Near-complete GUI

Here is the program so far:

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

public class PercentFat extends JFrame implements ActionListener
{
  JLabel title     = 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!");
    
  int calories ;  // input: total calories per serving
  int fatGrams ;  // input: grams of fat per serving
  double percent; // result: percent of calories from fat
    
  public PercentFat()   
  {  
    getContentPane().setLayout( new FlowLayout() );
   
    getContentPane().add( title );  
    getContentPane().add( fatLabel );  
    getContentPane().add( inFat );    
    getContentPane().add( calLabel );  
    getContentPane().add( inCal );    
    getContentPane().add( perLabel );   
    getContentPane().add( outPer );   
    outPer.setEditable( false );    
   
    getContentPane().add( doit );     
    doit.addActionListener( this );
    setDefaultCloseOperation( WindowConstants.EXIT_ON_CLOSE );   
  }
   
  . . . . .