Re: Top Ten Errors Java Programmers Make

From:
Patricia Shanahan <pats@acm.org>
Newsgroups:
comp.lang.java.help
Date:
Wed, 09 Dec 2009 08:01:22 -0800
Message-ID:
<xf6dnUgx5cJPVILWnZ2dnUVZ_jpi4p2d@earthlink.com>
Eric Sosman wrote:

On 12/9/2009 7:59 AM, Patricia Shanahan wrote:

RedGrittyBrick wrote:

[...]
Nowadays I try to avoid making comments that make work for myself.
It's a good thing to remove comments that merely restate what the code
says. These are the sorts of comments that are most likely to need
maintenance when code changes.


I agree with this except for two types of comments:

1. Interface descriptions, such as Javadoc comments. Someone who just
wants to know the preconditions and postconditions for a method should
not have to read its code, let alone the rest of the code in its class.

2. Descriptions of variables. Even if the meaning of a variable can be
deduced from the code, it is often much easier to read the code
knowing what its variables mean.


    May I add a third type? The "why" comment for a section
of code can be important, the comment that explains (or cites)
the reason for doing something (or for not doing it).

    /* Rebalance the Frammis to minimize the flimflam margin
     * (algorithm P from TAOCP section 3.1.4.1.6)
     */

    /* No need to synchronize here: We already hold the
     * Lochinvar lock
     */

    /* Make the defensive copy *after* instantiating the
     * MuggleModel
     */

... and so on. There's a fairly extreme example in a piece of
my own code, where a two-line method

    private static int peakUsers(int users, double frac) {
        double stdev = Math.sqrt(users * frac * (1 - frac));
        return (int)Math.round(users * frac + 2 * stdev);
    }

... also carries a thirty-two-line comment giving the reasoning
behind the two lines. The "what" is clear from the code; the
"why" requires explanation.


I agree that those comments are needed, but assumed that case was
covered by not being "comments that merely restate what the code says".
I think the interface and variable explanation comments should be there
even if they contain no data that could not be deduced from a detailed
examination of the code.

Patricia

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