How can I set size of a button?

Question

I put my buttons in a JPane with GridLayout. Then I put JPanel into another JPanel with BoxLayout.Y_AXIS. I want buttons in the GridLayout to be square. I use tmp.setSize(30,30) and it does not work. I also try to use new GridLayout(X, Y, 4, 4) but I cannot figure out what X and Y are. So, what is the correct way to do this stuff?

ADDED:

I still cannot solve the problem. Here is the code of what I am trying to do:

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

public class PanelModel {
    public static void main(String[] args) {
        JFrame frame = new JFrame("Colored Trails");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        JPanel mainPanel = new JPanel();
        mainPanel.setLayout(new BoxLayout(mainPanel, BoxLayout.Y_AXIS));

        JPanel firstPanel = new JPanel();
        firstPanel.setLayout(new GridLayout(4, 4));
        JButton btn;
        for (int i=1; i<=4; i++) {
            for (int j=1; j<=4; j++) {
                btn = new JButton();
                btn.setPreferredSize(new Dimension(100, 100));
                firstPanel.add(btn);
            }
        }

        JPanel secondPanel = new JPanel();
        secondPanel.setLayout(new GridLayout(5, 13));
        for (int i=1; i<=5; i++) {
            for (int j=1; j<=13; j++) {
                btn = new JButton();
                btn.setPreferredSize(new Dimension(40, 40));
                secondPanel.add(btn);
            }
        }

        mainPanel.add(firstPanel);
        mainPanel.add(secondPanel);
        frame.add(mainPanel);

        frame.setSize(400,600);
        frame.setVisible(true);
    }
}

The problem is that Java tries to make width of the firstPanel and secondPanel equal! Moreover, Java tries to to fill all height of the window. How can I remove this behavior?

Answer 1

The following bit of code does what you ask for. Just make sure that you assign enough space so that the text on the button becomes visible

JFrame frame = new JFrame("test");
frame.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
JPanel panel = new JPanel(new GridLayout(4,4,4,4));

for(int i=0 ; i<16 ; i++){
    JButton btn = new JButton(String.valueOf(i));
    btn.setPreferredSize(new Dimension(40, 40));
    panel.add(btn);
}
frame.setContentPane(panel);
frame.pack();
frame.setVisible(true);

The X and Y (two first parameters of the GridLayout constructor) specify the number of rows and columns in the grid (respectively). You may leave one of them as 0 if you want that value to be unbounded.

Edit

I've modified the provided code and I believe it now conforms to what is desired:

JFrame frame = new JFrame("Colored Trails");
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

JPanel mainPanel = new JPanel();
mainPanel.setLayout(new BoxLayout(mainPanel, BoxLayout.Y_AXIS));

JPanel firstPanel = new JPanel();
firstPanel.setLayout(new GridLayout(4, 4));
firstPanel.setMaximumSize(new Dimension(400, 400));
JButton btn;
for (int i=1; i<=4; i++) {
    for (int j=1; j<=4; j++) {
        btn = new JButton();
        btn.setPreferredSize(new Dimension(100, 100));
        firstPanel.add(btn);
    }
}

JPanel secondPanel = new JPanel();
secondPanel.setLayout(new GridLayout(5, 13));
secondPanel.setMaximumSize(new Dimension(520, 200));
for (int i=1; i<=5; i++) {
    for (int j=1; j<=13; j++) {
        btn = new JButton();
        btn.setPreferredSize(new Dimension(40, 40));
        secondPanel.add(btn);
    }
}

mainPanel.add(firstPanel);
mainPanel.add(secondPanel);
frame.setContentPane(mainPanel);

frame.setSize(520,600);
frame.setMinimumSize(new Dimension(520,600));
frame.setVisible(true);

Basically I now set the preferred size of the panels and a minimum size for the frame.