Classes and Objects

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Classes and Objects

• B.Ramamurthy

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Topics for Discussion

• Problem to Classes
• ScoreBoard class
• Designing and implementing classes
• Methods
• Data
• Using classes in an Application

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Problem Statement to Classes

• Underline the nouns in a problem statement.
• Using the problem context and general knowledge
  about the problem domain decide on the important
  nouns.
• Design and implement classes to represent the
  nouns.

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Examples

• Drawing package Design a user interface for
  drawing various shapes circle, square,
  rectangle.
• Football scores Keep track of football score.
• General purpose counter To keep of track of
  count for various applications.
• Library Books, different categories of books,
  details of student borrower, library personnel.

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Designing Classes

• A class represents a class of objects.
• A class contains the data declarations (parts)
  and methods (behaviors or capabilities ).
• OO Design
• Data Declarations are answers to What is it made
  of? (It has a ____, ____, etc.)
• Methods are answers to What can it do? (verbs
  in the problem)

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ScoreBoard class

• class scoreBoard
• //1. data
• int Score
• String Name // team name
• //2. Constructors to create objects
• ScoreBoard() //default constructor
• ScoreBoard(String teamName) // initializing
  constructor

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ScoreBoard class (contd.)

• //3. Interface Methods to perform operation
• void touchDown()
• void extraPoint()
• void twoPointConv()
• void fieldGoal()
• void safety()
• //4. Utility methods to get and set
• void setName(string teamName)
• int getScore()

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ScoreBoard class (contd.)

• //5. Predicate methods return true/false
• boolean shutDown()
• Implement the contents of the methods.

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Method Definition and Call
Method
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Method Definition Header
definition
Header
body
modifiers
parameter list
return type
method name
statements
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Parameters and Return Value
P2
P1
P3
aMethod
Assume P1, P2 and P3 are integers and aMethod
returns P3 which is the sum of P1 and P2. Then
definition of aMethod is int aMethod(int P1,
int P2) int P3 P3 P1 P2 return P3
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Items Accessible to a Method

• All the local data
• All the parameters
• All the data declared in the enclosing class
• All the methods in the enclosing class

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Using Classes

• An application creates objects it requires by
  instantiating them from classes.
• The statements of the application use the
  accessing the public data and by invoking the
  public methods of the objects.
• Example Application Simulate Super Bowl (Jan
  1999)

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Application

• class superBowl
• //main method
• //inside main method
• //1. create two object reference for the two
  teams
• scoreBoard Denver, Atlanta

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Application (contd.)

• // 2. instantiate objects two ways
• Denver new scoreBoard(Broncos)
• Atlanta new ScoreBoard()
• Atlanta.setName(Falcons)
• //3. Use them dot notation
• Devnver.touchDown()
• Denver.extraPoint()
• Denver.display()
• displayWinner( Denver, Atlanta)// is a method of
  the application

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Application (contd.)

• void displayWinner(scoreBoard team1, scoreBoard
  team2)
• System.out.println(The winner is)
• if (team1.getScore() team2.getScore())
• team1.display()
• else
• team2.display()

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Creating the Executable

• Enter ScoreBoard class in a file ScoreBoard.java
• javac ScoreBoard.java
• Enter SuperBowl application in a file
  SuperBowl.java
• javac SuperBowl.java
• Execute application by
• java SuperBowl

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Summary

• We discussed complete development cycle using
  java classes.
• Discussed (instance) methods and implementation
  of methods.
• Using classes for instantiating objects, and dot
  notation for accessing items in a class were
  illustrated.