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Thinking in Java
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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 work in many situations but that 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 the programmer is not vigilant—often the case 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 because you aren’t constantly forced to check for errors. In addition, a thrown exception is unlike an error value that’s returned from a method or a flag that’s set by a method 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 the program, you are often able to set things right and restore execution, 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 can sometimes make error handling much easier.

It’s worth noting that exception handling isn’t an object-oriented feature, although in object-oriented languages the exception is normally represented by an object. Exception handling existed before object-oriented languages.
Thinking in Java
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   Reproduced courtesy of Bruce Eckel, MindView, Inc. Design by Interspire