import java.awt.BorderLayout;
import java.awt.GridLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.JButton;
import javax.swing.JPanel;
import javax.swing.JTextField;

/**
   A component that lets the user enter a number, using 
   a button pad labeled with digits.
*/
public class KeyPad extends JPanel
{
   private JPanel buttonPanel;
   private JButton clearButton;
   private JTextField display;

   /**
      Constructs the keypad panel.
   */
   public KeyPad()
   {  
      setLayout(new BorderLayout());
   
      // Add display field
   
      display = new JTextField();
      add(display, "North");

      // Make button panel

      buttonPanel = new JPanel();
      buttonPanel.setLayout(new GridLayout(4, 3));
      
      // Add digit buttons
      
      addButton("7");
      addButton("8");
      addButton("9");
      addButton("4");
      addButton("5");
      addButton("6");
      addButton("1");
      addButton("2");
      addButton("3");
      addButton("0");      
      addButton(".");
      
      // Add clear entry button
      
      clearButton = new JButton("CE");
      buttonPanel.add(clearButton);

      class ClearButtonListener implements ActionListener
      {  
         public void actionPerformed(ActionEvent event)
         {  
            display.setText("");
         }
      }
      ActionListener listener = new ClearButtonListener();      

      clearButton.addActionListener(new 
            ClearButtonListener());      
      
      add(buttonPanel, "Center");
   }

   /**
      Adds a button to the button panel 
      @param label the button label
   */
   private void addButton(final String label)
   {  
      class DigitButtonListener implements ActionListener
      {  
         public void actionPerformed(ActionEvent event)
         {  

            // Don't add two decimal points
            if (label.equals(".") 
                  && display.getText().indexOf(".") != -1) 
               return;

            // Append label text to button
            display.setText(display.getText() + label);
         }
      }

      JButton button = new JButton(label);
      buttonPanel.add(button);
      ActionListener listener = new DigitButtonListener();
      button.addActionListener(listener);
   }

   /** 
      Gets the value that the user entered. 
      @return the value in the text field of the keypad
   */
   public double getValue()
   {  
      // Catch ill-formatted inputs.  In a proper implementation, an
      // error dialog would be displayed on the screen.  Here, the message is
      // written to the standard error stream.
      try {
         return Double.parseDouble(display.getText());
      }
      catch (NumberFormatException e) {
          System.err.println("ERROR: Ill-formanted numeric input.");
      }
      return 0;   
}
   
   /** 
      Clears the display. 
   */
   public void clear()
   {  
      display.setText("");
   }
}