[Chapter 14] 14.3 AppletContext Interface

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| Chapter 14 And Then There Were Applets |

## 14.3 AppletContext Interface

The `AppletContext` interface provides the means to control the browser environment where the applet is running. Methods

Some of these methods are so frequently used that they are also provided within the `Applet` class.

public abstract AudioClip getAudioClip (URL url)

  The `getAudioClip()` method loads the audio file located at `url`. `url` must be a complete and valid URL. Upon success, `getAudioClip()` returns an instance of a class that implements the `AudioClip` interface. You can then call methods in the `AudioClip` interface (see AudioClip Interface) to play the clip. If an error occurs during loading (e.g., because the file was not found or the URL was invalid), `getAudioClip()` returns `null`.

public abstract Image getImage (URL url)

  The `getImage()` method loads the image file located at `url`. `url` must be a complete and valid URL. The method returns a system-specific object that subclasses `Image` and returns immediately. The `Image` is not loaded until needed. A call to `prepareImage()`, `MediaTracker`, or `drawImage()` forces loading to start.

public abstract Applet getApplet (String name)

  The `getApplet()` method fetches the `Applet` from the current HTML page named `name`, which can be the applet's class name or the name provided in the `NAME` parameter of the `<APPLET>` tag. `getApplet()` returns `null` if the applet does not exist in the current context. This method allows you to call methods of other applets within the same context, loaded by the same `ClassLoader`. For example:

      MyApplet who = (MyApplet)getAppletContext().getApplet("hey");
      who.method();

  > TIP:

  Netscape Navigator 3.0 restricts which applets can communicate with each other. Internet Explorer seems to have a similar restriction. For applets to communicate, they must:

  - Have the same `CODEBASE`.

  - Have the same or no `ARCHIVES` tag.

  - Have `MAYSCRIPT` tags and appear in the same frame; alternatively, neither applet may have a `MAYSCRIPT` tag.

  If these conditions are not met and you try to cast the return value of `getApplet()` or `getApplets()` to the appropriate class, either the cast will throw a `ClassCastException`; or nothing will happen, and the method will not continue beyond the point of the failure.

public abstract Enumeration getApplets ()

  The `getApplets()` method gathers all the `Applets` in the current context, loaded by the same `ClassLoader`, into a collection and returns the `Enumeration`. You can then cycle through them to perform some operation collectively. For example:

      Enumeration e = getAppletContext().getApplets();
      while (e.hasMoreElements()) {
          Object o = e.nextElement();
          if (o instance of MyApplet) {
              MyApplet a = (Object)o;
              a.MyAppletMethod();
          }
      }

  > TIP:

  If you want communication between applets on one page, be aware that there is no guarantee which applet will start first. Communications must be synchronized by using a controlling class or continual polling.

public abstract void showDocument (URL url)

  The `showDocument()` method shows `url` in the current browser window. The browser may ignore the request if it so desires.

public abstract void showDocument (URL url, String frame)

  The `showDocument()` method shows `url` in a browser window specified by `frame`. Different `frame` values and the results are shown in Table 14.1. The browser may ignore the request, as appletviewer does.

      try {
          URL u = new URL (getDocumentBase(), (String) file);
          getAppletContext().showDocument (u, "_blank");
      } catch (Exception e) {
      }

  Table 14.1: Target Values
  Target String | Results
  --- | ---
  `_blank` | Show `url` new browser window with no name.
  `_parent` | Show `url` in the parent frame of the current window.
  `_self` | Replace current `url` with `url` (i.e., display in the current window).
  `_top` | Show `url` in top-most frame.
  `name` | Show `url` in new browser window named `name`.

  public abstract void showStatus (String message)

    The `showStatus()` method displays `message` on the browser's status line, if it has one. How to display this string is up to the browser, and the browser can overwrite it whenever it wants. You should use `showStatus()` only for messages that the user can afford to miss.

  AudioClip Interface |  | AppletStub Interface