import javax.swing.JFrame;
   import javax.swing.JTextField;
   import javax.swing.JLabel;
   import javax.swing.JButton;
   import java.awt.event.ActionListener;
   import java.awt.event.ActionEvent;


   public class HelloJFrame extends JFrame implements ActionListener     {
      private static final int ANSWER_LENGTH = 30;
      private JTextField ageBox = new JTextField (3);
      private JLabel ageLabel = new JLabel ("How old are you?");
      private JTextField answer = new JTextField (ANSWER_LENGTH);
      private JButton doItButton = new JButton ("Go ahead!");
   
   
      public static void main (String[] args)
         {
         JFrame.setDefaultLookAndFeelDecorated(true);
         JFrame theFrame = new HelloJFrame ("Hello JFrame");
         theFrame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
         theFrame.pack();
         theFrame.setVisible (true);
         }
   
    
      private HelloJFrame (String title) {
         super(title);
         java.awt.Container contents = this.getContentPane();
         contents.setLayout (new java.awt.FlowLayout());
         contents.add (this.ageLabel);		
         contents.add (this.ageBox);
         contents.add (this.doItButton);
         contents.add (this.answer);
         this.doItButton.addActionListener (this);
         }   

      public void actionPerformed (ActionEvent whatHappened) {
         try {
            int age = Integer.parseInt (this.ageBox.getText());
         
            if (age >= 18) {
               this.answer.setText ("" + age + " is old enough to vote.  Congratulations!");
               }
            else {
               this.answer.setText ("" + age + " is NOT old enough to vote.");
               }
            }
            catch (java.lang.NumberFormatException whatsWrong) {
               this.answer.setText ("I don't understand \"" + this.ageBox.getText() + "\"; it must be an integer.");
               }
         }
      }