Vollständiges Programm

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

public class PercentFatPanel extends JFrame implements ActionListener
{
  JLabel heading    = new JLabel("Percent of Calories from Fat");
  JLabel fatLabel   = new JLabel("Enter grams of fat:   ");
  JLabel calLabel   = new JLabel("Enter total calories: ");
  JLabel perLabel   = new JLabel("Percent calories from fat: ");

  JTextField inFat  = new JTextField( 7 );
  JTextField inCal  = new JTextField( 7 );
  JTextField outPer = new JTextField( 7 );

  JButton    doit   = new JButton("Do It!");

  JPanel hedPanel   = new JPanel();
  JPanel fatPanel   = new JPanel();
  JPanel calPanel   = new JPanel();
  JPanel perPanel   = new JPanel();
  JPanel butPanel   = new JPanel();

  int calories ;  // Input: Kalorien gesamt je Packung
  int fatGrams ;  // Input: Gramm Fett je Packung
  double percent; // Ergebnis: Prozent der Kalorien durch Fett

  public PercentFatPanel()
  {
    setTitle( "Calories from Fat" );
    outPer.setEditable( false );
    setLayout( new FlowLayout() );     // den Layoutmanager für den JFrame setzen

    // den Panels Komponenten hinzufügen
    hedPanel.add( heading );
    fatPanel.add( fatLabel );
    fatPanel.add( inFat );
    calPanel.add( calLabel );
    calPanel.add( inCal );
    perPanel.add( perLabel );
    perPanel.add( outPer );
    butPanel.add( doit );

    // dem Frame Panels hinzufügen
    add( hedPanel );
    add( heading );
    add( fatPanel );
    add( calPanel );
    add( perPanel );
    add( doit );
    add( butPanel );

    setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );
    doit.addActionListener( this );
  }

  // Die Application
  public void calcPercent( )
  {
    percent = ( (fatGrams * 9.0) / calories ) * 100.0 ;
  }

  public void actionPerformed( ActionEvent evt)
  {
    String userIn ;
    userIn    = inFat.getText()  ;
    fatGrams  = Integer.parseInt( userIn ) ;

    userIn    = inCal.getText()  ;
    calories  = Integer.parseInt( userIn ) ;
    calcPercent() ;

    outPer.setText( percent+" " );
    repaint();
  }

  public static void main ( String[] args )
  {
    PercentFatPanel fatApp  = new PercentFatPanel() ;
    fatApp.setSize( 300, 225 );
    fatApp.setResizable( false );
    fatApp.setVisible( true );
  }
}