Session 8

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 )); ... <code to println to the file> outputFile.flush(); // both of these are probably over kill outputFile.close(); } // end save

3. The Role of Inheritance in Java Graphics “Don’t call us. We’ll 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 don’t know enough OOP or Java yet. • Even if we could understand them (and we will eventually), we don’t care about them.

4. The Role of Inheritance in Java Graphics With inheritance, a BallWorld can act as a “regular” Frame when we don’t 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 didn’t 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 < 2000 ) repaint(); else System.exit(0); }

7. Example: The show(), paint(), and repaint() Methods // In BallWorldApplication BallWorld world=new BallWorld(Color.red); world.show(); • show() inherited from Frame • show() calls paint(Graphics g) • of BallWorld • theGraphics 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 • Ball’s 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 < 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. “Don’t call us. We’ll 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 Budd’s 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 < 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); // 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()); } // Adjust to slow simulation to the speed of the your computer try { Thread.sleep( 40 ); } catch (InterruptedException e) { System.exit( -1 ); } // finally, redraw the frame counter = counter + 1; if (counter < 2000) repaint(); else System.exit(0); } }

12. 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

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, 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 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; } protected void moveTo(int x, int y) {region().setLocation( x, y ); } } // end Ball

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; } protected double yMotion() { return dy; } } // end MovableBall

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() < 0) || (x() > maxWidth) ) setMotion( -xMotion(), yMotion() ); if ( (y() < 0) || (y() > maxHeight) ) 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?

20. 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 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.

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 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?