Exception Handling
his chapter examines Java's exception-handling mechanism. An exception is an abnormal
condition that arises in a code sequence at run time. In other words, an exception is a
run-time error. In computer languages that do not support exception handling, errors
must be checked and handled manually--typically through the use of error codes, and so
on. This approach is as cumbersome as it is troublesome. Java's exception handling avoids
these problems and, in the process, brings run-time error management into the object-
oriented world.
Exception-Handling Fundamentals
A Java exception is an object that describes an exceptional (that is, error) condition that has
occurred in a piece of code. When an exceptional condition arises, an object representing
that exception is created and thrown in the method that caused the error. That method may
choose to handle the exception itself, or pass it on. Either way, at some point, the exception
is caught and processed. Exceptions can be generated by the Java run-time system, or they
can be manually generated by your code. Exceptions thrown by Java relate to fundamental
errors that violate the rules of the Java language or the constraints of the Java execution
environment. Manually generated exceptions are typically used to report some error condition
to the caller of a method.
Java exception handling is managed via five keywords: try, catch, throw, throws, and
finally. Briefly, here is how they work. Program statements that you want to monitor for
exceptions are contained within a try block. If an exception occurs within the try block, it is
thrown. Your code can catch this exception (using catch) and handle it in some rational manner.
System-generated exceptions are automatically thrown by the Java run-time system. To
manually throw an exception, use the keyword throw. Any exception that is thrown out of
a method must be specified as such by a throws clause. Any code that absolutely must be
executed after a try block completes is put in a finally block.
This is the general form of an exception-handling block:
try {
// block of code to monitor for errors
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