求助 | Java 的 hello world 能运行，书上的案例却提示找不到或无法加载主类

package sizedFrame;

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

/**
 * @version 1.32 2007-04-14
 * @author Cay Horstmann
 */
public class SizedFrameTest
{
   public static void main (String[] args )
   {
      EventQueue.invokeLater (new Runnable ()
         {
            public void run ()
            {
               JFrame frame = new SizedFrame ();
               frame.setTitle ("SizedFrame");
               frame.setDefaultCloseOperation (JFrame.EXIT_ON_CLOSE );
               frame.setVisible (true );
            }
         });
   }
}

class SizedFrame extends JFrame
{
   public SizedFrame ()
   {
      // get screen dimensions

      Toolkit kit = Toolkit.getDefaultToolkit ();
      Dimension screenSize = kit.getScreenSize ();
      int screenHeight = screenSize.height;
      int screenWidth = screenSize.width;

      // set frame width, height and let platform pick screen location

      setSize (screenWidth / 2, screenHeight / 2 );
      setLocationByPlatform (true );

      // set frame icon

      Image img = new ImageIcon ("icon.gif").getImage ();
      setIconImage (img );      
   }
}