Saturday, February 01, 2014

The Java fake return trick

Sometimes we have a method that does nothing but throw an exception (after munging arguments a bit). For example:

static void fail(Bob bob, Fred fred, Exception cause) {
    throw new SadError(format(
            "Sorry, but {} and {} did not meet today",
            bob, fred), cause);
}

Great when used as:

void friends(Bob bob, Fred fred) {
    try {
        meetAgain(bob, fred);
    } catch (MissedTrainException e) {
        fail(bob, fred, e);
    }
}

But what about this?

PhoneNumber exchange(Bob bob, Fred fred {
    try {
        return beamBusinessCards(bob, fred);
    } catch (LostPhoneException e) {
        fail(bob, fred, e);
        return null; // Never reached
    }
}

There's a neat generics trick to help:

static <R> R fail(Bob bob, Fred fred, Exception cause) {
    // Code unchanged - return ignored
}

Now we can write:

PhoneNumber exchange(Bob bob, Fred fred {
    try {
        return beamBusinessCards(bob, fred);
    } catch (LostPhone e) {
        return fail(bob, fred, e);
    }
}

The main downside is readability for the casual reviewer. Unless expecting this technique, he may think the new catch body returns a value. It may help to limit use to Exception, forcing handling in the caller method, but not with RuntimeException.

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