Re: operator precedance

Thomas Hawtin <>
Thu, 07 Dec 2006 20:32:04 +0000
<457879f8$0$8757$> wrote:

I looked up that precedance should be + --> == --> ?: but still I am
confused, I was wondering
if anyone can tell me how this will be evaluated.

volume == null ? "" : volume + file == null ? "" : file + version ==
null ? "" : version;

It doesn't help that it is all one long line. You seem to have got the
precedence, but not the associativity.

a-b-c means (a-b)-c not a-(b-c)

a=b=c means a=(b=c) not (a=b)=c

a?b:c?d:e means a?b:(c?d:e) not (a?b:c)?d:e

That is to say, the ?: operator has right to left associativity.

Going back to the original example, we can write it more clearly:

String display =
     volume==null ? "" :
     volume+file==null ? "" :
     file+version==null ? "" :

Of course an expression a+b for any a, b will never result in null. So,
presumably what was meant was:

String display =
     (volume ==null ? "" : volume ) +
     (file ==null ? "" : file ) +
     (version==null ? "" : version);

Or more explicitly:

StringBuilder display = new StringBuilder();
if (volume != null) {
if (file != null) {
if (version != null) {

My approach to the ternary operator:
  o Layout the code out sensibly.
  o Line things up, where it makes sense to do so.
  o The condition is often of the form a==b. Don't use parentheses for
that, but do for anything complicated.
  o Generally don't put spaces in the condition subexpression.
  o For most expressions there is no issues of precedence between the ?
and :, so don't bother with parentheses there.
  o If the last expression is not a simple variable/literal/method add
  o Do not use ?: where it does not match the problem.
  o If things get too complicated, revert to if-else statements.

[Disclaimer: I have not compiled or tested any of this code.]

Tom Hawtin

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