Java Swing How to - Layout calculator like panel with GridLayout

Question

We would like to know how to layout calculator like panel with GridLayout.
Answer

import java.awt.GridLayout;
//  w  w w.  j av a  2  s.  c o  m
import javax.swing.BorderFactory;
import javax.swing.JButton;
import javax.swing.JComponent;
import javax.swing.JFrame;
import javax.swing.JPanel;

public class Main {
   private static final float BTN_FONT_SIZE = 20f; 
   private static final String[][] BTN_LABELS = {
      {"7", "8", "9", "-"},
      {"4", "5", "6", "+"},      
      {"1", "2", "3", "/"},
      {"0", ".", " ", "="}
   };
   private JPanel mainPanel = new JPanel();

   public Main() {
      int rows = BTN_LABELS.length;
      int cols = BTN_LABELS[0].length;
      int gap = 4;
      mainPanel.setBorder(BorderFactory.createEmptyBorder(gap, gap, gap, gap));
      mainPanel.setLayout(new GridLayout(rows, cols, gap, gap));
      for (String[] btnLabelRow : BTN_LABELS) {
         for (String btnLabel : btnLabelRow) {
            JButton btn = createButton(btnLabel);
            mainPanel.add(btn);
         }
      }
   }

   private JButton createButton(String btnLabel) {
      JButton button = new JButton(btnLabel);
      button.setFont(button.getFont().deriveFont(BTN_FONT_SIZE));
      return button;
   }

   public JComponent getMainComponent() {
      return mainPanel;
   }

   public static void main(String[] args) {
      Main mainPanel = new Main();

      JFrame frame = new JFrame("CalcEg");
      frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
      frame.getContentPane().add(mainPanel.getMainComponent());
      frame.pack();
      frame.setLocationByPlatform(true);
      frame.setVisible(true);
   }
}