Programming Principles using Java

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Programming Principles using Java
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Object Oriented Programs

• Object Oriented Programs consist of classes and
  objects
• We create objects from classes and then use the
  objects by sending messages to them

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Defining the Dog class

• What attributes do all dogs have in common?
• What behaviours do all dogs have in common?

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Class Diagram
Class name
Attribute name
Operations
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Dog class in Java

• class Dog
• int head1, tail1, legs4//dog attributes
• //dog behaviours
• void run()
• System.out.println(I am running)
• void sit()
• System.out.println(I am sitting)
• void bite()
• System.out.println(I am biting)

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What is a Class ?

• A class is a
• Template for objects
• Pattern for objects
• Blueprint for objects
• Factory for objects
• Description for objects
• Classes make objects in their own image
• Classes are abstract, objects are concrete

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Creating Objects from Classes

• Dog snoopy new Dog()
• Dog lassie new Dog()
• Dog scooby new Dog()
• Then we can make the objects do something by
  sending messages to them
• snoopy.sit()
• lassie.run()
• scooby.bite()

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Classes

• Classes facilitate code reuse
• Dog snoopy, lassie, scoobyetc
• BankAccount jim_ac, tim_ac, jane_ac..etc
• Classes facilitate code libraries
• java.awt, java.net, java.io, etc

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Inheritance reusing classes
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Inheritance

• Inheritance is a way of reusing classes
• reusing classes saves writing code
• Inheritance takes the attributes and methods of a
  class and adds to them

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Inheritance

• Inheritance implements an IS-A relationship
• Where ANIMAL is the superclass and
• DOG is a subclass
• A DOG IS-A ANIMAL
• So a dog inherits all the
• features of an animal

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Inheritance
Aeroplane
wings engines
take-off() fly() land()
PassengerJet
Fighter
passengers attendants
guns bombs
serve()
fire() drop()
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Inheritance uses extends

• public class PassengerJet extends Aeroplane
• //attributes
• private int passengers, attendants
• //method
• public void serve()
• System.out.println( attendants serve passengers
  meals)

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World Wide Web

• Internet - a network of networks
• World Wide Web - built on top of the Internet
• contains web pages, held on web servers, which
  can be downloaded onto web clients - web browsers
• web pages are written in HTML

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Applets

• Applets are Java programs that run in a web page
  in a browser
• all Applets are subclasses of the Applet class
• Applets do not have a main() method as Java
  applications do

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Web pages and APPLET tags

• ltHTMLgt
• ltBODYgt
• ltAPPLET CODELighting.class
• HEIGHT300 WIDTH400gt lt/APPLETgt
• lt/BODYgt
• lt/HTMLgt

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Applet methods

• init() is used for initialiation, it is called
  only once, when the applet is first run
• start() is run each time the web page is
  loaded
• stop() is run when the browser leaves the web
  page containing the applet
• destroy() used to relinquish any system
  resources it has allocated
• You may need to override these inherited methods

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• import java.awt.
• import java.awt.event. import
  java.applet.
•  
• public class Lighting extends Applet implements
  ActionListener
• private Button lightswitch
• private Label display
• private boolean state false
•  
• public void init()
•  
• lightswitch new Button(On/Off) //
  creates a Button object
• display new Label(OFF)
• lightswitch.addActionListener(this)
• //register the applet to receive and
  handle ActionEvent events from the button
•  
• add(lightswitch) //add the button to the
  applet
• add(display)

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private Button lightswitch private Label
display private boolean state false   public
void init()   lightswitch new
Button(On/Off) // creates a Button object
display new Label(OFF)
lightswitch.addActionListener(this)
//register the applet to receive and handle
ActionEvent events from the button  
add(lightswitch) //add the button to the applet
add(display)
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