Java GridBagLayout and JPanel Error: cannot add to layout: constraint must be a string (or null)

Question

I have researched this error and I cannot seem to find a solution. I am trying to create a grid of 800 JButtons with 40 columns and 20 rows. This will eventually be used to control a domino setting up robot I am making that will tip over dominoes. I have already successfully created a grid using GridLayout, but due to the nature of the project, I would like every other row to be offset by half a button. By this I mean like how keys on a computer keyboard are set up. (I would have added a helpful picture of what I am trying to explain, but apparently beginners who have trouble explaining things aren't allowed to add pictures, whatever).

I try to do this by creating a JPanel array of 20 panels called panel. Then I add to the panel the 40 JButtons. Then I use GridBagConstraints to offset every other row. I read that you shouldn't mix awt and swing so that could be the problem, but i don't know. Here is the code, I figured this out from youtube tutorials as I am a very beginner. Forgive me if anything I have said does not make sense. Code:

import java.awt.GridLayout;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;
import java.awt.GridBagConstraints;
import java.awt.GridBagLayout;
public class OffsetGrid {

    public static void main (String [] args){
        JFrame Frame = new JFrame();
        Frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        GridLayout grid= new GridLayout();


        GridBagConstraints gbca= new GridBagConstraints();
        GridBagConstraints gbcb= new GridBagConstraints();
        JPanel[] panel=new JPanel[20];


        for (int row=0;row<20; row++){
                panel[row]=new JPanel(new GridBagLayout()); 
                gbca.gridx=1;
                gbca.gridy=row;
                gbcb.gridx=0;
                gbcb.gridy=row;

        for (int y=0; y<40;y++){
                grid=new GridLayout(1,40);
                panel[row].setLayout(grid);
                JButton[] button = new JButton[40];
                button[y]=new JButton();
                button[y].setOpaque(true);
                panel[row].add(button[y]);

                }
            if (row%2==0){
                Frame.add(panel[row], gbcb);
            }
            else {
                Frame.add(panel[row], gbca);
            }

    }

        Frame.setVisible(true);
        Frame.setLocationRelativeTo(null);
        Frame.pack();
}
}

error:

Exception in thread "main" java.lang.IllegalArgumentException: cannot add to layout: constraint must be a string (or null)
    at java.awt.BorderLayout.addLayoutComponent(BorderLayout.java:426)
    at javax.swing.JRootPane$1.addLayoutComponent(JRootPane.java:531)
    at java.awt.Container.addImpl(Container.java:1120)
    at java.awt.Container.add(Container.java:998)
    at javax.swing.JFrame.addImpl(JFrame.java:562)
    at java.awt.Container.add(Container.java:966)
    at OffsetGrid.main(OffsetGrid.java:38)

Please help me figure out the problem and get it working. Thanks

edit: I am still confused on exactly how to use gridbagconstraints so I don't even know if gridy and gridx are even the right things to use here. Or even if i should use gridbagconstraints. Please offer any suggestions to get the job done. Thanks

edit: this seems to work.

import java.awt.GridLayout;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;
import java.awt.GridBagConstraints;
import java.awt.GridBagLayout;
import java.awt.Insets;
public class OffsetGrid {

    public static void main (String [] args){
        JFrame Frame = new JFrame();
        Frame.setLayout(new GridBagLayout());
        Frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        GridLayout grid= new GridLayout();


        GridBagConstraints gbca= new GridBagConstraints();
        GridBagConstraints gbcb= new GridBagConstraints();
        JPanel[] panel=new JPanel[20];


        for (int row=0;row<20; row++){
                panel[row]=new JPanel(new GridBagLayout()); 
                gbca.insets=new Insets(0,100,0,0);
                gbca.gridy=row;
                gbcb.insets=new Insets(0,0,0,0);
                gbcb.gridy=row;

        for (int y=0; y<40;y++){
                grid=new GridLayout(1,40);
                panel[row].setLayout(grid);
                JButton[] button = new JButton[40];
                button[y]=new JButton();
                button[y].setOpaque(true);
                panel[row].add(button[y]);

                }
            if (row%2==0){
                Frame.add(panel[row], gbcb);
            }
            else {
                Frame.add(panel[row], gbca);
            }

    }

        Frame.setVisible(true);
        Frame.setLocationRelativeTo(null);
        Frame.pack();
}
}

Answer 1

You did this:

Frame.add(panel[row], gbcb);

But you forgot to set the frame's layout:

JFrame Frame = new JFrame();
Frame.setLayout(new GridBagLayout());

Now the exception is not thrown. However......

I wonder if this is what you want:

I've made some change to my code in the case that user resize the window. You can still find the older code in revision

import static javax.swing.BorderFactory.createEmptyBorder;
import java.awt.GridLayout;
import java.awt.event.*;

import javax.swing.*;

public class OffsetGrid {
    public static final int ROWS = 10;
    public static final int COLUMNS = 10;

    public static void main(String[] args) {
        JFrame frame = new JFrame();
        frame.setLayout(new GridLayout(ROWS, 1));
        final JPanel[] panel = new JPanel[ROWS];
        final JButton[][] button = new JButton[ROWS][COLUMNS];
        for (int row = 0; row < ROWS; row++) {
            panel[row] = new JPanel(new GridLayout(1, COLUMNS));
            for (int y = 0; y < COLUMNS; y++) {
                button[row][y] = new JButton(row + "-" + y);
                button[row][y].setOpaque(true);
                panel[row].add(button[row][y]);
            }
            int padding = button[row][0].getPreferredSize().width / 2;
            if (row % 2 == 0)
                panel[row].setBorder(createEmptyBorder(0, 0, 0, padding));
            else
                panel[row].setBorder(createEmptyBorder(0, padding, 0, 0));
            frame.add(panel[row]);
        }
        frame.addComponentListener(new ComponentAdapter() {
            @Override
            public void componentResized(ComponentEvent e) {
                for (int row = 0; row < ROWS; row++) {
                    int padding = button[row][0].getSize().width / 2;
                    panel[row].setBorder(createEmptyBorder(0, 0, 0, padding));
                    if (++row == ROWS)
                        break;
                    padding = button[row][0].getSize().width / 2;
                    panel[row].setBorder(createEmptyBorder(0, padding, 0, 0));
                }
            }
        });
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.pack();
        frame.setLocationRelativeTo(null);
        frame.setVisible(true);
    }
}