Exception Rules II: The Wrath of Checked

Something I wrote seven years ago; I’m publishing now to see if I’ve learned anything.

This is part of a series where I review common wisdom about Java error handling. The series is in no particular order, but the first installment explains my categorization method.

Use checked exceptions when the client might recover
Importance: medium

The checked exception experiment tested a compelling ideal. Stated in Sun’s tutorial:

Any Exception that can be thrown by a method is part of the method’s public programming interface. Those who call a method must know about the exceptions that a method can throw so that they can decide what to do about them. These exceptions are as much a part of that method’s programming interface as its parameters and return value.

The more Java I write, the more convincing I find Bruce Eckel’s argument that the experiment proved its hypothesis false.

The tutorial writers tell us to use checked exceptions whenever our clients can do something useful to recover. They fail to mention the abstraction-destroying effects of checked exceptions.

To be fair, checked exceptions only destroy abstractions the way alcohol destroys families; if daddy stopped using so much we would be fine. But programmers are human, and humans are lazy. Especially programmers.

Laziness makes programmers suppress errors, but they hide exceptions for good reason. In the typical example, when trying to abstract away the database connection, avoid subjecting your client to SQLException. If SQLException were unchecked, abstractions that neglected its handling it would leak on error, but their programmers would not add the leakiness to their signatures.

Yes, checking the error code and determining whether to retry after waiting or email an administrator or call Ghostbusters is ideal, but only a small fraction of programs actually need that depth of error tolerance. For the majority, wrapping in RuntimeException is often best, but Sun’s tutorial will make you feel guilty about that:

Do not throw a RuntimeException or create a subclass of RuntimeException simply because you don’t want to be bothered with specifying the exceptions your methods can throw.

Ignore it. This is the sort of thinking that encourages silly specifications like FileNotFound, which indicates that the file does not exist. Or is read-only. Or locked. Or a directory. Or for some other reason inaccessible.

All languages I know other than Java work perfectly well without checked exceptions, implying that you can legitimately throw unchecked exceptions only and stop wasting brain cycles on whether you should make the exception checked or not. If, however, you still want to use checked exceptions, follow this simple guideline:

Use checked exceptions only when client code could not have anticipated the error.

FileInputStream, for example, makes itself more irritating by ignoring this advice. Its constructors should not throw FileNotFound because client code should have checked for the file’s existence before trying to open it. [Retraction: I don’t recommend check-then-act style so much any more. Better to ask forgiveness than get permission, thanks again, Python.]

I consider this guideline true even for multi-threaded use because errors of improper synchronization still land in the bucket of exceptions the client should have anticipated.

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