Java Swing - JPanel and GridLayout Margins/Padding

I'm working on building a chess game in Java, and I'm currently having a bit of trouble getting the GUI exactly the way I want it with Swing. I'm using a GridLayout to organize a grid of 8x8 ChessButtons (which override the JButton so that I can store extra information inside of them such as coordinates). Originally, the ChessButtons wouldn't appear unless I moused over them, but I solved that problem by placing each ChessButton inside a separate JPanel and setting each button's setPreferredSize() to a set height and width.

Now, my problem is that there seems to be a small margin or padding above (and/or below?) each button. I've made sure to set setHgap(0) and setVgap(0) for the GridLayout, so I'm pretty sure the mysterious margin is coming from either the buttons or the JPanels. But, I can't seem to get rid of them, and they seem to be causing each ChessButton to shift a little bit up/down whenever I mouse of them.

I realize this description of the problem might be a little hard to visualize, so I've taken a screenshot (using JButtons rather than ChessButtons so the gaps are slightly easier to recognize): http://img3.imageshack.us/img3/6656/jbuttonmargins.png

Here is the code I used to initialize each ChessButton:

    chessBoard = new JPanel(new GridLayout(8, 8, 0, 0));
    chessBoard.setBorder(BorderFactory.createEmptyBorder());

    for (int i = 0; i <= 65; i++) {
            //Create a n开发者_StackOverflowew ChessButton
            ChessButton button = new ChessButton("hi");
            button.setBorder(BorderFactory.createEmptyBorder());
            button.setPreferredSize(new Dimension(75, 75));
            button.setMargin(new Insets(0, 0, 0, 0));

            //Create a new JPanel that the ChessButton will go into
            JPanel buttonPanel = new JPanel();
            buttonPanel.setPreferredSize(new Dimension(75, 75));
            buttonPanel.setBorder(BorderFactory.createEmptyBorder());
            buttonPanel.add(button);

            //Add the buttonPanel to the grid
            chessBoard.add(buttonPanel);
    }

So, how can I get rid of these vertical spaces between buttons? I'm relatively new to Swing, so I'm sorry if the answer is extremely obvious, but I'd appreciate any help anyone might have to offer! Thanks in advance!

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Don't add an empty border; do use setBorderPainted(false).

Addendum: As @camickr notes, the panel's layout may include default gaps. The example below uses no-gap GridLayout accordingly.

import java.awt.Color;
import java.awt.Dimension;
import java.awt.EventQueue;
import java.awt.GridLayout;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;

/** @see http://stackoverflow.com/questions/4331699 */
public class ButtonBorder extends JPanel {

    private static final int N = 8;
    private static final int SIZE = 75;

    public ButtonBorder() {
        super(new GridLayout(N, N));
        this.setPreferredSize(new Dimension(N * SIZE, N * SIZE));
        for (int i = 0; i < N * N; i++) {
            this.add(new ChessButton(i));
        }
    }

    private static class ChessButton extends JButton {

        public ChessButton(int i) {
            super(i / N + "," + i % N);
            this.setOpaque(true);
            this.setBorderPainted(false);
            if ((i / N + i % N) % 2 == 1) {
                this.setBackground(Color.gray);
            }
        }
    }

    private void display() {
        JFrame f = new JFrame("ButtonBorder");
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        f.add(this);
        f.pack();
        f.setLocationRelativeTo(null);
        f.setVisible(true);
    }

    public static void main(String[] args) {
        EventQueue.invokeLater(new Runnable() {

            @Override
            public void run() {
                new ButtonBorder().display();
            }
        });
    }
}

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Originally, the ChessButtons wouldn't appear unless I moused over them, but I solved that problem by placing each ChessButton inside a separate JPanel and setting each button's setPreferredSize() to a set height and width

That is not the proper solution. There is no reason to use a JPanel to hold the buttons. In fact, this is probably the cause of the problem. The buttons should show up when you add them to a GridLayout. If they don't show up its probably because you added the buttons to the GUI after making the GUI visible. Components should be added to the GUI BEFORE it is made visible.

Now, my problem is that there seems to be a small margin or padding above (and/or below?) each button

I don't understand why there also isn't a horizontal gap. When you create a JPanel, by default it uses a FlowLayout which also contains a horizontal/vertical gap of 5 pixels. So I understand why you might have the vertical gap of 10 pixels. I don't understand why there is no horizontal gap.

If you need more help post your SSCCE demonstrating the problem. And the SSCCE should use regular JButtons. Get the basics working with standard components before you start playing with custom components. That way you know if the problem is with your custom code or not.

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Try adding chessBoard.setPreferredSize(600, 600) to create a JPanel for the board that only has room to fit the buttons (8 buttons each way * 75 size each way on the buttons).