Re: IllegalArg vs NullPtr Exceptions

From:

Tom McGlynn <tam@milkyway.gsfc.nasa.gov>

Newsgroups:

comp.lang.java.programmer

Date:

Wed, 30 Sep 2009 15:09:25 -0700 (PDT)

Message-ID:

<2f464af7-3db8-4d5f-9b61-7bd7172d21c7@e34g2000vbm.googlegroups.com>

On Sep 30, 1:26 am, "Scott A. Hightower" <VastEr...@SpamGourmet.com>
wrote:
....

A related question is whether to test for an empty List passed as an
argument value and throw an IllegalArgumentException or to let
NoSuchElementException through and document that it is *supposed to* happen.

The utility accepts Lists as arguments to several public methods. The only
thing that it does with each List is get an Iterator from it and extract the
elements of the list for comparison and storage.

A null List or a List with no elements is meaningless in the context of the
utility. The documentation explains why and currently warns that a
NullPointerException or NoSuchElementException will be thrown, respectively.

....

While it's very possible that your situation differs, I would be chary
of explicitly
precluding non-null lists of 0 length. I've frequently found that
being able to treat lists or arrays of 0 length the same as lists or
arrays with elements makes for cleaner logic. Are you making callers
of your utility write

   if (list.size() > 0) {
       utility(list);
   }

rather than just

   utility(list);

so that you are transferring the cost of understanding the special
status
of 0 length lists a level further up the chain? That may be right but
I've generally been happier with the other choice. I'd have my
utility method do nothing with an empty list.

Typically with the for each syntax, code handles the 0 length case
transparently.

    void myUtility(List<X> input) {
        for (X val: input) {
            ...
        }
    }

It sounds like you want to do something special with the first
element. Then perhaps

        boolean first = true;
        for (X val: input) {
            if (first) {
                first = false;
                ...
            }
            ...
        }

[Without the checks for nulls that have been discussed elsewhere in
this thread].

Just a thought. Ymmv.

    Regards,
    Tom McGlynn