Why doesn't this GridBagLayout look as planned?

I think I got this to work. I created a row with 22 components, each occupying one column:

import java.awt.*;
import javax.swing.*;
import javax.swing.border.EmptyBorder;

public class KeyBoardLayout {

    private JComponent ui = null;

    KeyBoardLayout() {
        initUI();
    }

    public void initUI() {
        if (ui!=null) return;

        ui = new JPanel(new GridBagLayout());
        ui.setBorder(new EmptyBorder(4,4,4,4));

        GridBagConstraints gbc = new GridBagConstraints();

        gbc.gridx = 0;
//        gbc.gridy = 0;
        gbc.gridy = 1;
        gbc.gridwidth = 3;
        gbc.fill = GridBagConstraints.HORIZONTAL;
        ui.add(new JButton("1,1 (3)"), gbc);

        gbc.gridx = 3;
        gbc.gridwidth = 2;
        ui.add(new JButton("2,1 (2)"), gbc);

        gbc.gridx = 5;
        ui.add(new JButton("3,1 (2)"), gbc);

        gbc.gridx = 7;
        ui.add(new JButton("4,1 (2)"), gbc);

        gbc.gridx = 9;
        ui.add(new JButton("5,1 (2)"), gbc);

        gbc.gridx = 11;
        ui.add(new JButton("6,1 (2)"), gbc);

        gbc.gridx = 13;
        ui.add(new JButton("7,1 (2)"), gbc);

        gbc.gridx = 15;
        gbc.gridwidth = 3;
        ui.add(new JButton("8,1 (3)"), gbc);

        gbc.gridx = 18;
        gbc.gridwidth = 4;
        ui.add(new JButton("9,1 (4)"), gbc);

        gbc.gridx = 0;
//        gbc.gridy = 1;
        gbc.gridy = 2;
        ui.add(new JButton("1,2 (4)"), gbc);

        gbc.gridx = 4;
        gbc.gridwidth = 2;
        ui.add(new JButton("2,2 (2)"), gbc);

        gbc.gridx = 6;
        ui.add(new JButton("3,2 (2)"), gbc);

        gbc.gridx = 8;
        ui.add(new JButton("4,2 (2)"), gbc);

        gbc.gridx = 10;
        ui.add(new JButton("5,2 (2)"), gbc);

        gbc.gridx = 12;
        ui.add(new JButton("6,2 (2)"), gbc);

        gbc.gridx = 14;
        ui.add(new JButton("7,2 (2)"), gbc);

        gbc.gridx = 16;
        ui.add(new JButton("8,2 (2)"), gbc);

        gbc.gridx = 18;
        gbc.gridwidth = 4;
        ui.add(new JButton("9,2 (4)"), gbc);

        gbc.gridx = 0;
//        gbc.gridy = 2;
        gbc.gridy = 3;
        gbc.gridwidth = 5;
        ui.add(new JButton("1,3 (5)"), gbc);

        gbc.gridx = 5;
        gbc.gridwidth = 2;
        ui.add(new JButton("2,3 (2)"), gbc);

        gbc.gridx = 7;
        ui.add(new JButton("3,3 (2)"), gbc);

        gbc.gridx = 9;
        ui.add(new JButton("4,3 (2)"), gbc);

        gbc.gridx = 11;
        ui.add(new JButton("5,3 (2)"), gbc);

        gbc.gridx = 13;
        ui.add(new JButton("6,3 (2)"), gbc);

        gbc.gridx = 15;
        ui.add(new JButton("7,3 (2)"), gbc);

        gbc.gridx = 17;
        ui.add(new JButton("8,3 (2)"), gbc);

        gbc.gridx = 19;
        gbc.gridwidth = 3;
        ui.add(new JButton("9,3 (3)"), gbc);

        gbc.gridx = 0;
//        gbc.gridy = 3;
        gbc.gridy = 4;
        gbc.gridwidth = 3;
        ui.add(new JButton("1,4 (3)"), gbc);

        gbc.gridx = 3;
        ui.add(new JButton("2,4 (3)"), gbc);

        gbc.gridx = 6;
        gbc.gridwidth = 10;
        ui.add(new JButton("3,4 (10)"), gbc);

        gbc.gridx = 16;
        gbc.gridwidth = 3;
        ui.add(new JButton("4,4 (3)"), gbc);

        gbc.gridx = 19;
        ui.add(new JButton("5,4 (3)"), gbc);

        gbc.gridx = 0;
        gbc.gridy = 4;
        gbc.gridwidth = 1;

        for (int i = 0; i < 22; i++)
        {
            gbc.gridx = i;
            gbc.gridy = 4;
            ui.add(new JButton());
//          ui.add(Box.createHorizontalStrut(15));
        }


    }

    public JComponent getUI() {
        return ui;
    }

    public static void main(String[] args) {
        Runnable r = new Runnable() {
            @Override
            public void run() {
                try {
                    UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
                } catch (Exception useDefault) {
                }
                KeyBoardLayout o = new KeyBoardLayout();

                JFrame f = new JFrame("Keyboard Layout");
                f.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
                f.setLocationByPlatform(true);

                f.setContentPane(o.getUI());
                f.pack();
                f.setMinimumSize(f.getSize());

                f.setVisible(true);
            }
        };
        SwingUtilities.invokeLater(r);
    }
}

      

        
        
        
      

    

Of course you don't want buttons to be displayed, so you can use them in invisible components as well.

Now I have my own question. Why do buttons appear in the first row even though I installed gridy = 4

?

Edit:

There is a better way to determine the number of columns in a grid without using dummy components. When using GridBagLayout, you can use an array to specify the number of columns and the minimum size for each column:

//ui = new JPanel(new GridBagLayout());

int[] columns = new int[22];
Arrays.fill(columns, 30);
GridBagLayout gbl = new GridBagLayout();
gbl.columnWidths = columns;
ui = new JPanel(gbl);

      

        
        
        
      

    

Then you remove the logic by creating mock components:

/*
    for (int i = 0; i < 22; i++)
    {
        gbc.gridx = i;
        gbc.gridy = 4;
        ui.add(new JButton());
        //ui.add(Box.createHorizontalStrut(15));
    }
*/