The Component Hierarchy

                                 a Frame
                                    |
                                   ...
                                    |
                               a Converter
                                    |
                  ----------------------------------
                  |                                |
        a ConversionPanel (metricPanel)  a ConversionPanel (usaPanel)
                  |                                |
          -------------------              -------------------
          |       |         |              |       |         |
       a Label    |      a Choice       a Label    |      a Choice
                  |                                |
            --------------                  --------------
            |            |                  |            |
       a TextField  a Scrollbar        a TextField  a Scrollbar


Classes in the Example Program:
 The example program defines three classes.
    two classes that inherit from AWT classes:
         Converter and ConversionPanel are subclasses of Panel
         Panel: public class Panel extends Container
         Container: A generic Abstract Window Toolkit(AWT) container
                   object is a component that can contain other AWT components.
    a data storing class: Unit


Converter:
  data members:
    Frame window;
    ConversionPanel metricPanel, usaPanel;
    Unit metricDistances[] = new Unit[3];
    Unit usaDistances[] = new Unit[4];
  member functions:
    public void init()    // Create the ConversionPanels
    void convert(ConversionPanel from)
     /* Does the conversion from metric to U.S., or vice versa, and
      * updates the appropriate ConversionPanel. */
    public void paint(Graphics g)
    // Paints the component. Draws a box around this panel
    public Insets insets()
     /* Constructs and initializes a new Inset with the specified top, left,
        bottom, and right insets.
        Puts a little breathing space between
        the panel and its contents, which lets us draw a box
        in the paint() method.
      */
    public static void main(String args[])
     /** The Converter class is the heart of the example program.
         It contains the program's main()
      */
    public static void main(String args[]) {
        Frame f = new Frame("Converter Applet/Application");
        /* public class Frame extends Window
           Window(Frame): Constructs a new Window initialized to an invisible
           state.
         */
        Converter converter = new Converter(); // constructor

        converter.init();
        /* -initialize Converter: Converter has 2 ConversionPanels, one
            for Metric System, the other for U.S. System.
           -the two systems have different units: Centimeters, Meters,
            Kilometers and Inches Feet, Yards, Miles.
         */

        f.add("Center", converter);
        /* add the specified component(converter) to the center of the
           container(frame)
         */
        f.pack();
        // Packs the components of the Window
        f.show();
        /* Shows the Window. This will bring the window to the front if the
           window is already visible.
         */
    }

ConversionPanel:
 data members:
    String title;
    // for the label. Label label = new Label(title, Label.CENTER);
    TextField textField;
    Scrollbar slider;
    Choice unitChooser;
    int min = 0;
    int max = 10000;
    Converter controller;
    Unit units[];

 member functions:
    ConversionPanel(Converter myController, String myTitle, Unit myUnits[])
    /* the constructor for ConversionPanel.
       Set up default constraints
       Add the label
       Add the text field
       Add the pop-up list (Choice)
       Add the slider
     */
    double getMultiplier()
    /* Returns the multiplier (units/meter) for the currently
       selected unit of measurement.
     */
    public void paint(Graphics g)
    /* Paints the component. Draws a box around this panel. */
    public Insets insets()
    /* Puts a little breathing space between
       the panel and its contents, which lets us draw a box
       in the paint() method.
     */
    double getValue()
    /* Gets the current value in the text field.
       That's guaranteed to be the same as the value
       in the scroller (subject to rounding, of course).
       handleEvent() guarantees the above.
     */

  /** Respond to user actions. */
    public boolean handleEvent(Event e){
        if (e.target instanceof Scrollbar) {
            textField.setText(String.valueOf(slider.getValue()));
            controller.convert(this);
            return true;
        } else if (e.target instanceof TextField) {
            setSliderValue(getValue());
            controller.convert(this);
            return true;
        } else if (e.target instanceof Choice) {
            controller.convert(this);
            return true;
        }
        return false;
    }

    void setValue(double f)
    /* Set the values in the slider and text field. */
    void setSliderValue(double f)
    /* Set the slider value. */

Unit:
  data members:
    String description;
    double multiplier;
  member functions:
    Unit(String description, double multiplier)
    // constructor
    public String toString()