bufferedtickermessage.java

北京大学出版社的

package examples.awt;
import java.applet.*;
import java.awt.*;

/** A class to demonstrate how an applet can use
  * a thread and parameters specified in HTML
  */
public class BufferedTickerMessage extends Applet 
                              implements Runnable {
   private String message;
   private int delay;
   private int stringWidth, x, y;
   private Thread scrollingThread;
   private static final Color textColor =
      Color.black;

   // the following fields were added to support
   // double buffering
   private Dimension offDim;
   private Image offImage;
   private Graphics offGraphics;
   private boolean keepScrolling;

   /** Initialize the applet and prepare to start
     */
   public void init() {
      Dimension mySize = getSize();

      // get the message and period parameter values
      // from the HTML file
      message = getParameter( "message" );
      if ( message == null ) {
         message = "missing parameter 'message'";
      }
      String p = getParameter( "period" );
      int period;
      if ( p == null ) {
         period = 20;
      } else {
         period = Integer.parseInt( p );
      }

      // calculate the delay between repaints based
      // on the size and specified period.  This is
      // only very approximate because the time needed
      // to do the painting is not accurately
      // accounted for.  Dividing the delay by an
      // adjustment factor is a simple attempt.
      delay = ( 1000 * period / mySize.width ) / 2;

      // set the font and calculate the width and
      // height using the font's metrics information
      Font f = new Font( "Helvetica", Font.BOLD, 18 );
      setFont( f );
      FontMetrics fm = getFontMetrics( f );
      stringWidth = fm.stringWidth( message );
      y = mySize.height / 2 + fm.getHeight() / 2;
      x = 0;
   }

   /** Start the ticker applet running, creating a
     * new thread as necessary
     */
   public void start() {
      if ( scrollingThread == null ) {
         scrollingThread = new Thread(this);
         keepScrolling = true;
         scrollingThread.start();
      }
   }

   /** Stop the ticker applet running
     */
   public void stop() {
      if ( scrollingThread != null ) {
         keepScrolling = false;
         scrollingThread = null;
      }
   }

   /** Run the ticker.  This method repaints the
     * the message repeatedly which will cause it
     * to move across the display.  To save CPU,
     * it will sleep between repaint operations.
     */
   public void run() {
      while ( keepScrolling ) {
         repaint();
         try { 
            Thread.sleep( delay ); 
         } catch ( Exception e ) {
            // ignore it
         }
      }
   }

   /** Paint the display area.  This is
     * called whenever the ticker starts
     * or restarts
     */
   public void paint( Graphics g ) {
      g.setColor( getBackground() );
      g.fillRect( 0, 0, 
                  getSize().width,
                  getSize().height );
   }

   /** Update the ticker on the display by
     * "erasing" it at its current position
     * and redrawing it at its new position
     * moved slightly to the left.
     */
   public void update( Graphics g ) {
      // set up the off-screen Graphics object
      Dimension d = getSize();
      if ( offGraphics == null
           || offDim.width != d.width
           || offDim.height != d.height ) {
         offDim = d;
         offImage = createImage( d.width,
                                 d.height );
         offGraphics = offImage.getGraphics();
      }

      // clear the off-screen Graphics object
      offGraphics.setColor( getBackground() );
      offGraphics.fillRect( 0, 0,
                            d.width, d.height );

      offGraphics.setColor( textColor );
      x--;
      if ( x < -stringWidth ) {
         x = getSize().width;
      }
      offGraphics.drawString( message, x, y );

      // paint the image onto the screen
      g.drawImage( offImage, 0, 0, this );
   }

   /** A very brief description of the applet
     * @return the applet description
     */
   public String getAppletInfo() {
      return "Demonstration applet";
   }

   /** Provide information about the applet
     * parameters
     * @return a two dimensional array containing
     *    parameter names, types, and
     *    descriptions
     */
   public String[][] getParameterInfo() {
      return new String[][] {
         { "message", 
           "String", 
           "The message to be displayed"
         },
         { "period", 
           "int", 
           "The time (in sec) to display the message"  
         }
      };
   }

}