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 they rely on programmer vigilance in following an agreed-upon convention that is not enforced by the language. If programmers are not vigilant, which often occurs when 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 recover reliably from a bad situation. Instead of just exiting the program, you are often able to set things right and restore the execution of a program, which produces much more robust systems.

It’s worth noting that exception handling isn’t an object-oriented feature, although in object-oriented languages the exception is normally represented with an object. Exception handling existed before object-oriented languages.

Exception handling is only lightly introduced and used in this Volume; Volume 2 (available from www.BruceEckel.com) has thorough coverage of exception handling.