Don't understand the code, about boucing ball

Question

this is a program intends to create a ball and let the ball bouncing until it reaches the right side of the console. It's in the stanford cs106a course.
I don't the part：
// assume bounce will move ball an amount above the
// floor equal to the amount it would have dropped
// below the floor.
double diff = ball.getY() - (getHeight() - DIAM_BALL);
ball.move(0, -2 * diff);does the ball actually reaches the bottom of the console, and them let the ball moves back upwards with the distance 2diff?
thanks a lot!
beside, I'm new in this forum, how to let the code show in the code mode? Cause it seems hard to read right now.
Here is the whole program:
* File: BouncingBall.java
* This program graphically simulates a bouncing ball.
import acm.program.*;
import acm.graphics.*;
public class BouncingBall extends GraphicsProgram {
/** Size (diameter) of the ball */
private static final int DIAM_BALL = 30;
/** Amount Y velocity is increased each cycle as a
* result of gravity */
private static final double GRAVITY = 3;
/** Animation delay or pause time between ball moves */
private static final int DELAY = 50;
/** Initial X and Y location of ball */
private static final double X_START = DIAM_BALL / 2;
private static final double Y_START = 100;
/** X Velocity */
private static final double X_VEL = 5;
/** Amount Y Velocity is reduced when it bounces */
private static final double BOUNCE_REDUCE = 0.9;
/** Starting X and Y Velocties */
private double xVel = X_VEL;
private double yVel = 0.0;
/* private instance variable */
private GOval ball;
public void run() {
setup();
// Simulation ends when ball goes off right hand
// end of screen
while (ball.getX() < getWidth()) {
moveBall();
checkForCollision();
pause(DELAY);
/** Create and place ball. */
private void setup() {
ball = new GOval(X_START, Y_START, DIAM_BALL, DIAM_BALL);
ball.setFilled(true);
add(ball);
/** Update and move ball */
private void moveBall() {
// increase yVelocity due to gravity on each cycle
yVel += GRAVITY;
ball.move(xVel,yVel);
/** Determine if collision with floor, update velocities
* and location as appropriate. */
private void checkForCollision() {
// determine if ball has dropped below the floor
if (ball.getY() > getHeight() - DIAM_BALL) {
// change ball's Y velocity to now bounce upwards
yVel = -yVel * BOUNCE_REDUCE;
// assume bounce will move ball an amount above the
// floor equal to the amount it would have dropped
// below the floor.
double diff = ball.getY() - (getHeight() - DIAM_BALL);
ball.move(0, -2 * diff);
}Edited by: 967521 on 2012-10-24 下午7:11
Edited by: 967521 on 2012-10-25 上午7:43
Edited by: Melissa13 on 2012-10-26 上午7:21

Answer 1

Put your code between tags and it'll show correctly (you should edit the post now).