Using ClassesObjects

1
Using Classes/Objects

• Chapter 3 part 2
• Dr. James Jiang

Using Methods, Classes, and Objects
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Classes

• Everything is an object
• Every object is a member of a class
• A Java program using classes is a class
• is-a relationships
• My desk in Schott 508 is a Desk
• My cat named Putty is a Cat

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Classes
Reusability

• An object is an instantiation of a class
• one tangible example of a class
• Objects inherit attributes from classes
• these attributes are predictable as members of
  certain classes
• reusability
• Every object that is an instance of a class has
  the methods associated with the class

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Object-oriented Programming Steps

• 1. Create the classes of objects from which
  objects will be instantiated
• 2. Write other classes to use the objects
• A program (class) that instantiates objects of
  another prewritten class is a class client (class
  user).

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Creating a Class

• Create a class header
• optional access modifier (public, final,
  abstract)
• public classes are accessible by all objects and
  may be extended (used as a basis for another
  class)
• keyword class
• name of the class

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class Fields
Greater control!
Information hiding
private data public access

• Include access modifier
• Example -
• private int empNum
• Allowable field modifiers
• private provides the highest level of security
• information hiding (private data/public method)

private public protected static final
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Using Instance Methods

• Instance methods - methods associated with
  individual objects (dont use static!)
• Instance methods require an instance of an object!

public int getSiteNumber() return
siteNumber
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public class EventSite private int
siteNumber public int getSiteNumber() retur
n siteNumber public void setSiteNumber(int
n) siteNumber n
This file CANNOT be run as a program. Why???
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Declaring Objects

• Declaring a class does not create an actual
  object
• To create an object that is an instance of a
  class
• Supply a type and an identifier
• Allocate memory for that object

Employee someEmployee someEmployee new
Employee() OR Employee someEmpe new
Employee()
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Declaring Objects - A Closer Look
Declare an instance of the Employee class
Employee someEmployee someEmployee new
Employee()
Constructor method that constructs an Employee
object
Allocate necessary memory
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Constructor Methods

• Method that constructs an object
• Write your own OR
• Java writes one for you
• Employee() - a constructor method written by Java

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Accessing an Instantiated Object

• someEmployee.changeSalary(350.00)
• Method (changeSalary) is applied to object
  (someEmployee) and the argument (350.00) is
  passed to the method

public void changeSalary(double
newAmount) salary newAmount
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Organizing Classes

• Place data fields in some logical order at the
  beginning of a class
• Store class methods in alphabetical order
• Use comments to document and describe

See Figure 3-29
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Using Constructors

• Employee chauffeur new Employee()
• Constructor methods supplied by Java provide
  specific initial values
• numeric fields set to 0
• character fields set to Unicode \u0000
• boolean fields set to false
• object type fields set to null

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Write your own Constructor

• Must have same name as class it constructs
• No return type
• Example

EventSite() siteNumber 999 managerName
ZZZ
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Exercise 3-6 (p.92)

• public class Cube
• public static void main (String args)
• System.out.println(The cube of 10 is
  
• 
  calCube(10.0)
• public static double calCube (double num)
• double cube
• cube num num num
• return cube