14 exception handling

ICS 313 - Fundamentals of Programming Languages 1
14. Exception Handling
14.1 Intro to Exception Handling
In a language without exception handling
When an exception occurs, control goes to the operating system, where a
message is displayed and the program is terminated
In a language with exception handling
Programs are allowed to trap some exceptions, thereby providing the
possibility of fixing the problem and continuing
Many languages allow programs to trap input / output errors (including
EOF)
An exception is any unusual event, either erroneous or not, detectable
by either hardware or software, that may require special processing
The special processing that may be required after detection of an
exception is called exception handling
The exception handling code unit is called an exception handler

14.1 Intro to Exception Handling (continued)
An exception is raised when its associated event occurs
A language that does not have exception handling capabilities can
still define, detect, raise, and handle exceptions
Alternatives:
Send an auxiliary parameter or use the return value to indicate the
return status of a subprogram
Pass a label parameter to all subprograms (error return is to the
passed label)
Pass an exception handling subprogram to all subprograms
Advantages of Built-in Exception Handling:
Error detection code is tedious to write and it clutters the program
Exception propagation allows a high level of reuse of exception
handling code
Design Issues for Exception Handling:
How and where are exception handlers specified and what is their
scope?
How is an exception occurrence bound to an exception handler?
Where does execution continue, if at all, after an exception handler
completes its execution?
How are user-defined exceptions specified?
Should there be default exception handlers for programs that do not
provide their own?
Can built-in exceptions be explicitly raised?
Are hardware-detectable errors treated as exceptions that can be
handled?
Are there any built-in exceptions?
How can exceptions be disabled, if at all?

Exception handling-control flow
14.4 Exception Handling in C++
Added to C++ in 1990
Design is based on that of CLU, Ada, and ML
Exception Handlers
Form:
try {
-- code that is expected to raise an exception
}
catch (formal parameter) {
-- handler code
}
...
catch (formal parameter) {
-- handler code
}
catch is the name of all handlers - it is an overloaded name, so the formal parameter of each must be
unique
The formal parameter need not have a variable - It can be simply a type name to distinguish the handler it
is in from others
The formal parameter can be used to transfer information to the handler
The formal parameter can be an ellipsis, in which case it handles all exceptions not yet handled

14.4 Exception Handling in C++ (continued)
Binding Exceptions to Handlers
Exceptions are all raised explicitly by the statement:
throw [expression];
The brackets are metasymbols
A throw without an operand can only appear in a handler;
when it appears, it simply reraises the exception, which is then
handled elsewhere
The type of the expression disambiguates the intended handler
Unhandled exceptions are propagated to the caller of the
function in which it is raised
This propagation continues to the main function
If no handler is found, the program is terminated
14.4 Exception Handling in C++ (continued)
Continuation
After a handler completes its execution, control flows to the first statement
after the last handler in the sequence of handlers of which it is an element
Other Design Choices
All exceptions are user-defined
Exceptions are neither specified nor declared
Functions can list the exceptions they may raise - Without a specification,
a function can raise any exception (the throw clause)
See program listing (pp. 577-578)
Evaluation
It is odd that exceptions are not named and that hardware- and system
software-detectable exceptions cannot be handled
Binding exceptions to handlers through the type of the parameter certainly
does not promote readability

14.5 Exception Handling in Java
Based on that of C++, but more in line with OOP philosophy
All exceptions are objects of classes that are descendants of
the Throwable class
The Java library includes two subclasses of Throwable :
Error
Thrown by the Java interpreter for events such as heap underflow
Never handled by user programs
Exception
User-defined exceptions are usually subclasses of this
Has two predefined subclasses, IOException and RuntimeException (e.g.,
ArrayIndexOutOfBoundsException and NullPointerException)
14.5 Exception Handling in Java (continued)
Java Exception Handlers
Like those of C++, except every catch requires a named
parameter and all parameters must be descendants of
Throwable
Syntax of try clause is exactly that of C++
Exceptions are thrown with throw, as in C++, but often the
throw includes the new operator to create the object, as in:
throw new MyException();
Binding an exception to a handler is simpler in Java than it is in
C++
An exception is bound to the first handler with a parameter is the same class as the
thrown object or an ancestor of it
An exception can be handled and rethrown by including a
throw in the handler (a handler could also throw a different
exception)

Continuation
If no handler is found in the try construct, the search is
continued in the nearest enclosing try construct, etc.
If no handler is found in the method, the exception is
propagated to the method’s caller
If no handler is found (all the way to main), the
program is terminated
To insure that all exceptions are caught, a handler can be
included in any try construct that catches all exceptions
Simply use an Exception class parameter
Of course, it must be the last in the try construct
Other Design Choices
The Java throws clause is quite different from the throw clause of C++
Exceptions of class Error and RunTimeException and all of their
descendants are called unchecked exceptions
All other exceptions are called checked exceptions
Checked exceptions that may be thrown by a method must be either:
Listed in the throws clause, or
Handled in the method
A method cannot declare more exceptions in its throws clause than the method
it overrides
A method that calls a method that lists a particular checked exception in its
throws clause has three alternatives for dealing with that exception:
Catch and handle the exception
Catch the exception and throw an exception that is listed in its own throws clause
Declare it in its throws clause and do not handle it

See Example program (pp. 582-583)
The finally Clause
Can appear at the end of a try construct
Purpose: To specify code that is to be executed, regardless of what
happens in the try construct
A try construct with a finally clause can be used outside
exception handling
Evaluation
The types of exceptions makes more sense than in the case of C++
The throws clause is better than that of C++ (The throw clause in
C++ says little to the programmer)
The finally clause is often useful
The Java interpreter throws a variety of exceptions that can be
handled by user programs

14 exception handling

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14 exception handling