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OOEDP Week 1 Lecture 1 1April 12

Object Oriented Programming in Java

Object Oriented and Event Driven Programming

Week 1 Lecture 1

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Objectives

In this lecture, we will

Discuss the module organisation

Discover why object-orientation is desirable

Review the basic object oriented concepts Define classes, create and use instances of classes

Examine the structure and syntax of a simple Javaprogram using classes

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Module introduction

this module follows on from the first Semester module,Introduction to Software Development (ISD)

we will build on the Java basics we learnt last semester

this module introduces object-orientation writing classes

creating and using instances of classes

object-oriented concepts

developing graphical user interfaces (GUIs) using classes

from the Java Swing and awt packages (libraries)

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Module organisation

see Blackboard site for details

you must attend 2 lectures and 2 practical sessions perweek

see your timetable for details

most of you will have the same tutor as ISD Semester 1

you will keep a portfolio of your tutorial exercisesthroughout the course

test based on the portfolio in Week 7

30% of module mark assignment, based on portfolio work, worth 70%

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Introduction to Objects and Classes

If you want to build a wall you find an object, a mancalled Bob, who knows how to build walls

Then you ask Bob to build the wall

You know what you would like done but not how to do it The object Bob is a specific type, or class, of object

He is an instance of a Builder

All instances of Builder know how to build walls

Object Oriented programming adopts the same approach!!

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Objects and Classes in Java

Java is an object-oriented language

all Java code belongs to a class

you have already written many Java classes

in most cases you have not constructed any instances of theclasses you wrote

instead you wrote static methods

public static void main(String[] args)

public static int addNums(int num1, int num2)

the keyword static means the method belongs to the class

as a whole, not to an individual instance (object)

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Objects and Classes in Java

Java provides the class Scanner that knows how to readinput from an input stream

you construct two objects of this class one object called kybd to read from System.in

one object called inFile to read from the file payroll.txt the Scanner class defines methods that can be used to read

the input

next()

nextInt()

nextDouble()

these methods can be called on any Scanner object

inFile.next() reads the next integer in payroll.txt

kybd.nextInt() reads the next integer from the keyboard

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Objects and Classes

This is analogous to the builder situation

Builder is a class that knows how to do something

Build walls

Scanner is a class that knows how to do something get input from an input stream

Bob is an instance of builder

We can ask Bob to build a wall

kybd is an instance ofScanner

We can askkybd to find the next integer typed in on the

keyboard

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Objects and Classes

Using classes makes performing complex tasks simple

In order to use a class you need to know:

Where the class is located

How to build an instance of the class an object What instances of the class can do

How to ask the instance to do something

A class has a set of methods that define the functionalitythat it can provide

For example nextInt is a method of the Scanner class

A method is invoked by sending a message to an object

num1 = kybd.nextInt();

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When is it useful to write a class?

consider the payroll example from the ISD lecture on tables

of data we wanted to store information about a list of employees

where each employee has a name, employee number,department, and salary

Name Employee

number

Department Salary

Pete 437 6 25000

Cathy 525 4 60000

Graham 214 3 40000

Claude 639 4 35000

Dave 309 5 15000

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When is it useful to write a class?

we store this information as

a one-dimensional array of type String, to store the names a two-dimensional array of type int

what are the drawbacks of this approach?

Name Employee

number

Department Salary

Pete 437 6 25000

Cathy 525 4 60000

Graham 214 3 40000

Claude 639 4 35000

Dave 309 5 15000

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The object-oriented approach to thepayroll problem

define a class Employee

an employee has an name, department, employee number andsalary

create objects of the Employee class

one object for each employee on the payroll

store the Employee objects in a 1-D array of type Employee

to access Employee information, loop through the array and callan appropriate method on each Employee object

to find the total salary, call a getSalary() method on each Employeeobject

add to a running total

we will learn how to do this over the next few weeks

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What is Object Oriented Programming?

Object oriented Programming an approach to programDesign and Implementation that focuses upon:

Organisation of data

Access of data

In contrast traditional programming focuses upon theorganisation of Algorithms

The three main characteristics of OOP are: Encapsulation

Inheritance

Polymorphism

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Characteristics of O.O.P.

Encapsulation

combining data structures with the functions that act upon them

hiding implementation

Member Variables

Member Functions (also called Member Methods)

CLASS is the unit of encapsulation

An Object is an instance of a class

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Inheritance

the ability to construct DERIVED classes from existingBASE classes

the derived class inherits the member variables andmember methods of the base class

the derived class can

add additional member variables

add new member methods or override or extend the

functionality of existing base class methods

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Polymorphism

one method can have a different function associated withit for each class in a hierarchy

which function to call can be determined at run timebased upon the class of the object against which themethod is called

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Designing Object Oriented Programs

The general approach for designing and developing

object oriented programs is:

determine the set of classes required

establish the essential instances of the classes specify the exchange of messages between instances

Create CLASSES

Create INSTANCES of classes (OBJECTS)

Specify the MESSAGES to be exchanged by objects

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Classes

ACLASS is a user defined type which encapsulates

the representation of the type

the operations which access or update objects of that type

operations for creating and deleting objects

A class is a structure that can have functions as membersas well as data elements

The functions are called:

member functions

or member methods

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Person

Person

name

age

setName

setAge

getName

getAge

displayDetails

attributes

member variables

member methods

member functions

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Person

setName setAge

getName getAge

displayDetails

name

age

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Classes in Java

public class Person {

String name;

int age;

public void setName (String theName) {name = theName;}

public void setAge (int theAge) {age = theAge;}

public String getName () {return name;}

public int getAge () {return age;}

public void displayDetails ()

{ System.out.println("Name is : " + name );System.out.println("Age is : " + age );

}

}

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Creating an Object

An object is an instance of a class

The tutor is an example of a person

has a name

has an age

In Java an object is constructed by instantiating a class:

construct an object

an instance of the class

Person theTutor = new Person( );

the class

a reference to

the object

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Using the Methods of an Object

The member functions of a class can be called against aninstance of the class:

theTutor.setName (Pete) ;

theTutor.setAge (53) ;

theTutor.displayDetails ( ) ;

This is often described as sending a message to the object

asking the object to do something

the object itself knows how to do what is asked

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The Heap and Garbage Collection

All objects in Java are created on the heap

Space is allocated dynamically using the keyword new

Consider the following code:

Person thePerson; The variable thePerson can hold a reference to an object

of type Person

Initially it is a null reference

An object can be constructed on the heap using new

thePerson = new Person ( );

A constructor is invoked when new is used

Constructs the object

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Testing a Class

One way to test a class is to build a simple driver class

The driver contains a main

In the main method the class to be tested is instantiated

Then each of the methods of the class are invoked

Why can the driver class method main be used withoutinstantiating the driver class?

In the driver main is:

public static void main (String [] args)

Methods that are static are at the class level and can beinvoked without instantiating the class

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Testing the class

public class TestPerson {

public static void main(String []args)

{

System.out.println("Testing Person class");

System.out.println("---------------------------------------");

// instantiating the class

// constructing an object of type Person

Person theTutor = new Person();

// using the set methodstheTutor.setName ("Pete") ;

theTutor.setAge (53) ;

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Testing the class (continued)

System.out.println("Testing displayDetails "+

" name Pete age 53 expected");

theTutor.displayDetails () ;

System.out.println("Testing getName - Pete expected");

System.out.println(theTutor.getName( ));

System.out.println("Testing getAge - 53 expected");

System.out.println(theTutor.getAge( ));

}

}

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Summary

In this lecture we have:

Discussed the module organisation

Introduced why object-orientation is desirable

Reviewed the basic object oriented concepts Defined classes, created and used instances of classes

Examined the structure and syntax of a simple Javaprogram using classes

OOEDP_wk1_L1

Documents

Transcript of OOEDP_wk1_L1