如何使用 Java 中的迭代器正确访问队列中的值？

Question

I used queue in adding values for my x and y which are the coordinates of where i clicked my mouse. However, I got an error when I try to access the values using Iterator . Can somebody help me? How do I do it the right way? Also, I don't know what Timer and Actionlistener does to my code (are they needed or do i have to remove this?) so a little explanation will be very useful to me. I hope you could help me fix and improve these codes. Thanks! :)

public class Canvas extends JPanel implements MouseListener, ActionListener{

Timer time;
int numberOfPoints;
Queue<Integer> x = new LinkedList<Integer>();
Queue<Integer> y = new LinkedList<Integer>();
Iterator itX = x.iterator();
Iterator itY = y.iterator();
public Canvas(){
    addMouseListener(this);
    time = new Timer(1, this);
    time.start();       
    numberOfPoints = 5;

}

public void paint (Graphics g){
    super.paint(g);
    Graphics2D g2d = (Graphics2D) g;
    g2d.setBackground(Color.black);
    g2d.drawString("Left: "+ numberOfPoints, 10, 20);
    g2d.fillOval(220, 170, 30, 30);
    g2d.setColor(Color.red);    
    while(itX.hasNext()){
        Integer xx = (Integer) itX.next();
        Integer yy = (Integer) itY.next();
        g2d.fillOval(xx, yy, 30, 30);
    }
}

public void mousePressed(MouseEvent e) {        
    if(numberOfPoints>0){
        numberOfPoints--;
        x.add(e.getX()-15);
        y.add(e.getY()-15);
        repaint();
    }

}

public void mouseReleased(MouseEvent e) {}

public void mouseClicked(MouseEvent e) {}

public void mouseEntered(MouseEvent e) {}

public void mouseExited(MouseEvent e) {}

@Override
public void actionPerformed(ActionEvent arg0) {
    repaint();

}

}

Anyway, here's my main class in case you want to run it.

public class ClikkPanikk extends JFrame{

public ClikkPanikk(){
    short width = 500;
    short height = 400;
    add(new Canvas());
    setTitle("ClikkPanikk");
    setVisible(true);
    setSize(width, height);
    setResizable(false);
    setLocationRelativeTo(null);
    setDefaultCloseOperation(EXIT_ON_CLOSE);
}

public static void main(String[] args) {
    new ClikkPanikk();
}

}

Answer 1

You are creating the iterator as a field rather than just before its use. Because of this, the iterator notices that the values have changed since it was created and throws a concurrent modification exception.

Initialize your iterator just before your while loop like:

itX = x.iterator();
itY = y.iterator();
while(itX.hasNext()){
    Integer xx = (Integer) itX.next();
    Integer yy = (Integer) itY.next();
    g2d.fillOval(xx, yy, 30, 30);
}

Also, out of context I may be wrong (I haven't gone through all of your code), but you shouldn't need to cast for an Integer object as you can just set an Integer object to the primitive int type:

Integer varName = 12;

will create an Integer object of value 12, so the unnecessary casting just leaves room for the compiler to miss fatal errors.