Java AWT GridBagLayout create calculator

import java.awt.GridBagConstraints;
import java.awt.GridBagLayout;
import java.awt.Insets;
import java.util.HashMap;
import java.util.Map;

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JTextField;

public class Main {

   public static void main(String[] args) {
      JFrame frame = new JFrame();
      frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
      frame.setTitle("GridLayout frame");

      frame.getContentPane().setLayout(new GridBagLayout());

      Map<String, JButton> buttons = makeButtons("7", "8", "9", "0", "4", "5", "6", "C", "1", "2", "3", "=", "+", "-",
            "*", "/");

      JTextField resultBox = new JTextField("*** Layout Demo, No Calcualtion ***");
      resultBox.setEditable(false);//w ww.j a  va2  s . com

      GridBagConstraints constraints = new GridBagConstraints();

      // Add number buttons
      constraints.fill = GridBagConstraints.BOTH;
      constraints.weightx = 1;
      constraints.weighty = 1;
      constraints.fill = GridBagConstraints.BOTH;

      // First row
      constraints.gridy = 1;
      constraints.gridx = 0;
      frame.add(buttons.get("7"), constraints);
      constraints.gridx = 1;
      frame.add(buttons.get("8"), constraints);
      constraints.gridx = 2;
      frame.add(buttons.get("9"), constraints);
      // Second row
      constraints.gridy = 2;
      constraints.gridx = 0;
      frame.add(buttons.get("4"), constraints);
      constraints.gridx = 1;
      frame.add(buttons.get("5"), constraints);
      constraints.gridx = 2;
      frame.add(buttons.get("6"), constraints);
      // Third row
      constraints.gridy = 3;
      constraints.gridx = 0;
      frame.add(buttons.get("1"), constraints);
      constraints.gridx = 1;
      frame.add(buttons.get("2"), constraints);
      constraints.gridx = 2;
      frame.add(buttons.get("3"), constraints);

      // Add text field on row above
      constraints.gridy = 0;
      constraints.gridx = 0;
      constraints.gridwidth = 4;
      constraints.anchor = GridBagConstraints.PAGE_START;
      constraints.insets = new Insets(10, 4, 10, 4);
      frame.add(resultBox, constraints);

      // Add bottom buttons
      constraints.gridy = 4;
      constraints.gridwidth = 1;
      constraints.anchor = GridBagConstraints.PAGE_END;
      constraints.insets = new Insets(10, 0, 0, 0);
      constraints.gridx = 0;
      frame.add(buttons.get("+"), constraints);
      constraints.gridx = 1;
      frame.add(buttons.get("-"), constraints);
      constraints.gridx = 2;
      frame.add(buttons.get("*"), constraints);
      constraints.gridx = 3;
      frame.add(buttons.get("/"), constraints);

      // Add buttons to the right
      constraints.anchor = GridBagConstraints.LINE_END;
      constraints.gridx = 3;
      constraints.gridy = 1;
      constraints.insets = new Insets(0, 0, 0, 0);
      frame.add(buttons.get("0"), constraints);

      constraints.insets = new Insets(0, 10, 0, 0);
      constraints.gridy = 2;
      frame.add(buttons.get("C"), constraints);
      constraints.gridy = 3;
      frame.add(buttons.get("="), constraints);

      frame.pack();
      frame.setVisible(true);
   }

   private static Map<String, JButton> makeButtons(String... strings) {
      Map<String, JButton> buttons = new HashMap<>();
      for (String label : strings)
         buttons.put(label, new JButton(label));
      return buttons;
   }
}