Best way to check if all elements in a List are unique
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From: laredotornado on 1 Mar 2010 17:29 Hi, I"m using Java 1.5. Given a java.util.List that I know to have at least one element, what is the best way to check that all elements in the list are unique ? Thanks, - Dave
From: Lew on 1 Mar 2010 17:43 laredotornado wrote: > I"m using Java 1.5. Given a java.util.List that I know to have at > least one element, what is the best way to check that all elements in > the list are unique ? That depends largely on what you mean by "best", and somewhat on what you mean by "check". If the List contains exactly one element, then you already know that it's unique, so the real problem doesn't come about until it has at least two elements. One way is to dump the List into a Set as you iterate through the List, and for each element see if it's already in the Set. (This is indicated by the Set#add() returning false.) If all you need to do is ensure that each element is unique, you can dump the contents into a Set and use the Set. (The Set implementations with which I'm familiar have constructors that take a Collection as the argument.) If you have a choice about whether to use a List in the first place, just don't, and use a Set instead. Whether any of these constitute "best" I cannot tell from your question. -- Lew
From: Mike Schilling on 1 Mar 2010 17:44 laredotornado wrote: > Hi, > > I"m using Java 1.5. Given a java.util.List that I know to have at > least one element, what is the best way to check that all elements in > the list are unique ? > > Thanks, - Dave boolean areListElementsUnique(List<?> l) { return l.size() == new HashSet<Object>(l).size(); }
From: John B. Matthews on 1 Mar 2010 17:47 In article <95bd0b1b-e372-4981-a6cf-eed5a58e4461(a)u19g2000prh.googlegroups.com>, laredotornado <laredotornado(a)zipmail.com> wrote: > I'm using Java 1.5. Given a java.util.List that I know to have at > least one element, what is the best way to check that all elements in > the list are unique ? You should be able to construct a Set, say TreeSet, of the List elements and see if the sizes match. -- John B. Matthews trashgod at gmail dot com <http://sites.google.com/site/drjohnbmatthews>
From: Lew on 1 Mar 2010 17:49
laredotornado wrote: >> I"m using Java 1.5. Given a java.util.List that I know to have at >> least one element, what is the best way to check that all elements in >> the list are unique ? Mike Schilling wrote: > boolean areListElementsUnique(List<?> l) > { > return l.size() == new HashSet<Object>(l).size(); > } Sweet. You could also generify that: <T> boolean areListElementsUnique( List<T> list ) { if (list == null || list.size() == 1 ) { return true; } return list.size() == new HashSet <T> (list).size(); } In this case the generics add nothing except a sense for the programmer that they're being clever. The 'null' check is important, I swan. -- Lew |