Chapter 9 Active Objects
+ Boxball Project Design
• Animations!
• What are they?
– Snapshots of objects in different positions
– When viewed quickly appear to be moving
A Virtual Flipbook:
Animating an Object
• First create an object
• Then
– Move it a little
– Pause for the user to see it
– Repeat
FallingBall
public class FallingBall extends ActiveObject {
public FallingBall( Location initialLocation, DrawingCanvas aCanvas ){
canvas = aCanvas;
ballGraphic = new FilledOval (initialLocation, SIZE, SIZE, canvas );
start();
}
public void run() {
while ( ballGraphic.getY() < canvas.getHeight() ) {
ballGraphic.move( 0, Y_STEP );
pause( DELAY_TIME );
}
ballGraphic.removeFromCanvas();
}
}
Defining an Active Object
• define a class that extends ActiveObject
• ensure the class includes a run method
• pause occasionally within the run method
• include start(); usually as the last
statement in the constructor
Using Active Objects
public class FallingBallController extends WindowController {
private FallingBall droppedBall;
public void begin() {
new Text("Click to make a falling ball…”,
INSTR_LOCATION, canvas );
}
public void on MouseClick( Location point ) {
droppedBall = new FallingBall( point, canvas );
}
}
What if…
• We wanted the falling object to look like a
raindrop
Image and VisibleImage Classes
• In the controller class:
private Image rainPicture;
rainPicture = getImage("raindrop.gif" );
• Can be used anywhere:
new VisibleImage( rainPicture, 0,
canvas.getWidth()/2, canvas );
A Controller Class
public class FallingRainPicController extends WindowController {
private Image rainPicture;
public void begin() {
new Text("Click to make a falling raindrop…",
INSTR_LOCATION, canvas );
rainPicture = getImage( "raindrop.gif" );
}
public void onMouseClick( Location point ) {
new FallingRainDrop( rainPicture, point, canvas );
}
}
A Falling Raindrop
public class FallingRainDrop extends ActiveObject {
private static final int DELAY_TIME = 33;
private static final double Y_SPEED=4;
private VisibleImage ballGraphic;
private DrawingCanvas canvas;
public FallingRainDrop( Image rainPic, Location initialLocation, DrawingCanvas aCanvas ) {
canvas = aCanvas;
rainGraphic = new VisibleImage( rainPic, initialLocation, canvas );
start();
}
public void run() {
while ( rainGraphic.getY() < canvas.getHeight() ) {
rainGraphic.move( 0, Y_SPEED );
pause ( DELAY_TIME );
}
rainGraphic.removeFromCanvas();
}
}
Active Object Interactions
• Accomplished with methods and
parameters
• A RainCloud:
– RainCloud continuously generates drops
– Drops fall and disappear at the bottom
The RainCloud Class
public class RainCloud extends ActiveObject {
public RainCloud( DrawingCanvas aCanvas, Image aRainPic) {
rainPic = aRainPic;
canvas = aCanvas;
start();
}
public void run() {
RandomIntGenerator xGenerator = new RandomIntGenerator( 0, canvas.getWidth() );
int dropCount = 0;
while ( dropCount < MAX_DROPS ) {
new FallingRainDrop( rainPic, new Location( xGenerator.nextValue(), 0 ), canvas);
pause( DELAY_TIME );
dropCount++;
}
}
}
Interactions Between Active and
Non-Active Objects
• An Active Object:
– Droplet in the shape of an oval
• Created at the top of canvas, falls to the bottom
• A Non-Active Object:
– Waterline in the shape of a FilledRect
• Rises as drops fall
A New Kind of Raindrop
1. Created at the top of the screen
2. Falls to the bottom
3. Raises the waterline
Falling Droplet
public class FallingDroplet extends ActiveObject {
private FilledOval dropletGraphic;
private FilledRect collector;
public FallingDroplet( Location initialLocation, DrawingCanvas canvas, FilledRect aCollector ) {
dropletGraphic = new FilledOval( initialLocation, SIZE, SIZE, canvas );
collector = aCollector;
start();
}
public void run() {
while ( dropletGraphic.getY() < collector.getY() ) {
dropletGraphic.move( 0, Y_STEP );
pause( DELAY_TIME );
}
dropletGraphic.removeFromCanvas();
if ( collector.getY() > 0 ) {
collector.setHeight( collector.getHeight() + SIZE/4 );
collector.move( 0, -SIZE/4 );
}
}
}
Boxball
Classes break problem up
• Boxball Class:
– sets up arena, title, status message "Let's Play"
– sets up bar and easy/med/hard buttons
– sets up target box
– onMouseClick
• if on easy/med/hard button, moves bar to different level
• if clickPoint is above bar creates a new Ball
Boxball design, continued
• Box Class:
– manages the FilledRect that is the target
– methods to
•
•
•
•
•
Construct a Box
moveBox move to different x-position
setSize
change the size of the box
getLeft
retrieve the x position of left side of box
getRight retrieve the x position of right side
Box Class examples
• target = new Box(100, 50, canvas);
– creates a new Box at x position 100, width 50
• target.setSize(35);
– changes width of box to 35
• target.setSize(BALL_SIZE*1.5);
– width set to 50% larger than ball
• target.moveBox();
– moves box to a random x position
Boxball Design, continued
• Ball Class
– constructor to create a new Ball
• remember the canvas in instance variable
– run method to animate ball so it drops
• while ball is above the bottom of the arena,
– advance ball by 10 pixels or so
– pause 50 milliseconds or so
• after the ball reaches bottom, check if it is within
left and right sides of box
Ball class constructor examples
• Part 3
– new Ball(BALL_SIZE, point, canvas);
• ball will be created at point and fall.
• Part 4
– new Ball(BALL_SIZE, point, target, canvas);
• ball will also be able to access target
• to check if within left and right edges
• Another possibility—
– new Ball(BALL_SIZE, point, arena, target, canvas);
• ball will also be able to access arena to tell when to stop
– each new variation has to add actual parameters
Have fun with challenges
• Check with instructor frequently to stay on
track
• Email code over weekend for help
• This is a crucial component to master
• Reading textbook can help with concepts