//  SumApplet.java  (c) 2005 Kari Laitinen

//  2005-09-27  File created.
//  2005-10-03  Latest modification.

/*  This applet demonstrates the following things:
      - an applet made with Swing classes JApplet, JPanel, JTextField, JLabel
      - DocumentListener interface
      - GridLayout
      - Scanner class
      - using null layout in a panel and placing a component into
        an absolute numerical position within a Panel object
*/

import java.awt.* ;
import java.awt.event.* ;
import javax.swing.* ;
import javax.swing.event.* ;
import java.util.Scanner ;

class SumPanel extends JPanel
               implements DocumentListener
{
   JTextField  first_text_field, second_text_field ;
   JLabel  result_label ;

   SumPanel()
   {
      setLayout( null ) ;

      Font font_for_numbers  =  new  Font( "Courier", Font.BOLD, 20 ) ;

      first_text_field  =  new  JTextField( "" ) ;
      first_text_field.setHorizontalAlignment( JTextField.RIGHT ) ;
      first_text_field.setFont( font_for_numbers ) ;
      first_text_field.getDocument().addDocumentListener( this ) ;
      second_text_field  =  new  JTextField( "" ) ;
      second_text_field.setHorizontalAlignment( JTextField.RIGHT ) ;
      second_text_field.setFont( font_for_numbers ) ;
      second_text_field.getDocument().addDocumentListener( this ) ;

      result_label  =  new  JLabel( "0", JLabel.RIGHT ) ;
      result_label.setFont( font_for_numbers ) ;

      JPanel  calculation_panel  =  new JPanel( new GridLayout( 3, 2, 10, 10 ) ) ;

      //  The following statement specifies that the position
      //  of the panel is ( 100, 50 ), and its width is 300 pixels
      //  and its height is 150 pixels.

      calculation_panel.setBounds( 100, 50, 300, 150 ) ;

      calculation_panel.add( new JLabel( "First integer:" ) ) ;
      calculation_panel.add( first_text_field ) ;
      calculation_panel.add( new JLabel( "Second integer:" ) ) ;
      calculation_panel.add( second_text_field ) ;
      calculation_panel.add( new JLabel( "Calculated sum:" ) ) ;
      calculation_panel.add( result_label ) ;

      add( calculation_panel ) ;
   }

   void calculate_sum()
   {
      //  The texts from the text fields are converted to Scanner objects
      //  from which it is easy to scan data. The Scanner class has
      //  the hasNextInt() method which returns false if no valid
      //  int value is present in the text being scanned.

      Scanner first_text_to_scan  =  new Scanner( first_text_field.getText() ) ;
      Scanner second_text_to_scan  =  new Scanner( second_text_field.getText());

      //  The following two if constructs ensure that empty text fields
      //  are treated as if they contained a zero.

      if ( ! first_text_to_scan.hasNext() )
      {
         first_text_to_scan  =  new  Scanner( "0" ) ;
      }

      if ( ! second_text_to_scan.hasNext() )
      {
         second_text_to_scan  =  new  Scanner( "0" ) ;
      }

      if ( first_text_to_scan.hasNextInt() &&
           second_text_to_scan.hasNextInt() )
      {
         int first_integer  =  first_text_to_scan.nextInt() ;
         int second_integer  =  second_text_to_scan.nextInt() ;
         int sum_of_two_integers  =  first_integer  +  second_integer ;
         result_label.setText( "" + sum_of_two_integers  ) ;
      }
      else
      {
         result_label.setText( "INPUT ERROR!" ) ;
      }
   }

   public void insertUpdate( DocumentEvent event )
   {
      calculate_sum() ;
   }

   public void removeUpdate( DocumentEvent event )
   {
      calculate_sum() ;
   }

   public void changedUpdate( DocumentEvent event )
   {
   }  //  This kind of update is not possible in the case of JTextField
}

public class SumApplet extends JApplet
{
   public void init()
   {
      getContentPane().add( new SumPanel() ) ;
   }
}