learning
• Loose coupling
• change in one module wont affect others

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The key to LARGE SCALE DEVELOPMENT

• Different parts are manufactured by different
  companies from different places in the world
• Modularity, encapsulation, abstraction, and loose
  coupling are boiled down to one important
  principle in good engineering design
• The Interface

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Example of good interface

• World-wide web
• Network level interface HTTP protocol
• Human level interface HTML/XML
• Internet
• Interfaces - IP
• Airplane, plumbing, car, . . .
• 2nd language (English) is more important than
  mother tongue for business communication

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The high input impedance / low output impedance
principle in circuit design

• B has high input impedance and low output
  impedance, can change B without affecting the
  rest of the system
• Essential ideas are the same
• Divide and conquer
• With good interface, one can change a part
  without affecting the rest of the system

Box A
Box B
Box C
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The most important principle in (software)
engineering

• Separation
• of
• Interface
• and
• Implementation

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Interface and implementation
Interface versus implementation
Disk drive CDROM Printer Digital
camera Future products . . .
USB bus
Polymorphism many different forms
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• USB is an abstraction of the hardware
• Design the PC system around the USB interface,
  not to a particular device
• The first principle in OOP (in Gammas Design
  Pattern)
• programming to an interface, not an
  implementation

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From C, C to Java/C

• Basic principles are the same
• The separation of interface from implementation
• The differences
• On granularity
• C at sub-system level (coarser scale)
• C, C - at object level (finer scale)
• C is complicated
• C (and Java) is simpler (no pointer)

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Abstract versus concrete

• Interface is abstract, implementation is concrete
• Top designers design the interface, they dont
  code !!
• Implementation is done by programmers working
  independently all over the world
• OO is about an abstract style of programming,
  focus on the interface

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Which one is a more successful design from OO
point of view?
Morning ?
Twins