ConcurrentModificationException when painting component

[Kees Jacobsen]

Hi,

I'm making a small card game in Java with Swing, and I've run into a problem.
I have the following code when you drag with your mouse:

@Override
	public void mouseDragged(MouseEvent e) {
		firstClick = 0;
		if (!isDragging &&
				!isDraggingStack &&
				e.getSource() instanceof ResultDisplay && 
				searchPanel.getCard(e.getX(), e.getY()) != null) {
			
			isDragging = true;
			draggingCard = searchPanel.getCard(e.getX(), e.getY());
			offsets = searchPanel.getOffsets(e.getX(), e.getY());
			dragPanel.drag(e.getX() + offsets[0], e.getY() + offsets[1], draggingCard);
			
		} else if (!isDragging &&
				!isDraggingStack &&
				e.getSource() instanceof DeckBuilderPanel &&
				deckBuilderPanel.getCard(e.getX(), e.getY()) != null) {
			
			isDraggingStack = true;
			offsets = deckBuilderPanel.getOffsets(e.getX(), e.getY());
			draggingCardStack = deckBuilderPanel.removeCardStack(e.getX(), e.getY());
			dragPanel.dragCards(e.getX() + offsets[0], e.getY() + offsets[1] + searchPanel.getHeight(), draggingCardStack);
			
		} else if (isDragging && !isDraggingStack) {
			dragPanel.drag(e.getX() + offsets[0], e.getY() + offsets[1], draggingCard);
		} else if (!isDragging && isDraggingStack) {
			dragPanel.dragCards(e.getX() + offsets[0], e.getY() + offsets[1] + searchPanel.getHeight(), draggingCardStack);          //Things go wrong here
		}
	}

What it's supposed to do, is when you first click on a stack of cards, it removes the stack of cards from the field, and then puts it on the drag panel.
This is the DragPanel class:

public class DragPanel extends JComponent{
	
	private final int CARD_STACK_SPACING;
	private int x, y;
	private List<Card> cards;
	
	public DragPanel(int cardStackSpacing) {
		x = -1;
		y = -1;
		cards = null;
		CARD_STACK_SPACING = cardStackSpacing;
	}
	
	public void drag(int x, int y, Card card) {
		this.x = x;
		this.y = y;
		cards = new ArrayList<Card>();
		cards.add(card);
		repaint();
	}
	public void dragCards(int x, int y, List<Card> cards) {
		this.x = x;
		this.y = y;
		this.cards = cards;
		repaint();
	}
	public void stopDragging() {
		x = -1;
		y = -1;
		cards = null;
		repaint();
	}
	
	@Override
	public void paintComponent(Graphics g) {
		System.out.println("Painting component");
		if (x > 0 && y > 0 && cards != null) {
			Graphics2D g2d = (Graphics2D) g;
			g2d.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);
			int adjustedY = y;
			for (Card card : cards) {                                     //StackTrace says the problem lies with this line.
				CardImager.drawCard(card, g2d, x, adjustedY);
				adjustedY += CARD_STACK_SPACING;
			}
		}
	}
}

Which is calling a method from CardImager, which doesn't modify the card in any way.
Do any of you have any insight?

[Henry Wong]

Kees Jacobsen wrote:
Which is calling a method from CardImager, which doesn't modify the card in any way.
Do any of you have any insight?

It is not "card" -- it is "cards". You are not allowed to modify the List of cards while you are iterating through it. If you want to modify the list, you are only allowed to do it via the iterator, which you haven't exposed in your code.

Henry

[Kees Jacobsen]

It's also not modifying the list of cards - it's completely independent on input.

[Henry Wong]

Kees Jacobsen wrote:It's also not modifying the list of cards - it's completely independent on input.

Well, something is modifying that list. That error occurs when the list is modified while iteration is taking place -- and that modification can come from your code, or some other code not related to your method call, or thread of execution.

Henry

[Kees Jacobsen]

Yes, the problem is, I don't know what and why.
Without any body, the for loop where the stackTrace points to still causes problems.
Doing anything else, like printing the size of the 'cards' array, also causes a ConcurrentModificationException.