JavaServer Pages (JSPs) - An Introduction to Network Programming with Java - page 284

Give these classes empty constructor bodies, defi ne the toString method for
each to return a meaningful error message and place the classes in a package
called errorExceptions . Create a subdirectory of classes called errorExceptions
and save these Exception classes (at least the .class fi les) into this directory.
(ii) Modify the JSP of the preceding exercise so that it imports package
errorExceptions and throws an object of one of the three Exception classes just
defi ned, according to whether the fi rst operand, the second operand or both is/
are non-numeric. (This involves some interesting logic that includes nested
try blocks.) If you are going to change the name of your error page (which
will require modifi cation, as noted below), remember to change its name in this
JSP's page directive.
(iii) Modify the associated JSP error page so that it makes use of the toString
method of the implicit exception object (if it didn't already do so) and allows
return to the correct JSP.