Help with "Exception in thread "Thread-4" java.lang.NullPointerException"

Question

I am new to Java and I am trying a simple bouncing ball example from Stanford. Here is their simple example that works.
import acm.program.*;
import acm.graphics.*;
import java.awt.*;
public class BouncingBallWorking extends GraphicsProgram {
/** sets diameter */
private static final int DIAM_BALL = 30;
/** amount y velocity is increased each cycle */
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 ball*/
private static final double X_START = DIAM_BALL / 2;
private static final double Y_START = 100;
private static final double X_VEL = 3;
private static final double BOUNCE_REDUCE = 0.9;
private double xVel = X_VEL;
private double yVel = 0.0;
private GOval BallA;
public void run() {
setup(X_START,Y_START);
//         Simulation ends when ball goes off right hand
//         end of screen
while (BallA.getX() < getWidth()) {
moveBall(BallA);
checkForCollision(BallA);
pause(DELAY);
private void setup(double X_coor, double Y_coor) {
BallA = new GOval(X_coor, Y_coor, DIAM_BALL, DIAM_BALL);
add(BallA);
private void moveBall(GOval BallObject) {
yVel += GRAVITY;
BallObject.move(xVel,yVel);
private void checkForCollision(GOval BallObject) {
if(BallObject.getY() > getHeight() - DIAM_BALL){
yVel = - yVel * BOUNCE_REDUCE;
double diff = BallObject.getY() - (getHeight() - DIAM_BALL);
BallObject.move(0, -2 * diff);
} Now I am trying to modify "setup" so it I can create several balls. I made a simple modification to "setup" and now I am getting an error.
"Exception in thread "Thread-4" java.lang.NullPointerException
at BouncingBallNotWorking.run(BouncingBallNotWorking.java:36)
at acm.program.Program.runHook(Program.java:1592)
at acm.program.Program.startRun(Program.java:1581)
at acm.program.AppletStarter.run(Program.java:1939)
at java.lang.Thread.run(Unknown Source)"
Can you describe why I am getting an error? Thanks.
Here is what I changed.
Before:
     private void setup(double X_coor, double Y_coor) {
          BallA = new GOval(X_coor, Y_coor, DIAM_BALL, DIAM_BALL);
          add(BallA);
     }After:
     private void setup(double X_coor, double Y_coor, GOval BallObject) {
          BallObject = new GOval(X_coor, Y_coor, DIAM_BALL, DIAM_BALL);
          add(BallObject);
     }Here is the complete code.
* File:BouncingBall.java
* This program graphically simulates a bouncing ball
import acm.program.*;
import acm.graphics.*;
import java.awt.*;
public class BouncingBallNotWorking extends GraphicsProgram {
/** sets diameter */
private static final int DIAM_BALL = 30;
/** amount y velocity is increased each cycle */
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 ball*/
private static final double X_START = DIAM_BALL / 2;
private static final double Y_START = 100;
private static final double X_VEL = 3;
private static final double BOUNCE_REDUCE = 0.9;
private double xVel = X_VEL;
private double yVel = 0.0;
private GOval BallA;
public void run() {
setup(X_START,Y_START, BallA);
//         Simulation ends when ball goes off right hand
//         end of screen
while (BallA.getX() < getWidth()) {
moveBall(BallA);
checkForCollision(BallA);
pause(DELAY);
private void setup(double X_coor, double Y_coor, GOval BallObject) {
BallObject = new GOval(X_coor, Y_coor, DIAM_BALL, DIAM_BALL);
add(BallObject);
private void moveBall(GOval BallObject) {
yVel += GRAVITY;
BallObject.move(xVel,yVel);
private void checkForCollision(GOval BallObject) {
if(BallObject.getY() > getHeight() - DIAM_BALL){
yVel = - yVel * BOUNCE_REDUCE;
double diff = BallObject.getY() - (getHeight() - DIAM_BALL);
BallObject.move(0, -2 * diff);
} Edited by: TiredRyan on Mar 19, 2010 1:47 AM

Answer 1

TiredRyan wrote:
That is great! Thanks. I've got two question though.
1.) Now I want to have 100 bouncing balls. Is the best way to achieve this is through an array of GOvals? I haven't gotten to the lecture on arrays on Stanford's ITunesU site yet so I am not sure what is possible with what I know now.An array would work, or a List. But it doesn't matter much in this case.
2.) Are references being passed by value only applicable to Java or is that common to object-oriented programming? Also is there a way to force Java to pass in "BallA" instead of the value "null"?I can't say about all OO languages as a whole, but at least C++ can pass by reference.
And no, there's no way to pass a reference to the BallA variable, so the method would initialize it and it wouldn't be null anymore.
When you call the method with your BallA variable, the value of the variable is examined and it's passed to the method. So you can't pass the name of a variable where you'd want to store some object you created in the method.
But you don't need to either, so there's no harm done.