Advanced Programming in Java

Presented by: Mojtaba Khezrian

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Different ContextsProblem Space

the place where the problem existssuch as a business

Solution Spacethe place where you’re implementing that

solutionsuch as a computer

The effort required to perform this mapping

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Library ProblemSuppose you want to write a library programWhat are the elements of your program?We think about functions and variables…

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Object Oriented ApproachOO approach goes a step further Lets the programmer represent problem

space elementsThe programmer 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”

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OOP

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

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OOP (2)

OOP allows you to describe the problem in terms of the problem

Rather than in terms of the computer Objects in your code are similar to real

objects

Recall the sample programs: phonebook and library

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OOP vs. Procedural ApproachElements of procedural programming

Functions VariablesFunction invocation

Elements of OOPObjectsMessage passing between objects

The way of thinkingThinking about functions and computer structureThinking about objects and relations

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OOP Characteristics

1. Everything is an object 2. A program is a bunch of objects telling each

other what to do by sending messages

3. Each object has its own memory made up of other objects

4. Every object has a type 5. All objects of a particular type can receive the

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Everything is an object You can take any conceptual component in the

problem dogs, buildings, books, people, …

And represent it as an object in your program. Example

Person p; Book book;

Think of an object as a variableIt stores dataBut you can make requests to that object

asking it to perform operations on itself. Fall 2014

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Object MessagesTo make a request of an object, you send a

message to that object. Message = invoking a method of an object. Example

Book b;….if(b.isReserved())…

Person p;….p.setPhoneNumber(66166601)

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Each object has its own memory

You can create a new kind of object by making a package containing existing objects

Thus, you can build complexity into a program while hiding it behind the simplicity of objects

Book{String name;Person reservedTo;

}

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Every object has a typeEach object is an instance of a classclass is synonymous with type The most important distinguishing

characteristic of a class is What messages can you send to it?

Person p;Person q;Person[] people;

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SubstitutabilityAll objects of a particular type can receive the same

messagesAn object of type circle is also an object of type

shapeA circle is guaranteed to accept shape messages You can write code that talks to shapes and

automatically handle anything that fits the description of a shape

This substitutability is one of the powerful concepts in OOP. InheritancePolymorphism

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Description of an ObjectAn object has state and behaviorAn object (may) have internal data

which gives it stateAn object (may) have methods

to produce behaviorAnd each object can be uniquely

distinguished from every other objectEach object has a unique address in memory

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Abstract Data TypesEach programming language has some

predefined data typesint, double, char, …

Creating abstract data types is a fundamental concept in object-oriented programming

Abstract Data Type = ClassProgrammer defines a class to fit a problem You extend the programming language by

adding new data types specific to your needs

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MessagesE.g.

Assign a book to a personSet phone number of a personCalculate area of a shapecomplete a transactiondraw something on the screenturn on a switch

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InterfaceEach object can satisfy only certain requestsThe requests you can make of an object are

defined by its interfaceThe type is what determines the interface

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Representation of a light bulb:

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UML Diagram

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Person in an Education System

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New names in OOP

Function Method, ServiceVariable Property, State

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EncapsulationCommercial products are encapsulated

Remote controlTVCell phone

They are Black BoxesHidden ImplementationsPublic interface

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Why Encapsulation?Simplified use

Even for the producerOpen implementation bad useHiding the implementation reduces bugsIt is more beautiful!

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Object EncapsulationEncapsulation of a problem-space concept

into a class of objectsDefine interfaceHide the implementationBlack boxThe client may see the implementationBut can not use it directlyThis is better even for the producer

(programmer)

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Access ControlAccess to some parts of the class is restrictedPublic and Private area of the classThe client of a class can use only public areaPublic area = class interface

Public methodsPublic variables

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Example: RectangleLets encapsulate a rectangleWhat is a rectangle?

An objectWhich has length and width (properties)Lets you specify its length and widthCan calculate its area and perimeter

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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;}

}Fall 2014

Private area: hidden implementation

Public area :the interface

Class Declaration

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How to Use Rectangle?

Rectangle rect = new Rectangle();rect.setWidth(2);rect.setLength(7);System.out.println(rect.calculateArea());System.out.println(rect.calculatePerimeter());

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Object creation or instantiation

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More ExercisesEncapsulate these concepts:

StudentFootbal-PlayerDog

name, age : properties bite, bark : messages (actions)

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ConclusionOOP brings us a new abstractionOOP allows you to describe the problem in

terms of the problemThink in terms of the structure of the

problemRather than the structure of the computer

Object has state, behavior and identityObject has data and interface

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Further ReadingGoogle these queries and read some pages

(Wikipedia is preferred)Object Oriented ProgrammingHistory of Object Oriented ProgrammingSmalltalkUMLEncapsulationInheritancePolymorphism

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