Re: Best way to check if all elements in a List are unique

From:

"Mike Schilling" <mscottschilling@hotmail.com>

Newsgroups:

comp.lang.java.programmer

Date:

Wed, 3 Mar 2010 13:33:46 -0800

Message-ID:

<hmmkjs$m56$1@news.eternal-september.org>

Tom Anderson wrote:

On Tue, 2 Mar 2010, Lew wrote:

Mike Schilling wrote:

boolean areListElementsUnique(List<?> l)
{
return l.size() == new HashSet<Object>(l).size();
}

laredotornado wrote:

Winner! -

Lew wrote:

Don't forget to null-check the argument!

Tom Anderson wrote:

The method already does that - if it's null, you get a
NullPointerException.

Umm, the point of my comment was to *avoid* the NPE, duhh. Why
cause trouble for the method's client?

I'm hoping that you're being sarcastic. Or that i've misunderstood - i
read your comment as suggesting that the method should null-check the
incoming argument.

The right thing for this method to do if passed a null argument is to
throw an exception. There is no uncertainty about that. And it already
does that, so no check is necessary.

Sometimes the check is necessary, of course. Consider
Collections.unmodifiableList(), for which a naive implementation goes
something like

    public List unmodifaibleList(List list)
    {
        class UnmodifiableList implments List
        {
            private List list;

            UnmodifiableList(List list)
            {
                this.lst = list;
            }

            public get(int i)
            {
                return list.get(i);
            }

            // etc.
        }

        return new UnmodifiableList(list);
    }

That will work, but it's much beter for the UnmodifiableList constructor to
be

            UnmodifiableList(List list)
            {
                if (list == null)
                    throw new NullPointerException();
                this.lst = list;
            }

so that the exception is thrown immediately rather than waiting until the
new list is first accessed.