ArrayList issues

Question

I am supposed to create a few random balls on a canvas, those balls being stored within an ArrayList (I hope an ArrayList is suitable here: the alternative options to choose from were HashSet and HashMap). Now, whatever I do, I get the differently colored balls at the top of my canvas, but they just get stuck there and refuse to move at all. Apart from that I now get a ConcurrentModificationException, when running the code:

java.util.ConcurrentModificationException
at java.util.AbstractList$Itr.checkForComodification(AbstractList.java:372)
at java.util.AbstractList$Itr.next(AbstractList.java:343)
at BallDemo.bounce(BallDemo.java:109)

Line 109 being: bBall.draw();

Reading up on that exception, I found out that one can make sure ArrayList is accessed in a threadsafe manner by somehow synchronizing access. But since I have remember fellow students doing without synchronizing, my guess is, that it would actually be the wrong path to go.

Maybe you guys could help me get this to work, I at least need those stupid balls to move ;-)

 /**
 * Simulate random bouncing balls
 */
public void bounce(int count)
{
 int ground = 400; // position of the ground line
 System.out.println(count);
 myCanvas.setVisible(true);
 // draw the ground
 myCanvas.drawLine(50, ground, 550, ground);
 // Create an ArrayList of type BouncingBalls
 ArrayList<BouncingBall>balls = new ArrayList<BouncingBall>();
 for (int i = 0; i < count; i++){
 Random numGen = new Random(); 
 // Creating a random color. 
 Color col = new Color(numGen.nextInt(256), numGen.nextInt(256), numGen.nextInt(256));
 // creating a random x-coordinate for the balls
 int ballXpos = numGen.nextInt(550);
 BouncingBall bBall = new BouncingBall(ballXpos, 80, 20, col, ground, myCanvas);
 // adding balls to the ArrayList
 balls.add(bBall);
 bBall.draw();
 boolean finished = false;
 }
 for (BouncingBall bBall : balls){
 bBall.move();
 }
}

This would be the original unmodified method we got from our teacher, which only creates two balls:

 /**
 * Simulate two bouncing balls
 */
public void bounce()
{
 int ground = 400; // position of the ground line
 myCanvas.setVisible(true);
 myCanvas.drawLine(50, ground, 550, ground);
 // draw the ground
 // crate and show the balls
 BouncingBall ball = new BouncingBall(50, 50, 16, Color.blue, ground, myCanvas);
 ball.draw();
 BouncingBall ball2 = new BouncingBall(70, 80, 20, Color.red, ground, myCanvas);
 ball2.draw();
 // make them bounce
 boolean finished = false;
 while(!finished) {
 myCanvas.wait(50); // small delay
 ball.move();
 ball2.move();
 // stop once ball has travelled a certain distance on x axis
 if(ball.getXPosition() >= 550 && ball2.getXPosition() >= 550) {
 finished = true;
 }
 }
 ball.erase();
 ball2.erase();
}

}

So I just modified my code as follows

public void bounce(int count)
 {
 int ground = 400; // position of the ground line
 System.out.println(count);
 myCanvas.setVisible(true);
 // draw the ground
 myCanvas.drawLine(50, ground, 550, ground);
 // Create an ArrayList of type BouncingBalls
 ArrayList<BouncingBall>balls = new ArrayList<BouncingBall>();
 Random numGen = new Random(); 
 for (int i = 0; i < count; i++){
 // Creating a random color. 
 Color col = new Color(numGen.nextInt(256), numGen.nextInt(256), numGen.nextInt(256));
 // creating a random x-coordinate for the balls
 int ballXpos = numGen.nextInt(550);
 BouncingBall bBall = new BouncingBall(ballXpos, 80, 20, col, ground, myCanvas);
 // adding balls to the ArrayList
 balls.add(bBall);
 bBall.draw();
 }
 boolean finished = false;
 while(!finished)
 {
 myCanvas.wait(50); // small delay
 for(BouncingBall ball : balls)
 {
 ball.move();
 // once one ball has travelled the distance, they all have
 if(ball.getXPosition() >= 550)
 finished = true;
 }
 }
 for(BouncingBall ball : balls)
 ball.erase();
}

But that only moves the balls very shortly and then creates the same exception as above.

• Bozho: the stacktrace shows which line?
• jottr: oh sorry forgot to include that: java.util.ConcurrentModificationException at java.util.AbstractList$Itr.checkForComodification(AbstractList.java:372) at java.util.AbstractList$Itr.next(AbstractList.java:343) at BallDemo.bounce(BallDemo.java:109)
• Jan K.: It would help if you edited that into your post.
• Michael Mrozek: There doesn't appear to be a ConcurrentModificationException in the snippet you posted; I second showing which line you're getting it on (line numbers don't help much since this is an excerpt -- add a comment next to the line in the code block)
• Kathy Van Stone: I suspect that line is for (BouncingBall bBall : balls) which calls Iterator.next implicitly. I don't see how the current code does that as the list does not appear to be modified inside the loop.

Answer 1

You are missing the while(!finished) part.
With the for you are adding you are iterating only once through the balls, that's why you are not seeing them move.

Edit: The new version can be over really soon if the random X position of any ball (between 0 and 550) is near 550.

• jottr: hm. it seems I just don't see it right now. Doesn't my first for-loop add the balls to the ArrayList, and shouldn't for (BouncingBall bBall : balls){ bBall.move(); } make them move?
• Poindexter: The second for loop (the BouncingBall loop) will only move them once and not until they hit the bound.
• Kathy Van Stone: for (Bouncing bBall: balls) makes them move once, and immediately after they were created so you may not even see the single move.
• Kathy Van Stone: @BlairHippo Calling bounce will recreate more balls and go into an infinite recursive loop.
• jottr: @Kathy Van Stone, yeah that seems to be exactly the problem now, as I have modified the code after Justin Niessners example.

Answer 2

Are you being forced to use ONLY one of those three structures? An ArrayList barfing-out ConcurrentModificationExceptions is pretty much an engraved invitation to try replacing it with a CopyOnWriteArrayList.

• Michael Mrozek: He really shouldn't need synchronization for this though, unless I'm missing part of the assignment. He's making an ArrayList of sprites and then looping over them repeatedly to move them around; it shouldn't be changing after the initial population
• jottr: yes, we are only supposed to chose from ArrayList, HashSet and HashMap.
• Kathy Van Stone: Usually a ConcurrentModificationException is created by modifying a list while you are iterating it and does not imply a multi-threaded environment.

Answer 3

public void bount(int count)
{
 int ground = 400;
 myCanvas.setVisible(true);
 myCanvas.drawLine(50, ground, 550, ground);
 ArrayList<BouncingBall> balls = new ArrayList<BouncingBall>(); 
 for(int i = 0; i < count; i++)
 {
 // set up colors and position
 BouncingBall newBall = new BouncingBall(ballXpos, 80, 
 20, col, ground, myCanvas);
 newBall.Draw();
 balls.Add();
 }
 boolean finished = false;
 while(!finished)
 {
 myCanvas.wait(50); // small delay
 for(BouncingBall ball : balls)
 {
 ball.Move();
 // once one ball has travelled the distance, they all have
 if(ball.getXPosition() >= 550)
 finished = true;
 }
 }
 for(BouncingBall ball : balls)
 ball.erase();
}

Answer 4

You should create the Random object before the for loop. see below.This may position your balls correctly

 Random numGen=new Random();
 for (int i = 0; i < count; i++){ 
 // Creating a random color. 
 Color col = new Color(numGen.nextInt(256), numGen.nextInt(256), numGen.nextInt(256)); 
 // creating a random x-coordinate for the balls 
 int ballXpos = numGen.nextInt(550); 
 BouncingBall bBall = new BouncingBall(ballXpos, 80