Question

I’m having a problem where I can’t properly access my instance Point data.

I create an multi-dimensional array of GridPanels, and instantiate each with a Point.
When first created, everything works as expected.

pic1 http://img.skitch.com/20100218-fciwr7t73ci2gajafmfxa2yf9q.jpg

When I click on a GridPanel however, the Listener class always receives the Point from the last GridPanel that was created ( (3, 3) in this case.)

When I pass an int instead of a Point however, the int for the GridPanel that was clicked is shown (like you’d expect).

Anyone know what’s going on here?

Thanks

import javax.swing.JFrame;

/**
 * Driver class.
 */
public class Test {
    /**
     * The main method.
     * @param args Command line arguments.
     */
    public static void main(String[] args) {
        JFrame frame = new JFrame("TEST");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        TestPanel panel = new TestPanel();
        frame.getContentPane().add(panel);

        frame.pack();
        frame.setVisible(true);
    }
}

---

import java.awt.GridLayout;
import java.awt.Point;
import javax.swing.JPanel;

/**
 * Creates a 4 by 4 grid of GridPanels.
 */
public class TestPanel extends JPanel {
    static final int ROW_SIZE = 4;
    static final int COL_SIZE = 4;
    private GridPanel[][] g = new GridPanel[ROW_SIZE][COL_SIZE];

    public TestPanel() {
        Point coords = new Point();

        setLayout(new GridLayout(ROW_SIZE, COL_SIZE));

        for (int i = 0; i < ROW_SIZE; i++) {
            for (int j = 0; j < COL_SIZE; j++) {
                coords.setLocation(i, j);
                g[i][j] = new GridPanel(coords);
                add(g[i][j]);
            }
        }
    }
}

---

import java.awt.Dimension;
import java.awt.Point;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import javax.swing.JLabel;
import javax.swing.JPanel;

/**
 * Contains the MouseListener.
 */
public class GridPanel extends JPanel {

    private JLabel label;
    private Point p;

    public GridPanel(Point p) {
        this.p = p;
        label = new JLabel("" + p);

        add(label);

        setBackground(Color.WHITE);
        setPreferredSize(new Dimension(200, 50));

        addMouseListener(new SelectListener());
    }

    private class SelectListener extends MouseAdapter {
        public void mousePressed(MouseEvent e) {
            label.setText("" + p);
        }
    }
}

Answer 1

The problem is that you are re-using the same point, stored in coords. You need to create a new point for each grid element. It looks to you as if each panel has a different point value stored in it because each panel has a different label. But in the line

label = new JLabel("" + p);

you are creating a String that contains the current value of p. But p can change later, and the label won’t change with it.

So the easiest fix for your problem is to change the line

this.p = p;

to

this.p = new Point(p); // Create a defensive copy.

It looks like you may be currently somewhat confused about the difference between objects and fields. For example,

Point p = new Point(3, 4);
Point p2 = p;
p.x = 7;
System.out.println(p2.x);

will yield 7, as there is only one point being manipulated, though it’s pointed to by two fields. Using = doesn’t create a copy of the point.

(Apologies if I’m explaining things you already know!)