Stepping through an image map applet

Spice up any Web site with this reusable Java code

By Maria Winslow, JavaWorld.com, 12/01/96

Overview of the problem

One suggestion is to use below-background image placement to provide textual information relating to the link; interesting graphical effects can be achieved with Image overlays. If this applet is designed to be as generic as possible, then it can be written and compiled once, and reused many times with differing effects.

Global elements

import java.awt.*;
import java.applet.*;
import java.util.*;
import java.net.*;
public class ImageMap extends Applet {
  Image background;
  boolean overlay;
  Vector areas = new Vector();
  Vector images = new Vector();
  Vector urls = new Vector();
  int current = -1;

Vectors are used so that any number of regions may be defined. A Vector is a dynamically allocated array that grows as needed. This is more efficient than a large array for most cases.

Initialization and parameter collection

  public void init() {
    // get the parameters
    background = getImage(getDocumentBase(), getParameter("main"));
    prepareImage(background, this);
    overlay = Boolean.valueOf(getParameter("overlay")).booleanValue();
    String bgcolor = getParameter("bgcolor");
    if (bgcolor != null)
      setBackground (new Color (Integer.parseInt (bgcolor.substring (1), 16)));
    boolean ok = true;
    for (int i = 1; ok; ++i) {
      String areaStr = getParameter("area"+i);
      String imageStr = getParameter("image"+i);
      String urlStr = getParameter("url"+i);
      if ((areaStr != null) && (imageStr != null) && (urlStr != null)) {
        try {
          StringTokenizer str = new StringTokenizer(areaStr);
          Rectangle area = new Rectangle(Integer.parseInt(str.nextToken()),
                                         Integer.parseInt(str.nextToken()),
                                         Integer.parseInt(str.nextToken()),
                                         Integer.parseInt(str.nextToken()));
          Image image = getImage(getDocumentBase(), imageStr);
          URL url = new URL(getDocumentBase(), urlStr);
          areas.addElement(area);
          images.addElement(image);
          urls.addElement(url);
          prepareImage (image, this);
        } catch (MalformedURLException ex) {
          System.out.println ("Invalid format for URL " + i + " : " + urlStr);
        } catch (NoSuchElementException ex) {
          System.out.println ("Invalid format for area " + i + " : " + areaStr);
        } // end try
      } else {
        ok = false;
      } // end if
    } // end for
  } // end init

There are several parameters that we need to collect from the HTML APPLET tag. A typical tag will look like this:

&ltapplet code=ImageMap width=461 height=585>
&ltparam name="main" value="jwbackcolors.gif">
&ltparam name="bgcolor" value="#000000">
&ltparam name="overlay" value="true">
&ltparam name="area1" value="1 3 460 138">
&ltparam name="image1" value="images/jw1.gif">
&ltparam name="url1" value="http://prominence.com/">
&ltparam name="area2" value="8 149 243 188">
&ltparam name="image2" value="jw2.gif">
&ltparam name="url2" value="http://prominence.com/">
&ltparam name="area3" value="256 147 196 26">
&ltparam name="image3" value="images/jw3.gif">
&ltparam name="url3" value="http://prominence.com/">
&ltparam name="area4" value="255 171 205 34">
&ltparam name="image4" value="images/jw4.gif">
&ltparam name="url4" value="http://prominence.com/">
</applet >

The first three parameters define the general features of the image map. The "main" parameter is the background image, and the "bgcolor" is the background color. The "overlay" parameter should have a value of true or false, and determines whether the images are drawn on top of the background image over their respective areas or centered below. We use the type wrapper objects, Boolean and Integer, to convert strings to the primitive types. The Boolean class has a static method, valueOf, that takes a string as a parameter and returns the corresponding Boolean whose booleanValue method we call. The Integer class has a method, parseInt, that takes a string and returns an integer. We want the integer and boolean values as primitive types, not reference types, so we use these classes to perform manipulation of the strings in order to get integer and boolean primitive types.

When we collect the background image into the variable background, we call the applet's prepareImage method with the image as the parameter. This method begins downloading the image at that time; otherwise, the image would not be downloaded until we need to paint it for the first time, causing more flicker. This is a simple optimization that can be employed when images are used.

The rest of the parameters define distinct regions of the image map with a string representing the Rectangle coordinates (x , y, width, height), and the images and URLs associated with each region. The parameter names start with area, image, and url, respectively, and numbers are added to the end, beginning with 1. Since we are using Vectors to store these values, there is no limit on the number of regions that can be defined.

Validity checking makes the applet more robust, and we require that each region have a valid image, URL, and coordinate set. We collect the parameters in logical groups of three, checking to make sure that none of the Strings are null. If any of these are null, we set the ok boolean to false to indicate that it was the last fully correct region in the sequence, and the for loop will terminate. A try .. catch statement is then used to ensure that everything is correct when the parameters are converted to their usable types.

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