Beginner question: ArrayList can't seem to get it right! Pls help
- by elementz
I have been staring at this code all day now, but just can't get it right.
ATM I am just pushing codeblocks around without being able to concentrate anymore, with the due time being within almost an hour...
So you guys are my last resort here.
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)
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();
}
}
