how to sort a Collection<T>?

Question

Hi All

I have a generic Collection and am trying to work out how I can sort the items contained within it. Ive tried a few things but I cant get any of them working.

Answer 1

You can't if T is all you get. You have to have the it injected by the provider via:

Collection<T extends Comparable>

or pass in the Comparator via the Collections.sort(...) method

Answer 2

A Collection does not have an ordering, so wanting to sort it does not make sense. You can sort List instances and arrays, and the methods to do that are Collections.sort() and Arrays.sort()

Answer 3

Collections by themselves do not have a predefined order, therefore you must convert them to a java.util.List. Then you can use one form of java.util.Collections.sort

Collection< T > collection = ...;

List< T > list = new ArrayList< T >( collection );

Collections.sort( list );
 // or
Collections.sort( list, new Comparator< T >( ){...} );

// list now is sorted

Answer 4

You have two basic options provided by java.util.Collections:

• <T extends Comparable<? super T>> void sort(List<T> list)
  • Use this if T implements Comparable and you're fine with that natural ordering
• <T> void sort(List<T> list, Comparator<? super T> c)
  • Use this if you want to provide your own Comparator.

Depending on what the Collection is, you can also look at SortedSet or SortedMap.

Answer 5

If your collections object is a list, I would use the sort method, as proposed in the other answers.

However, if it is not a list, and you don't really care about what type of Collection object is returned, I think it is faster to create a TreeSet instead of a List:

TreeSet sortedSet = new TreeSet(myComparator);
sortedSet.addAll(myCollectionToBeSorted);