My way to get X and Y index of buttons inside Grid

This is related to How to get X and Y index of element inside GridLayout? post and its answers.

For whatever reason none of them suggested to extend JButton to include its position in the grid and in associated array of buttons.

I have made the following illustration that simply displays button's coordinates when it's clicked.

Extended JButton:

package buttons_array;

import javax.swing.*;

@SuppressWarnings("serial")
public class ButtonWithCoordinates extends JButton {

    int coordX;
    int coordY;

    public ButtonWithCoordinates(String buttonText, int coordX, int coordY) {
        super(buttonText);
        this.coordX = coordX;
        this.coordY = coordY;
    }

    /**
     * @return the coordX
     */
    public int getCoordX() {
        return coordX;
    }

    /**
     * @return the coordY
     */
    public int getCoordY() {
        return coordY;
    }
}

sample GUI:

package buttons_array;

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

public class ButtonsArray implements ActionListener {

    private ButtonWithCoordinates buttons[][];
    private int nRows;
    private int nCols;

    private JFrame frame;
    private JPanel panel;

    public ButtonsArray(int x, int y) {
        if (x > 0 && y > 0) {
            nRows = x;
            nCols = y;
            buttons = new ButtonWithCoordinates[nRows][nCols];
            for (int i=0; i < nRows; ++i) {
                for (int j=0; j < nCols; ++j) {
                    buttons[i][j] = new ButtonWithCoordinates("        ", i, j);
                    buttons[i][j].addActionListener(this);
                }
            }
        } else {
            throw new IllegalArgumentException("Illegal array dimensions!!!");
        }
    }

    @Override
    public void actionPerformed(ActionEvent e) {

        // TODO Auto-generated method stub
        ButtonWithCoordinates button = (ButtonWithCoordinates) e.getSource();
        button.setText(button.getCoordX() + ", " + button.getCoordY());

    }

    public void GUI() {

        if (buttons == null) { throw new NullPointerException("Array is not initialized!!!"); }

        frame = new JFrame();
        panel = new JPanel();

        frame.setContentPane(panel);
        panel.setLayout(new GridLayout(nRows, nCols));
        for (int i=0; i < nRows; ++i) {
            for (int j=0; j < nCols; ++j) {
                panel.add(buttons[i][j]);
            }
        }

        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.pack();
        frame.setVisible(true);

    }

    public static void main(String[] args) {

        SwingUtilities.invokeLater(new Runnable() {

            @Override
            public void run() {
                new ButtonsArray(3, 5).GUI();
            }
        });         

    }

}

Now my questions:

Have I been reinventing the wheel here? I mean, is there a more straightforward way to achieve the same?
Is it in any way inferior to searching through the array each time we need to find the coordinates?

The original version of this example used extension:

GridButton extends JButton

The updated version was predicated on the colloquy seen here. While extension may be appropriate in some contexts, a few alternatives are mentioned here; a client property is particularly convenient. Identifying a button from its grid coordinates is also easy:

private static final int N = 5;
List<JButton> list = new ArrayList<>();
…
private JButton getGridButton(int r, int c) {
    int index = r * N + c;
    return list.get(index);
}