Exception handling:dealing with errors

Ever since the beginning of programming languages, error handling has been one of the
most difficult issues. Because it’s so hard to design a good error-handling scheme, many
languages simply ignore the issue, passing the problem on to library designers who come up
with halfway measures that can work in many situations but can easily be circumvented,
generally by just ignoring them. A major problem with most error-handling schemes is that
6 Note that this is true only for objects that are created on the heap, with new. However, the problem
described, and indeed any general programming problem, requires objects to be created on the heap.
7 According to a technical reader for this book, one existing real-time Java implementation
(www.newmonics.com) has guarantees on garbage collector performance.
56 Thinking in Java www.BruceEckel.com
they rely on programmer vigilance in following an agreed-upon convention that is not
enforced by the language. If the programmer is not vigilant, which is often if they are in a
hurry, these schemes can easily be forgotten.
Exception handling wires error handling directly into the programming language and
sometimes even the operating system. An exception is an object that is “thrown” from the
site of the error and can be “caught” by an appropriate exception handler designed to handle
that particular type of error. It’s as if exception handling is a different, parallel path of
execution that can be taken when things go wrong. And because it uses a separate execution
path, it doesn’t need to interfere with your normally-executing code. This makes that code
simpler to write since you aren’t constantly forced to check for errors. In addition, a thrown
exception is unlike an error value that’s returned from a function or a flag that’s set by a
function in order to indicate an error condition, These can be ignored. An exception cannot
be ignored so it’s guaranteed to be dealt with at some point. Finally, exceptions provide a
way to reliably recover from a bad situation. Instead of just exiting you are often able to set
things right and restore the execution of a program, which produces much more robust
programs.
Java’s exception handling stands out among programming languages, because in Java,
exception-handling was wired in from the beginning and you’re forced to use it. If you don’t
write your code to properly handle exceptions, you’ll get a compile-time error message. This
guaranteed consistency makes error-handling much easier.
It’s worth noting that exception handling isn’t an object-oriented feature, although in objectoriented
languages the exception is normally represented with an object. Exception handling
existed before object-oriented languages.