Java JOptionPane.ERROR_MESSAGE

import java.awt.Color;
import java.awt.Dimension;
import java.awt.Font;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

import javax.swing.BorderFactory;
import javax.swing.Box;
import javax.swing.BoxLayout;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JOptionPane;
import javax.swing.JTextField;
import javax.swing.SwingUtilities;

public class Main extends JFrame {
   //www .ja  va 2s  .  com
   private final JTextField txtMass = makePrettyTextField();
   private final JTextField txtHeight = makePrettyTextField();
   private final JButton btnCalc = makePrettyButton("Calculate BMI");
   
   private final Main self = this;

   public Main() {
      super();
      setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
      setTitle("BMI Calculator");
      
      getContentPane().setLayout(new BoxLayout(getContentPane(), BoxLayout.PAGE_AXIS));
      
      txtMass.setPreferredSize(new Dimension(200,30));
      txtHeight.setPreferredSize(new Dimension(200,30));
      txtMass.setMaximumSize(txtMass.getPreferredSize());
      txtHeight.setMaximumSize(txtHeight.getPreferredSize());
      
      getContentPane().setBackground(new Color(232, 240, 255));
      
      getContentPane().add(makePrettyLabel("Your mass (kg):"));
      getContentPane().add(Box.createRigidArea(new Dimension(5,5)));
      getContentPane().add(txtMass);
      getContentPane().add(Box.createRigidArea(new Dimension(5,5)));
      
      getContentPane().add(Box.createVerticalGlue());
      
      getContentPane().add(makePrettyLabel("Your height (cm):"));
      getContentPane().add(Box.createRigidArea(new Dimension(5,5)));
      getContentPane().add(txtHeight);
      getContentPane().add(Box.createRigidArea(new Dimension(5,5)));
      
      getContentPane().add(Box.createVerticalGlue());
      getContentPane().add(btnCalc);
      getContentPane().add(Box.createRigidArea(new Dimension(5,5)));
      
      // Add BMI calculation
      btnCalc.addActionListener(new ActionListener() {
         @Override
         public void actionPerformed(ActionEvent arg0) {
            double mass;
            double height;
            try {
               mass = Double.parseDouble(txtMass.getText());
               height = Double.parseDouble(txtHeight.getText());
            } catch (NumberFormatException e) {
               JOptionPane.showMessageDialog(self,
                     "Please enter a valid number for mass and height.",
                     "Input error",
                     JOptionPane.ERROR_MESSAGE);
               return;
            }
            double result = calculateBMI(mass, height);
            JOptionPane.showMessageDialog(self,
                  "Your BMI is: " + (Math.round(result*100.0)/100.0),
                  "Your BMI result",
                  JOptionPane.PLAIN_MESSAGE);
         }        
      });
      
      pack();
      setVisible(true);
   }
   
   protected double calculateBMI(double mass, double height) {
      return mass / Math.pow(height/100.0, 2.0);
   }

   private JButton makePrettyButton(String title) {
      JButton button = new JButton(title);
      button.setFont(new Font(Font.SANS_SERIF, Font.PLAIN, 16));
      button.setBorder(BorderFactory.createRaisedBevelBorder());
      button.setBackground(Color.white);
      button.setForeground(new Color(53, 124, 255));
      return button;
   }

   private JTextField makePrettyTextField() {
      JTextField field = new JTextField();
      field.setFont(new Font(Font.SANS_SERIF, Font.ITALIC, 14));
      field.setHorizontalAlignment(JTextField.RIGHT);
      field.setBorder(BorderFactory.createLoweredBevelBorder());
      return field;
   }
   
   private JLabel makePrettyLabel(String title) {
      JLabel label = new JLabel(title);
      label.setFont(new Font(Font.SANS_SERIF, Font.BOLD, 14));
      label.setForeground(new Color(53, 124, 255));
      return label;
   }
   
   public static void main(String[] args) {
      SwingUtilities.invokeLater(new Runnable() {
         public void run() {
            new Main();
         }
      });
   }
}