How can I display a regular grid quickly?

Question

I want to display a grid of 400 identically-sized JPanels. The usual approach seems to be to create and lay out all the panels, and then actually display them. In my application, however, most of the panels actually start out hidden (think "minesweeper", but with much more complicated panels), so I'd love to be able to display an "empty" grid, and then add the panels to it as I need them. Two approaches I've considered:

1. Dispense with a layout manager and simply add panels at the appropriate absolute coordinates as necessary.

2. Use a layout manager, but start off filling up the table with dummy components and replace them with the complicated ones as I go.

Using either of these approaches, however, I seem to need to know the panel size in advance, which I don't. I could fix this by building a sample panel and measuring it, but that seems rather ugly, and duplicates a bunch of code. Is there some other way to do this?

Answer 1

Use the flyweight pattern to render only visible panels. The approach is illustrated in JTable renderers and outlined here.

Answer 2

I would not use panels or custom painting here. Instead:

• Component: JToggleButton
• Layout: GridLayout
• Tiles: Icon (standard, focused, pressed, selected etc.)

E.G.

import java.awt.*;
import java.net.URL;
import javax.imageio.ImageIO;
import javax.swing.*;

class MineSweeper {

    public static final int COLS = 20;
    public static final int ROWS = 20;

    public static void main(String[] args) throws Exception {
        URL urlDefault = new URL("http://i.stack.imgur.com/in9g1.png");
        URL urlPressed = new URL("http://i.stack.imgur.com/1lgtq.png");
        URL urlSelected = new URL("http://i.stack.imgur.com/wCF8S.png");
        final Image imgDefault = ImageIO.read(urlDefault);
        final Image imgPressed = ImageIO.read(urlPressed);
        final Image imgSelected = ImageIO.read(urlSelected);
        Runnable r = new Runnable() {

            @Override
            public void run() {
                JPanel gui = new JPanel(new GridLayout(ROWS, COLS, 2, 2));
                ImageIcon iiDefault = new ImageIcon(imgDefault);
                for (int ii = 0; ii < COLS; ii++) {
                    for (int jj = 0; jj < ROWS; jj++) {
                        JToggleButton tb = new JToggleButton(iiDefault);
                        tb.setContentAreaFilled(false);
                        tb.setMargin(new Insets(0,0,0,0));
                        tb.setPressedIcon(new ImageIcon(imgPressed));
                        tb.setSelectedIcon(new ImageIcon(imgSelected));
                        gui.add(tb);
                    }
                }

                JOptionPane.showMessageDialog(null, gui);
            }
        };
        // Swing GUIs should be created and updated on the EDT
        // http://docs.oracle.com/javase/tutorial/uiswing/concurrency
        SwingUtilities.invokeLater(r);
    }
}