How to sort a Collection<T>?

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

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()

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.