The Principles of Abstraction Encapsulation Information-hiding and Modularity

1Department of Information Engineering

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


• 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/document/msdnaa.pdf

– Ignore the CD part, don’t need my signature, sign it and return it to tutors


•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


Interface&

Implementation


What is software engineering?

How to build large-scale software

by a large team of people quickly

and inexpensively?


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


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


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?


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


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


Levels of reuse

• Source code reuse – But vendors don’t 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


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


疱丁解牛


To divide a system into parts

• A decomposed cow

Head

Body

FrontLeg

ReadLeg


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 – 牽一發而動全身


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


Interface – so that parts can be changed independently• USB interface

Camera

PC

Phone Disk

USB Interface


To connect to a new camera

New Camera

PC

Phone Disk

USB Interface


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


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)


Separation of interface and implementation

• Interface and implementation are two different things

New Camera(data)

PC

(data)

Interface (USB)Implementation


What is a USB interface?

• Just a specification on paper

• Abstract concept, not concrete product

• Interface, not implementation


What is the use of header file in C?

Interface implementation

.h file

int foo(int x);

.c file

int foo(int x) { . . .}


The reason of using header file

• Separation of interface and implementation

Rest of the system

Can only see the header file, so that the implementationcan be changed freely

.h file

.c file


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!!

Rest of the system

Can only see the header file, so that the code can be changed freely

.h file

New .c file


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 classRest of the system

Can only see the base class (interface), so that the subclass(implementation) can be changed freely

Interface class

Implementation


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 people’s interface

and implementation

• UML notation

A

B


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


Reusable software

• Inheritance reuse– By inheritance

• Implementation reuse– By delegation

•Reuse other people’s code by using other people’s object to do the work for you

• Inheritance and delegation are the two key concepts in OO


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)


Inheritance

• Example– Interface

•USB specification (just a piece of paper, totally abstract)

– Implementation•MP3 player•Digital camera•Mobile phone


Inheritance and implementation

USB

MP3 DigitalCamera

MobilePhone

Rest of theSystem

Interface

Implementation


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 won’t affect others


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


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


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


The most important principle in (software) engineering

Separation of

Interface and

Implementation


Interface and implementation

Disk drive

CDROM

Printer

Digital camera

Future products . . .

USB bus

Interface versus implementation

Polymorphism – many different forms


• 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 Gamma’s Design Pattern)

programming to an interface, not an implementation


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)


Abstract versus concrete

• Interface is abstract, implementation is concrete

• Top designers design the interface, they don’t code !!– Implementation is done by programmers

working independently all over the world

• OO is about an abstract style of programming, focus on the interface


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

Twins Morning 娘

The Principles of Abstraction Encapsulation Information-hiding and Modularity

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