This is a class that is used to report errors. It provides a
set of static methods that are called to report errors. However, the
actual reporting of the errors is deferred to an instance of this class
that is set using the setMessageHandler() method. Normally there
is only one instance, set up by the application, so the class is
a singleton. But this is not enforced.
This base class simply writes the errors to System.err.
When an applet or application starts up, it may wish to set a subclass
of this class as the message handler, to allow a nicer way of
reporting errors. For example, a swing application will probably
want to report errors in a dialog box, using for example
the derived class GraphicalMessageHandler.