Object Oriented Design

Advanced Programming

SUT • Spring 2019

Outline

  • Object Oriented Programming

  • Characteristics of objects

  • Interface

  • Encapsulation

Abstraction

Abstraction

  • Machine language

  • Assembly: an abstraction of the machine language

  • Many languages are abstraction of assembly language

    • Fortran, Basic, C

  • Big improvement

  • But they still require you to think in terms of the structure of the computer

  • Rather than the structure of the problem

Different Contexts

  • Problem Space

    • the place where the problem exists

    • such as a business

  • Solution Space

    • the place where you’re implementing that solution such as a computer

  • The effort required to perform this mapping

Library Problem

  • Suppose you want to write a library program

  • What are the elements of your program?

  • We think about functions and variables…

Object Oriented Approach

  • A step further

  • Represent problem space elements

  • This representation is general enough

  • Is not constrained to any particular type of problem.

  • The elements in the problem space and their representations in the solution space are referred to as “objects”

Object Oriented Principle

  • The program is allowed to adapt itself to the lingo of the problem

  • by adding new types of objects

  • when you read the code, you’re reading words that also express the problem.

  • This is a more flexible and powerful language abstraction

Object Oriented Languages

  • Smalltalk

    • The first successful object-oriented language

    • One of the languages upon which Java is based

  • Java

  • C++

  • C##

OOP vs. Procedural Approach

  • Elements of OOP

    • Objects

    • Message passing between objects

  • Elements of procedural programming

    • Functions

    • Variables

    • Function invocation

  • The way of thinking

    • Thinking about objects and relations

    • Thinking about functions and computer structure

OOP Characteristics

  • basic characteristics of Smalltalk:

    • Everything is an object

    • A program is a bunch of objects telling each other what to do

      • by sending messages

    • Each object has its own memory

      • made up of other objects

    • Every object has a type

    • All objects of a particular type can receive the same messages

Booch’s description of an Object

  • An object has state, behavior and identity

  • Booch added identity to the description

    • An object (may) have internal data

  • which gives it state

  • An object (may) have methods

    • to produce behavior

  • And each object can be uniquely distinguished from every other object

    • Each object has a unique address in memory

Interface

  • Each object can satisfy only certain requests

  • The requests you can make of an object are defined by its interface

  • The type is what determines the interface

Representation of a light bulb

public class Main {
    public static void main(String[] args) {
        // Turn On the Light
        Light lt = new Light();
        lt.on();
    }
}

Person in an Education System

public class Main {
    public static void main(String[] args) {
        // Person in Education System
        Person person = new Person();
        person.setName("Taghi Taghavi");
        person.setPhoneNumber(66166601L);
        person.showInformation();
    }
}

New names in OOP

  • Function: Method, Service

  • Variable: Property, State

Encapsulation

  • Commercial products are encapsulated

    • Remote control

    • TV

    • Cell phone

  • They are Black Boxes

  • Hidden Implementations

  • Public interface

Why Encapsulation?

  • Simplified use

  • Even for the producer

  • Open implementation: bad use

  • Hiding the implementation reduces bugs

  • It is more beautiful!

Object Encapsulation

  • Encapsulation of a problem-space concept into a class of objects

  • Define interface

  • Hide the implementation

  • Black box

  • The client may see the implementation

  • But can not use it directly

  • This is better even for the producer (programmer)

Access Control

  • Access to some parts of the class is restricted

  • Public and Private area of the class

  • The client of a class can use only public area

  • Public area = class interface

  • Public methods

  • Public variables

Example: Rectangle

  • Lets encapsulate a rectangle

  • What is a rectangle?

    • An object

    • Which has length and width (properties)

    • Lets you specify its length and width

    • Can calculate its area and perimeter

Class Declaration

public class Rectangle {
    private int width, length;

    public void setWidth(int w) {
        width = w;
    }

    public void setLength(int l) {
        length = l;
    }

    public int calculateArea() {
        return width * length;
    }

    public int calculatePerimeter() {
        return (width + length) * 2;
    }
}

How to Use Rectangle?

public class Main {
    public static void main(String[] args) {
        // Rectangle Usage
        Rectangle rect = new Rectangle();
        rect.setWidth(2);
        rect.setLength(7);
        System.out.println(rect.calculateArea());
        System.out.println(rect.calculatePerimeter());
    }
}

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