Classes and Objects

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Classes and Objects

• CSC 171 FALL 2004
• LECTURE 2

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LABS

• Labs start this week.
• Go to your 2nd (Wed or Thurs) lab
• Expect something like the programming exercises
  (P1.1 P1.4) from chapter 1
• You may use notepad and the command prompt

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History

• The abacus

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Questions

• What is an object?
• What is a class?
• What is the difference between an object and a
  class?
• How do I write a class?
• How do I make an object?

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What are objects?

• Object entity that you can manipulate in your
  programs (by invoking methods)
• Each object belongs to a class
• Class Set of objects with the same behavior
• Class determines legal methods"Hello".println()
  // Error"Hello".length() // OK

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Classes and Objects

• Object oriented programs
• Define classes of objects
• Make specific object out of class definitions
• Run by having the objects interact (communicate
  pass messages)
• A class is a type of thing
• Instructor
• An object is a specific thing
• Ted
• An object is an instance of a class

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ANALOGY

• RUBBER STAMPING
• The STAMP is the Class definition
• The IMPRESSION is the object

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DESIGN EXERCISE

• Think about the class of things we call
  students
• What is the data?
• What are the behaviors?

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EXAMPLE
(5,10)

• A rectangle has a location, width, and height

30
20
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Making an object CONSTRUCTION

• new Rectangle(5,10,20,30)

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Reference to an object

• Rectangle cerealBox new Rectangle(5,10,20,30)

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Using references

• System.out.println(The location is (
• cerealBox.x , cerealBox.y)

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Multiple references

• Rectangle cerealBox new Rectangle(5,10,20,30)
• Rectangle r cerealBox

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Programming Example

• The Instructor class
• Instructors have names

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Review A Computer Program

• //Really, A Class definition!
• public class myFirstProgram
• public static void main(String args)
• System.out.println(Hello, CSC171)

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Questions

• What is an object?
• An instance of a class
• What is a class?
• A definition of a set of objects
• What is the difference between an object and a
  class?
• How do I write a class?
• With the class pattern
• How do I make an object?
• By invoking a constructor