Session 9
MultiballWorld, Refactoring Ball
using Inheritence, and Intro. to
CannonWorld
Notes on HW #3
• Due date changed to Friday, 9/23 at 11:59 PM
• 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 ));
... <code to println to the file>
outputFile.flush();
outputFile.close();
} // end save
// both of these are probably over kill
Multiple Instances of “things”
• What do we need to do if we want…
– Multiple BallWorlds?
– Multiple Balls?
• Refer to your textbook for Budd’s
examples. In class we will play with the
code.
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();
}
}
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 < BallArraySize; i++) {
ballArray[i] = new Ball(10, 15, 5);
ballArray[i].setColor (ballColor);
ballArray[i].setMotion (3.0+i, 6.0-i);
}
} // end MultiBallWorld constructor
public void paint (Graphics g) {
for (int i = 0; i < BallArraySize; i++) {
ballArray[i].paint (g);
Multiple Balls
...
public void paint (Graphics g) {
for (int i = 0; i < BallArraySize; i++) {
ballArray[i].paint (g);
// then move it slightly
ballArray[i].move();
if ((ballArray[i].x()<0)|| (ballArray[i].x() > FrameWidth))
ballArray[i].setMotion (-ballArray[i].xMotion(),
ballArray[i].yMotion());
if ((ballArray[i].y()<0)||(ballArray[i].y() > FrameHeight))
ballArray[i].setMotion (ballArray[i].xMotion(),
-ballArray[i].yMotion());
} // end for
...
} // end paint
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.
Refactoring BallWorld
• Both 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.
– 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.
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?
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, 2*r, 2*r );
color
= Color.blue;
}
public void paint( Graphics g ) {
g.setColor( color );
g.fillOval( location.x, location.y, location.width,
location.height );
}
...
protected
protected
protected
protected
protected
} // end Ball
int radius() { return location.width / 2; }
int x() { return location.x + radius(); }
int y() { return location.y + radius(); }
Rectangle region() { return location; }
void moveTo(int x, int y) {region().setLocation( x, y ); }
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; }
protected double yMotion() { return dy; }
} // end MovableBall
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() < 0) || (x() > maxWidth) )
setMotion( -xMotion(), yMotion() );
if ( (y() < 0) || (y() > maxHeight) )
setMotion( xMotion(), -yMotion() );
}
}
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?
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.
An Exercise
Define a ShadowBall class. A ShadowBall is a
Ball that becomes darker after every
twentieth time it paints itself.
The ball’s 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.
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!
The CannonGame Application
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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 don’t 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?