Lab 4 comments, MultiballWorld, Refactoring Ball using Inheritence, and Intro. to CannonWorld

1
Session 8

• Lab 4 comments, MultiballWorld, Refactoring Ball
  using Inheritence, and Intro. to CannonWorld

2
Notes on HW 3

• You must close the file or you might get an empty
  file on save.
• public void save( String saveToFile ) throws
  IOException
• PrintWriter outputFile new PrintWriter(new
  FileWriter(saveToFile ))
• ... ltcode to println to the filegt
• outputFile.flush() // both of these are
  probably over kill
• outputFile.close()
• // end save

3
The Role of Inheritance in Java Graphics

• Dont call us. Well call you.
• Without inheritance, we would have to understand
  many details of how windows work and how they
  interact with the operating system.
• Some of those details are over our heads at this
  point. We dont know enough OOP or Java yet.
• Even if we could understand them (and we will
  eventually), we dont care about them.

4
The Role of Inheritance in Java Graphics

• With inheritance, a BallWorld can act as a
  regular Frame when we dont care about the
  details and as a special kind of Frame when we
  do.
• But there is more to it than that. Not only does
  BallWorld inherit a lot of individual methods
  that we use but many of the methods in Frame
  call the methods we implement in BallWorld!

5
Example Frame manipulation

• Your BallWorld should be able to handle
• Frame relocation
• Frame resizing
• Minimizing/Maximizing
• Etc.
• But wait, I didnt write any code to handle this

6
Example The show() and paint() Methods

• But there is more to it than that. Not only does
  BallWorld inherit a lot of individual methods
  that we use but many of the methods in Frame
  call the methods we implement in BallWorld!
• Consider how the BallWorld program displays its
  output
• // BallWorld
• public void paint( Graphics g )
• aBall.paint( g )
• aBall.move ()
• ...
• counter counter 1
• if ( counter lt 2000 )
• repaint()
• else
• System.exit(0)

7
Example The show(), paint(), and repaint()
Methods
// In BallWorldApplication BallWorld worldnew
BallWorld(Color.red) world.show()

• show() inherited from Frame
• show() calls paint(Graphics g)
• of BallWorld
• the Graphics object g passed by
• show() has the ability to draw a
• host of items to the Frame
• the Graphics object g is passed
• to paint(g) of aBall
• Balls paint uses the Graphics
• object to put a fillOval on the
• screen

• // In BallWorld
• public void paint( Graphics g )
• aBall.paint( g )
• aBall.move ()
• ...
• counter counter 1
• if ( counter lt 2000 )
• repaint()
• else
• System.exit(0)

// In Ball public void paint( Graphics g )
g.setColor( color ) g.fillOval(
location.x, location.y,
location.width, location.height )
8
Dont call us. Well call you.

• This is a simple example of... multiple objects
  collaborating to solve a problem
• It is also an even better example of... writing a
  program by filling in the details (e.g., paint())
  of another program (e.g., Frame) that already
  does a lot of work
• The Java AWT is also an example of a framework, a
  group of classes that work together to provide
  generic solutions in an application domain, which
  programmers can extend to provide specific
  solutions.

9
Multiple Instances of things

• What do we need to do if we want
• Multiple BallWorlds?
• Multiple Balls?
• Refer to your textbook for Budds examples. In
  class we will play with the code.

10
Multiple BallWorlds

• import java.awt.Color
• public class MultipleBallWorldsApp
• public static void main( String args )
• BallWorld world new BallWorld(
  Color.green )
• world.show()
• BallWorld world2 new BallWorld( Color.red
  )
• world2.show()

11
Multiple Balls

• public class MultiBallWorld extends Frame
• ...
• private Ball ballArray
• private static final int BallArraySize 6
• ...
• public MultiBallWorld (Color ballColor)
• ...
• // initialize object data field
• ballArray new Ball BallArraySize
• for (int i 0 i lt BallArraySize i)
• ballArrayi new Ball(10, 15, 5)
• ballArrayi.setColor (ballColor)
• ballArrayi.setMotion (3.0i, 6.0-i)
• // end MultiBallWorld constructor
• public void paint (Graphics g)
• for (int i 0 i lt BallArraySize i)
• ballArrayi.paint (g)

12
Multiple Balls

• ...
• public void paint (Graphics g)
• for (int i 0 i lt BallArraySize i)
• ballArrayi.paint (g)
• // then move it slightly
• ballArrayi.move()
• if ((ballArrayi.x()lt0) (ballArrayi.x()
  gt FrameWidth))
• ballArrayi.setMotion (-ballArrayi.xMotio
  n(),
• 
  ballArrayi.yMotion())
• if ((ballArrayi.y()lt0)(ballArrayi.y()
  gt FrameHeight))
• ballArrayi.setMotion (ballArrayi.xMoti
  on(),
• 
  -ballArrayi.yMotion())
• // end for
• ...
• // end paint

13
Refactoring BallWorld

• In our current implementation, a ball has two
  very different kinds of responsibilities
• to keep track of its position and size and draw
  it on the screen, and
• to move around the window.
• I can imagine using a ball that doesn't move,
  say, as a part of a stationary picture.
• So these responsibilities should reside in
  different classes.
• But we don't want to duplicate any code, so we
  will want to use inheritance.

14
Refactoring BallWorld

• The original BallWorld and MultiBallWorld
  duplicated the bounds-checking code!
• If we must build the same functionality into two
  different programs, that is a sure sign that an
  object should be providing that functionality as
  a service.
• Maintain in single place, e.g., adding Insets
  checking
• What's worse, because the BallWorld and the
  MultiBallWorld do the bounds checking, they have
  to know the values of the Ball's instance
  variables -- and then change them.
• We want to design objects that provide services
  which don't require the client to (have to) know
  about the object's data. Each object should
  manipulate its own state. So we would prefer for
  the Ball to monitor its own location and control
  its own magnitudes.

15
Refactoring Ball Using Inheritance

• Ball - to keep track of its position and size and
  draw it on the screen
• MovableBall extends Ball - allows Ball to move
• BoundedBall extends MovableBall - allows ball to
  bounce off the sides of the frame
• How will the Ball know about the frame?

16
Refactoring Ball Using Inheritance

• public class Ball
• private Rectangle location
• private Color color
• public Ball( int x, int y, int r )
• location new Rectangle( x-r, y-r, 2r,
  2r )
• color Color.blue
• public void paint( Graphics g )
• g.setColor( color )
• g.fillOval( location.x, location.y,
  location.width,
• location.height )
• ...
• protected int radius() return
  location.width / 2
• protected int x() return location.x
  radius()
• protected int y() return location.y
  radius()
• protected Rectangle region() return
  location

17
Refactoring Ball Using Inheritance

• public class MovableBall extends Ball
• private double dx
• private double dy
• public MovableBall( int x, int y, int r,
  double dx, double dy )
• super( x, y, r )
• this.dx dx
• this.dy dy
• public void move() region().translate(
  (int) dx, (int) dy )
• protected void setMotion( double ndx, double
  ndy )
• dx ndx
• dy ndy
• protected double xMotion() return dx

18
Refactoring Ball Using Inheritance

• import java.awt.Frame
• public class BoundedBall extends MovableBall
• private Frame myWorld
• public BoundedBall( int x, int y, int r,
• double dx, double dy,
  Frame aWorld )
• super( x, y, r, dx, dy )
• myWorld aWorld
• public void move()
• super.move()
• int maxHeight myWorld.getHeight()
• int maxWidth myWorld.getWidth()
• if ( (x() lt 0) (x() gt maxWidth) )
• setMotion( -xMotion(), yMotion() )

19
The Identity of an Object

• How can an object refer to itself?
• Why would an object ever want to do this?
• How can an object refer to itself as an instance
  of its superclass?
• Why would an object ever want to do this?

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Solutions

• this
• Send message to self explicitly?
• Refer to an instance variable with the same name
  as a temporary variable.
• Pass itself as an argument with a message.
• super
• Refer to an inherited method with the same name
  as a method defined in the class. Respond as
  if...
• Initialize inherited instance variables using an
  inherited constructor.

21
An Exercise

• Define a ShadowBall class. A ShadowBall is a Ball
  that becomes darker after every twentieth time it
  paints itself.
• The balls Color object can help the ShadowBall
  do its task. Java Colors respond to the following
  message
• public Color darker()
• By creating a darker version of this color.

22
How Do I Test My Solution?

• A ShadowBall is a Ball. So, I can use a
  ShadowBall any place that I use a Ball. So, I can
  test my new class in any Ball application, such
  as MultiBallWorld...
• Testing is necessary.
• Test early and often. Test only small changes.
• Make testing be as simple as possible.
• Use what you have available!

23
The CannonGame Application

• What color is the cannon ball?
• What is the role of dy()?
• How does the CannonGame draw the cannon?
• What do lv, lh, sv, and sh mean? What about sx
  and sy?
• How does the cannon ball follow the prescribed
  angle, when we dont pass the angle to the cannon
  ball?
• How does the cannon ball reverse direction?
• How does the game know that the ball has hit
  something (either the target or the floor)?
• How does the program terminate?