Re: About using assertion

Robert Klemme <>
Tue, 10 May 2011 07:23:42 +0200
On 10.05.2011 00:49, Lew wrote:

Robert Klemme wrote:

Lew, I am not sure what you tried to convey with this posting. I for
my part
would say that the assert is a tad too much here since the if clause
that gives me enough "confidence" that arg is actually >= 0 at that
line. If

First, the example is deliberately simple to highlight the structural
location for 'assert' statements. Formally, they go where invariants are.

The exception gives that confidence, yes, but only just now at
code-inspection time. At this level, the assertion provides not much
more than compiler-enforced documentation of the invariant. However,
refactoring, inheritance and other future actions could violate the
invariant. This would manifest in production, where source code is not
immediately convenient.

OK, I see what you mean. Only trouble with refactoring (as with any
code changes) is that you need to watch out which parts you extract as
method. The assert could end in less useful than optimal place. :-)

Ops guys can use assertions to locate which invariants were violated,
helping diagnose and triage the problem.

it isn't then I have bigger problems than calculating square roots of
numbers. :-)

True but only at one moment in time. Assertions live ongoingly and can
be re-enabled at trouble time.

It's probably a different story if the calculation is done by a
private method
in which case I'd probably add the assert to the beginning of that
method just
to be sure the caller (which can only be in the same class or nested
did not make a mistake.

There is art in the decision of which invariants to document. I like to
document all of them. Why not? Others only document a few. Why? I've
been on the ops end of production code quite a few times, so I find
assertions valuable. I studied math way back when, and I appreciate
their formal value. Pragmatically there is no reason to avoid them, and
good reasons to use them liberally, if strategically.

Absolutely! As you said: The fact that you can enable and disable them
at will *after* compile time is a big plus of Java.

There is no real art to deciding where assertions go if you do use them.
They go at the algorithmic invariant points. I assert that you should
use them wherever they support operations, and in the largest proportion
of invariant points consistent with that goal. That is a matter of your
strategy and style.

Thanks for clarifying, Lew!



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