Complete Program

Here is the new version of the percent fat program. You may copy it to an editor, save it, and run it. When you run it, try resize the frame. The main() method has made the frame non-resizeable. Change this if you want and observe the effect.

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: total calories per serving
  int fatGrams ;  // input: grams of fat per serving
  double percent; // result: percent of calories from fat
  
  public PercentFatPanel()  
  { 
    setTitle( "Calories from Fat" );
    outPer.setEditable( false );   
    setLayout( new FlowLayout() ); 
   
    // Add components to Panels
    hedPanel.add( heading );
    fatPanel.add( fatLabel );
    fatPanel.add( inFat );
    calPanel.add( calLabel );
    calPanel.add( inCal );
    perPanel.add( perLabel );
    perPanel.add( outPer );
    butPanel.add( doit );

    // Add Panels to Frame     
    add( hedPanel );
    add( heading );
    add( fatPanel );
    add( calPanel );
    add( perPanel );
    add( doit );
    add( butPanel );
  
    setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );   
    doit.addActionListener( this );   
  }
  
  // The 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+"      ").substring(0,6) );
    repaint();                 
  }
  
  public static void main ( String[] args )
  {
    PercentFatPanel fatApp  = new PercentFatPanel() ; 
    fatApp.setSize( 300, 225 );  
    fatApp.setResizable( false );
    fatApp.setVisible( true );        
  }
}