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Java in a Nutshell

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Applets
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6.6 Images and Sounds

Example 6.5 shows a Java applet that implements a simple client-side imagemap, which has the capability to highlight the "hot spots" in the image and play a sound clip when the user clicks on the image. Figure 6.4 shows what this applet might look like, when configured with an appropriate image.

Figure 6.4: An imagemap applet

[Graphic: Figure 6-4]

This applet demonstrates quite a few important applet techniques:

The following HTML fragment shows an example of the properties read by this applet:

<APPLET code="Soundmap.class" width=288 height=288>
<PARAM name="image" value="image.gif">
<PARAM name="sound" value="sound.au">
<PARAM name="rect0" value="114,95,151,33,#part1">
<PARAM name="rect1" value="114,128,151,33,#part2">
<PARAM name="rect2" value="114,161,151,33,#part3">
<PARAM name="rect3" value="114,194,151,33,#part4">
<PARAM name="rect4" value="114,227,151,33,#part5">
</APPLET>

Example 6.5: An Imagemap Applet

import java.applet.*;
import java.awt.*;
import java.awt.event.*;
import java.net.*;
import java.util.*;
/**
 * A Java applet that simulates a client-side imagemap.
 * Plays a sound whenever the user clicks on one of the hyperlinks.
 */
public class Soundmap extends Applet {
  protected Image image;      // The image to display.
  protected Vector rects;     // A list of rectangles in it.
  protected AudioClip sound;  // A sound to play on user clicks in a rectangle.
  /** Initialize the applet. */
  public void init() {
    // Look up the name of the image, relative to a base URL, and load it.
    // Note the use of three Applet methods in this one line.
    image = this.getImage(this.getDocumentBase(), this.getParameter("image"));
    // Lookup and parse a list of rectangular areas and the URLs they map to.
    // The convenience routine getRectangleParameter() is defined below.
    rects = new Vector();
    ImagemapRectangle r;
    for(int i = 0; (r = getRectangleParameter("rect" + i)) != null; i++)
      rects.addElement(r);
    // Look up a sound to play when the user clicks one of those areas.
    sound = this.getAudioClip(this.getDocumentBase(),
                  this.getParameter("sound"));
    // Specify an "event listener" object to respond to mouse button
    // presses and releases.  Note that this is the Java 1.1 event model.
    // Note that it also uses a Java 1.1 inner class, defined below.
    this.addMouseListener(new Listener());
  }
  /** Called when the applet is being unloaded from the system.
   * We use it here to "flush" the image we no longer need. This may
   * result in memory and other resources being freed more quickly. */
  public void destroy() { image.flush(); }
  /** To display the applet, we simply draw the image. */
  public void paint(Graphics g) { g.drawImage(image, 0, 0, this); }
  /** We override this method so that it doesn't clear the background
   * before calling paint().  No clear is necessary, since paint() overwrites
   * everything with an image.  Causes less flickering this way. */
  public void update(Graphics g) { paint(g); }
  /** Parse a comma-separated list of rectangle coordinates and a URL.
   * Used to read the imagemap rectangle definitions from applet parameters. */
  protected ImagemapRectangle getRectangleParameter(String name) {
    int x, y, w, h;
    URL url;
    String value = this.getParameter(name);
    if (value == null) return null;
    try {
      StringTokenizer st = new StringTokenizer(value, ",");
      x = Integer.parseInt(st.nextToken());
      y = Integer.parseInt(st.nextToken());
      w = Integer.parseInt(st.nextToken());
      h = Integer.parseInt(st.nextToken());
      url = new URL(this.getDocumentBase(), st.nextToken());
    }
    catch (NoSuchElementException e) { return null; }
    catch (NumberFormatException e) { return null; }
    catch (MalformedURLException e) { return null; }
    return new ImagemapRectangle(x, y, w, h, url);
  }
  /**
   * An instance of this inner class is used to respond to mouse events.
   */
  class Listener extends MouseAdapter {
    /** The rectangle that the mouse was pressed in. */
    private ImagemapRectangle lastrect;
    /** Called when a mouse button is pressed. */
    public void mousePressed(MouseEvent e) {
      // On button down, check if we're inside one of the specified rectangles.
      // If so, highlight the rectangle, display a message, and play a sound.
      // The utility routine findrect() is defined below.
      ImagemapRectangle r = findrect(e);
      if (r == null) return;
      Graphics g = Applet.this.getGraphics();
      g.setXORMode(Color.red);
      g.drawRect(r.x, r.y, r.width, r.height);  // highlight rectangle
      Applet.this.showStatus("To: " + r.url);   // display URL
      sound.play();                             // play the sound
      lastrect = r;   // Remember the rectangle so it can be un-highlighted.
    }
    /** Called when a mouse button is released. */
    public void mouseReleased(MouseEvent e) {
      // When the button is released, un-highlight the rectangle.  If the
      // mouse is still inside it, ask the browser to go to the URL.
      if (lastrect != null) {
    Graphics g = Applet.this.getGraphics();
    g.setXORMode(Color.red);
    g.drawRect(lastrect.x, lastrect.y, lastrect.width, lastrect.height);
    Applet.this.showStatus("");   // Clear the message.
    ImagemapRectangle r = findrect(e);
    if ((r != null) && (r == lastrect))  // If still in the same rectangle
      Applet.this.getAppletContext().showDocument(r.url); // Go to the URL
    lastrect = null;
      }
    }
    /** Find the rectangle we're inside. */
    protected ImagemapRectangle findrect(MouseEvent e) {
      int i, x = e.getX(), y = e.getY();
      for(i = 0; i < rects.size(); i++)  {
    ImagemapRectangle r = (ImagemapRectangle) rects.elementAt(i);
    if (r.contains(x, y)) return r;
      }
      return null;
    }
  }
  /**
   * A helper class.  Just like java.awt.Rectangle, but with a URL field.
   * Note the use of a nested top-level class for neatness.
   */
  static class ImagemapRectangle extends Rectangle {
    URL url;
    public ImagemapRectangle(int x, int y, int w, int h, URL url) {
      super(x, y, w, h);
      this.url = url;
    }
  }
}


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