Objects and Classes, Lecture 2 Definition

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Objects and Classes, Lecture 2Definition

• an object is a combination of some data
  (attributes) and some actions (methods).
• Hopefully the data and methods are closely
  related NOT randomly thrown together
• Creating Software Objects in Java

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Example of an object

• A Ball Could have
• Attributes
  xPosition, yPosition
• 
  diameter, colour
• 
  graphics context
• Constructor
• Methods display
• changeXPosition, changeYPosition
• moveXPosition, moveXPosition,
• slowMoveXPosition,
• slowMoveYPosition

(xPosition, yPosition)
diameter
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Classes

• Java provides many of its own classes.BUT for
  something like a Ball, a Triangle etc, the
  programmer must define what a ball or triangle
  is ( ie what it looks like)
• A class is like a template and an instance is
  an object constructed from this template.
• Constructor a special method used to create or
  construct an object, most often used to
  initialize variables So whats so special
  about it Same name as class, never returns a
  value.

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• //shortened version of class
• public class Ball
• //attributes
• private int xPosition
• private int yPosition
• private int diameter
• private Color col
• private Graphics g
• //constructor
• public Ball(int xPosIn, int yPosIn, int diamIn,
• Color colIn, Graphics gIn)
• //public method headers or signatures
• public void display()
• public void changeXPosition(int xPosIn)
• public void changeYPosition(int yPosIn)
• public void moveXPosition(int xMove)
• public void moveYPosition(int yMove)

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public or private

• If private, the programmer has control,
• that is no-one outside the class can see, ie
  directly change the variables.
• It is usual to make attributes private and write
  public methods to enable necessary changes
• This is Encapsulation or information hiding

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• The constructor methods are made public so they
  can be seen, ie used from a different class, any
  private methods cannot be used from a different
  class.
• pinkBall new Ball(50,100,20,Color.pink,g)

diameter
xPosition
yPosition
colour
Graphics object
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Creating Multiple Instances?

• // declare
• Ball orangeBall, pinkBall
• //construct
• orangeBall new
• Ball(20,40,30,Color.orange,g)
• pinkBall new
• Ball(50,100,20,Color.pink,g)
• //as many balls as you want/need all instances
  or objects

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• A second class CreateTwoBalls which extends
  Applet, is used to construct, display and
  manipulate the Ball objects by using their
  methods
• pinkBall.moveXPosition(50)
• pinkBall.moveYPosition(40)
• pinkBall.slowMoveXPosition(80)

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Class CreateTwoBalls (three main objects)

• 1. User interface object created from the class
  CreateTwoBalls . This is created by the browser
  or AppletViewer.
• 2. Ball objects, orangeBall and pinkBall, created
  from the class Ball. These are created by the
  user interface object of CreateTwoBalls .