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From: Andreas Leitgeb <avl@logic.at>
Newsgroups: comp.lang.java.programmer
Subject: Re: How to handle same sanity checks
Date: Thu, 22 Jun 2017 11:26:36 -0000 (UTC)
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Cecil Westerhof <Cecil@decebal.nl> wrote:
> In my code I have several methods that start with:
>     if ((n < MIN_VAL) || (n > MAX_VAL)) {
>         throw new InvalidParameterException(String.format(ERROR_N, n));
>     }
> That is not really DRY.
> I could write a method like:
>     private void checkN(n) {
>         if ((n < MIN_VAL) || (n > MAX_VAL)) {
>             throw new InvalidParameterException(String.format(ERROR_N, n));
>         }
>     }
> And then replace all the checks with a call to checkN. But then the
> exception is generated in the method checkN instead of the method that
> has a wrong parameter.

I would not worry about where the exception gets thrown from, as long
as the call-stack includes the "real culprit".

If you really want it thrown at the call-site, there are these slightly
more verbose variants:

Let checkN() *return* the exception (or null if all was ok), then:
   for (Exception e=checkN(...);e!=null;) { throw e; }
Yes, it's weird to use a "for" here, but "if" doesn't allow ad-hoc
declaration of the Exception var. If you declare "e" ahead, then it
becomes: (not much better, imho)
   Exception e; if ((e=checkN(...))!=null) { throw e; }
(You could also combine this with Martin Gregorie's suggestions.)

Another approach is, to wrap creation of the exception and the check
separately:
   private Exception invalPar(...) {
      return new InvalidParameterException(String.format( ... ));
   }
   private boolean checkN(...) { return ((n < MIN_VAL) || (n > MAX_VAL)); }

and at call-site:

   if (checkN(...)) { throw invalPar(...); }