630 Chapter 14 Exception Handling and Event Handling
T
his chapter discusses programming language support for two related parts of
many contemporary programs: exception handling and event handling. Both
exceptions and events can occur at times that cannot be predetermined,
and both are best handled with special language constructs and processes. Some of
these constructs and processes—for example, propagation—are similar for exception
handling and event handling.
We first describe the fundamental concepts of exception handling, including
hardware- and software-detectable exceptions, exception handlers, and the raising
of exceptions. Then, the design issues for exception handling are introduced and
discussed, including the binding of exceptions to exception handlers, continuation,
default handlers, and exception disabling. This section is followed by a description
and an evaluation of the exception-handling facilities of three programming lan-
guages: Ada, C++, and Java.
The latter part of this chapter is about event handling. We first present an
introduction to the basic concepts of event handling. This is followed by discussions
of the event-handling approaches of Java and C#.14.1 Introduction to Exception Handling
Most computer hardware systems are capable of detecting certain run-time
error conditions, such as floating-point overflow. Early programming lan-
guages were designed and implemented in such a way that the user program
could neither detect nor attempt to deal with such errors. In these languages,
the occurrence of such an error simply causes the program to be terminated
and control to be transferred to the operating system. The typical operating
system reaction to a run-time error is to display a diagnostic message, which
may be meaningful and therefore useful, or highly cryptic. After displaying the
message, the program is terminated.
In the case of input and output operations, however, the situation is some-
what different. For example, a Fortran Read statement can intercept input
errors and end-of-file conditions, both of which are detected by the input
device hardware. In both cases, the Read statement can specify the label of
some statement in the user program that deals with the condition. In the case
of the end-of-file, it is clear that the condition is not always considered an error.
In most cases, it is nothing more than a signal that one kind of processing is
completed and another kind must begin. In spite of the obvious difference
between end-of-file and events that are always errors, such as a failed input
process, Fortran handles both situations with the same mechanism. Consider
the following Fortran Read statement:Read(Unit=5, Fmt=1000, Err=100, End=999) WeightThe Err clause specifies that control is to be transferred to the statement
labeled 100 if an error occurs in the read operation. The End clause speci-
fies that control is to be transferred to the statement labeled 999 if the read