Hello experts!

As this program is now it's works perfectly when running as a application or
as an Applet.
Now to my question if I just change this row "public class Converter extends
Applet " in class Converter below
that it extends from a Frame instead from an Applet
then the whole program krasch with message "An exception
'java.lang.IllegalArgumentException' has occured in ...
I mean that if I run this program as an application I should be able to set
that it extends from Frame. I don't want to have extends Applet because I
will not run it as an applet.
So what must be changed before being able to run the program with extends
Frame?
If I comment this row out "f.add("Center", converter); " from method main in
the Converter class
the application starts but without an empty window.

The program is rather big but I don't think you have to look at the whole
program for solve my problem.

public class Converter extends Applet
{

import java.awt.*;
import java.awt.event.*;
import java.util.*;
import java.applet.Applet;

public class Converter extends Applet
{
  ConversionPanel metricPanel, usaPanel;
  Unit[] metricDistances = new Unit[3];
  Unit[] usaDistances = new Unit[4];

  /**
   * Create the ConversionPanels (one for metric, another for U.S.).
   * I used "U.S." because although Imperial and U.S. distance
   * measurements are the same, this program could be extended to
   * include volume measurements, which aren't the same.
   */
  public void init()
  {
     //Use a GridLayout with 2 rows, as many columns as necessary,
     //and 5 pixels of padding around all edges of each cell.
     setLayout(new GridLayout(2,0,5,5));

     //Create Unit objects for metric distances, and then
     //instantiate a ConversionPanel with these Units.
     metricDistances[0] = new Unit("Centimeters", 0.01);
     metricDistances[1] = new Unit("Meters", 1.0);
     metricDistances[2] = new Unit("Kilometers", 1000.0);
     metricPanel = new ConversionPanel(this, "Metric System",
        metricDistances);

     //Create Unit objects for U.S. distances, and then
     //instantiate a ConversionPanel with these Units.
     usaDistances[0] = new Unit("Inches", 0.0254);
     usaDistances[1] = new Unit("Feet", 0.305);
     usaDistances[2] = new Unit("Yards", 0.914);
     usaDistances[3] = new Unit("Miles", 1613.0);
     usaPanel = new ConversionPanel(this, "U.S. System", usaDistances);

     //Add both ConversionPanels to the Converter.
     add(metricPanel);
     add(usaPanel);
  }

  /**
   * Does the conversion from metric to U.S., or vice versa, and
   * updates the appropriate ConversionPanel.
   */
  void convert(ConversionPanel from)
  {
     ConversionPanel to;

     if (from == metricPanel)
        to = usaPanel;
     else
        to = metricPanel;

     double multiplier = from.getMultiplier() / to.getMultiplier();
     to.setValue(multiplier * from.getValue());
  }

  /** Draws a box around this panel. */
  public void paint(Graphics g)
  {
     Dimension d = getSize();
     g.drawRect(0,0, d.width - 1, d.height - 1);
  }

  /**
   * Puts a little breathing space between
   * the panel and its contents, which lets us draw a box
   * in the paint() method.
   */
  public Insets getInsets()
  {
     return new Insets(5,5,5,5);
  }

  /** Executed only when this program runs as an application. */
  public static void main(String[] args)
  {
     //Create a new window.
     Frame f = new Frame("Converter Applet/Application");
     f.addWindowListener(new WindowAdapter()
     {
        public void windowClosing(WindowEvent e)
        {
           System.exit(0);
        }
     });

     //Create a Converter instance.
     Converter converter = new Converter();

     //Initialize the Converter instance.
     converter.init();

     //Add the Converter to the window and display the window.
     f.add("Center", converter); //???????
     f.pack();        //Resizes the window to its natural size.
     f.setVisible(true);
  }
}

class ConversionPanel extends Panel
  implements ActionListener,
  AdjustmentListener,
  ItemListener
{
  TextField textField;
  Choice unitChooser;
  Scrollbar slider;
  int max = 10000;
  int block = 100;
  Converter controller;
  Unit[] units;

  ConversionPanel(Converter myController, String myTitle, Unit[] myUnits)
  {
     //Initialize this ConversionPanel to use a GridBagLayout.
     GridBagConstraints c = new GridBagConstraints();
     GridBagLayout gridbag = new GridBagLayout();
     setLayout(gridbag);

     //Save arguments in instance variables.
     controller = myController;
     units = myUnits;

     //Set up default layout constraints.
     c.fill = GridBagConstraints.HORIZONTAL;

     //Add the label.  It displays this panel's title, centered.
     Label label = new Label(myTitle, Label.CENTER);
     c.gridwidth = GridBagConstraints.REMAINDER; //It ends a row.
     gridbag.setConstraints(label, c);
     add(label);

     //Add the text field.  It initially displays "0" and needs
     //to be at least 10 columns wide.
     textField = new TextField("0", 10);
     c.weightx = 1.0;  //Use maximum horizontal space...
     c.gridwidth = 1; //The default value.
     gridbag.setConstraints(textField, c);
     add(textField);
     textField.addActionListener(this);

     //Add the pop-up list (Choice).
     unitChooser = new Choice();
     for (int i = 0; i < units.length; i++)
     { //Populate it.
        unitChooser.add(units[i].description);
     }
     c.weightx = 0.0; //The default value.
     c.gridwidth = GridBagConstraints.REMAINDER; //End a row.
     gridbag.setConstraints(unitChooser, c);
     add(unitChooser);
     unitChooser.addItemListener(this);

     //Add the slider.  It's horizontal, and it has the maximum
     //value specified by the instance variable max.  Its initial
     //and minimum values are the default (0).  A click increments
     //the value by block units.
     slider = new Scrollbar(Scrollbar.HORIZONTAL);
     slider.setMaximum(max + 10);
     slider.setBlockIncrement(block);
     c.gridwidth = 1; //The default value.
     gridbag.setConstraints(slider, c);
     add(slider);
     slider.addAdjustmentListener(this);
  }

  /**
   * Returns the multiplier (units/meter) for the currently
   * selected unit of measurement.
   */
  double getMultiplier()
  {
     int i = unitChooser.getSelectedIndex();
     return units[i].multiplier;
  }

  /** Draws a box around this panel. */
  public void paint(Graphics g)
  {
     Dimension d = getSize();
     g.drawRect(0,0, d.width - 1, d.height - 1);
  }

  /**
   * Puts a little breathing space between
   * the panel and its contents, which lets us draw a box
   * in the paint() method.
   * We add more pixels to the right, to work around a
   * Choice bug.
   */
  public Insets getInsets()
  {
     return new Insets(5,5,5,8);
  }

  /**
   * Gets the current value in the text field.
   * It's guaranteed to be the same as the value
   * in the scroller (subject to rounding, of course).
   */
  double getValue()
  {
     double f;
     try
     {
        f = (double)Double.valueOf(textField.getText()).doubleValue();
     }
     catch (java.lang.NumberFormatException e)
     {
        f = 0.0;
     }
     return f;
  }

  public void actionPerformed(ActionEvent e)
  {
     setSliderValue(getValue());
     controller.convert(this);
  }

  public void itemStateChanged(ItemEvent e)
  {
     controller.convert(this);
  }

  /** Respond to the slider. */
  public void adjustmentValueChanged(AdjustmentEvent e)
  {
     textField.setText(String.valueOf(e.getValue()));
     controller.convert(this);
  }

  /** Set the values in the slider and text field. */
  void setValue(double f)
  {
     setSliderValue(f);
     textField.setText(String.valueOf((float)f));
  }

  /** Set the slider value. */
  void setSliderValue(double f)
  {
     int sliderValue = (int)f;

     if (sliderValue > max)
        sliderValue = max;
     if (sliderValue < 0)
        sliderValue = 0;
     slider.setValue(sliderValue);
  }
}

class Unit
{
  String description;
  double multiplier;

  Unit(String description, double multiplier)
  {
     super();
     this.description = description;
     this.multiplier = multiplier;
  }

  public String toString()
  {
     String s = "Meters/" + description + " = " + multiplier;
     return s;
  }
}