CS2

1
CS2

• Module 5
• Category Elements of Java
• Topic What's an object?
• Objectives
• Second look at elements of Java

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CS 2

• Introduction to
• Object Oriented Programming
• Module 5
• Elements of Java
• What's an Object?

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Object Oriented Programming

• OOP is all about programming using something
  called an object
• We'll get much more deeply into this topic later
  (after some basic stuff)
• But just in case you can't wait...

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Previously...

• public class HelloWorld
• public void greet()
• System.out.println("Hello World!")
• public static void main(String argv)
• HelloWorld hw new HelloWorld()
• hw.greet()

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A Class
A class will be used to define how objects made
from this class will operate

• class Greeter
• String greeting
• public Greeter(String newGreeting)
• greeting newGreeting
• public void greet(String name)
• System.out.println(greeting ", " name)

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Field
A class typically will have one or more
fields which are the data items which the object
holds

• class Greeter
• String greeting
• public Greeter(String newGreeting)
• greeting newGreeting
• public void greet(String name)
• System.out.println(greeting ", " name)

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A Constructor
Classes can have Constructors which contain code
only run when an object is created. Useful for
initialization

• class Greeter
• String greeting
• public Greeter(String newGreeting)
• greeting newGreeting
• public void greet(String name)
• System.out.println(greeting ", " name)

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A Method
Once created an object can be asked to run a
"method" which will do something
normally involving data stored by the
object (plus sometimes data passed in.)

• class Greeter
• String greeting
• public Greeter(String newGreeting)
• greeting newGreeting
• public void greet(String name)
• System.out.println(greeting ", " name)

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Another Class
This class is just being used to hold a main
method. Here we make two Greeter objects and use
them

• class Driver
• public static void main(String args)
• Greeter g1 new Greeter("Hello")
• Greeter g2
• g2 new Greeter("Good morning")
• g1.greet("Bob")
• g2.greet("Mary")
• g1.greet("Sam")
• g2.greet("Joan")

Cgtjava Driver Hello, Bob Good morning,
Mary Hello, Sam Good morning, Joan Cgt
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We can also...

• class Greeter
• String greeting
• public Greeter(String newGreeting)
• greeting newGreeting
• public void greet(String name)
• System.out.println(greeting ", " name)
• public void setGreeting(String chgGreeting)
• greeting chgGreeting
• public String getGreeting()
• return greeting

We can even write methods to change or retrieve
the value of the fields.
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A Third Class
Another class just holding a main method. Here
we make a Greeter object and modify it.

• class Driver2
• public static void main(String args)
• Greeter g2
• g2 new Greeter("Good morning")
• g2.greet("Bob")
• System.out.println("Current "g2.getGreeting())
  
• g2.setGreeting("Good evening")
• g2.greet("Mary")

Cgtjava Driver2 Good morning, Bob Current Good
morning Good evening, Mary Cgtgt
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OOP

• OOP consists of designing a bunch of classes most
  of which will be used as templates to make
  objects
• The objects will be used to hold data and will be
  initialized with constructors and perform useful
  functions by being asked to run their methods
• As we will see they can even be used to make
  sophisticated graphical applications

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Demo
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Questions?
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