Oracle有一个有用的教程Creating a GUI With Swing。跳过使用NetBeans IDE学习Swing部分。请特别注意Performing Custom Painting部分。
正如我在评论中所说的，为什么不创建一个图像并将其放置在JButton中呢？
这是我想出的GUI。

要为所有JButtons创建图像，只需在循环中调用图像创建方法。
下面是完整的可运行代码。

import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.FlowLayout;
import java.awt.Font;
import java.awt.FontMetrics;
import java.awt.Graphics2D;
import java.awt.Rectangle;
import java.awt.RenderingHints;
import java.awt.image.BufferedImage;

import javax.swing.BorderFactory;
import javax.swing.ImageIcon;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.SwingUtilities;

public class JButtonImageExample implements Runnable {

    public static void main(String[] args) {
        SwingUtilities.invokeLater(new JButtonImageExample());
    }

    private BufferedImage image;

    public JButtonImageExample() {
        Font font = new Font(Font.DIALOG, Font.PLAIN, 36);
        this.image = createLetterImage(70, 70, font, 'G', 7);
    }

    @Override
    public void run() {
        JFrame frame = new JFrame("Example");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        frame.add(createButtonPanel(), BorderLayout.CENTER);

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

    private JPanel createButtonPanel() {
        JPanel panel = new JPanel(new FlowLayout());
        panel.setBorder(BorderFactory.createEmptyBorder(100, 100, 100, 100));

        JButton button = new JButton(new ImageIcon(image));
        panel.add(button);

        return panel;
    }

    private BufferedImage createLetterImage(int width, int height, Font font,
            char letter, int value) {
        BufferedImage image = new BufferedImage(width, height,
                BufferedImage.TYPE_INT_ARGB);
        Graphics2D g2d = (Graphics2D) image.getGraphics();
        g2d.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
                RenderingHints.VALUE_ANTIALIAS_ON);
        g2d.setRenderingHint(RenderingHints.KEY_TEXT_ANTIALIASING,
                RenderingHints.VALUE_TEXT_ANTIALIAS_ON);
        g2d.setRenderingHint(RenderingHints.KEY_FRACTIONALMETRICS,
                RenderingHints.VALUE_FRACTIONALMETRICS_ON);
        g2d.setFont(font);

        g2d.setColor(Color.WHITE);
        g2d.fillRect(0, 0, width, height);

        g2d.setColor(Color.BLACK);
        String s = Character.toString(letter);
        Rectangle rect = new Rectangle(0, 0, width, height);
        drawCenteredString(g2d, s, rect, font);

        g2d.setColor(Color.BLUE);
        s = Integer.toString(value);
        drawRightAlignedString(g2d, s, width, font);

        g2d.dispose();

        return image;
    }

    private void drawRightAlignedString(Graphics2D g2d, String text, int width,
            Font font) {
        int pointSize = font.getSize() / 2;
        g2d.setFont(font.deriveFont((float) pointSize));
        FontMetrics metrics = g2d.getFontMetrics(g2d.getFont());
        int margin = 2;
        int x = width - metrics.stringWidth(text) - margin;
        int y = metrics.getAscent() - margin;
        g2d.drawString(text, x, y);
    }

    private void drawCenteredString(Graphics2D g2d, String text, Rectangle rect,
            Font font) {
        FontMetrics metrics = g2d.getFontMetrics(font);
        int x = rect.x + (rect.width - metrics.stringWidth(text)) / 2;
        int y = rect.y + ((rect.height - metrics.getHeight()) / 2)
                + metrics.getAscent();
        g2d.setFont(font);
        g2d.drawString(text, x, y);
    }

}