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java.lang.ClassCastException Thrown From the AWT Event Dispatching Thread When Mouse Moves Over an Applet Frame

Symptoms

While running an applet in a browser by using the Sun Java™ Runtime Environment (JRE™) implementation, a java.lang.ClassCastException is thrown from the Abstract Window Toolkit (AWT) event-dispatching thread when the mouse moves over the applet's frame, as shown in the following code:

        java.lang.ClassCastException: sun.plugin....
         at MyApplet.mouseExit(Unknown Source)
         at java.awt.Component.handleEvent(Unknown Source)
         at java.awt.Component.postEvent(Unknown Source)
         at java.awt.Component.dispatchEventImpl(Unknown Source)
         at java.awt.Container.dispatchEventImpl(Unknown Source)
         at java.awt.Component.dispatchEvent(Unknown Source)
         at java.awt.EventQueue.dispatchEvent(Unknown Source)
         at java.awt.EventDispatchThread.pumpOneEventForHierarchy(Unknown Source)
         at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
         at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
         at java.awt.EventDispatchThread.run(Unknown Source)

The applet runs without any error with the Microsoft Virtual Machine (VM).

Cause

This exception has two possible causes:

• To track mouse events on the applet's frame, the applet tries to register a MouseListener event on the frame. In the Microsoft VM implementation, the applet's direct parent in the AWT hierarchical component tree is the frame. So, some applets rely on the following code:
  
  public void foo()
{
        Frame f = (Frame) getParent();
        ....
}
   
  But, applet's direct parent is specific to the implementation and might change. Because the Sun JRE has other containers between the applet and the frame, getParent() method does not return a Frame object, and the above code results in java.lang.ClassCastException error.
   
• In the Microsoft VM implementation, java.applet.AppletContext interface is implemented by the applet frame. So, some applets rely on the following code:
  
  public void foo()
{
        Frame f = (Frame) getParent();
        ....
        AppletContext ac = (AppletContext) f;
        ....
}

  But applet's direct parent is specific to the implementation and might change. Because the Sun JRE implements AppletContext using a different object, the above code results in a java.lang.ClassCastException.

Resolution

In the first cause, navigate the entire AWT hierarchical component tree from the applet to locate the frame, instead of relying on a frame at a particular level. The following code shows how you can navigate the entire component tree:

public void foo()
{
      // Navigate component tree
      Container c = getParent();
      while (c != null && (c instanceof Frame) == false)
              c = c.getParent();

      // Cast Container to Frame
      if (c instanceof Frame)
      {
          Frame f = (Frame) c;
               ...
      }
}

In the second cause, access AppletContext using the Applet.getAppletContext() method as shown in the following code:

        public void foo()
    {
         ....
         AppletContext ac = (AppletContext) getAppletContext();
         ...
    }
 

Related Information

        None.

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