Sorting Values of Set

Question

I am trying to sort elements of a set but unable to do so far. here is my code which i am trying to do

public static void main(String [] args){     Set<String> set=new HashSet<String>();     set.add("12");     set.add("15");     set.add("5");     List<String> list=asSortedList(set); }  public static <T extends Comparable<? super T>> List<T> asSortedList(Collection<T> c) {   List<T> list = new ArrayList<T>(c);   Collections.sort(list);   return list; } 

but this or other way is not working since its all time giving me the same order in which they have been filled 12,15,5

Answer 1

Use a SortedSet (TreeSet is the default one):

SortedSet<String> set=new TreeSet<String>(); set.add("12"); set.add("15"); set.add("5"); List<String> list=new ArrayList<String>(set); 

No extra sorting code needed.

Oh, I see you want a different sort order. Supply a Comparator to the TreeSet:

new TreeSet<String>(Comparator.comparing(Integer::valueOf)); 

Now your TreeSet will sort Strings in numeric order (which implies that it will throw exceptions if you supply non-numeric strings)

Reference:

• Java Tutorial (Collections Trail):
  • Object Ordering
  • The SortedSet interface
• Javadocs: TreeSet
• Javadocs: Comparator

Answer 2

If you sort the strings "12", "15" and "5" then "5" comes last because "5" > "1". i.e. the natural ordering of Strings doesn't work the way you expect.

If you want to store strings in your list but sort them numerically then you will need to use a comparator that handles this. e.g.

Collections.sort(list, new Comparator<String>() {     public int compare(String o1, String o2) {         Integer i1 = Integer.parseInt(o1);         Integer i2 = Integer.parseInt(o2);         return (i1 > i2 ? -1 : (i1 == i2 ? 0 : 1));     } }); 

Also, I think you are getting slightly mixed up between Collection types. A HashSet and a HashMap are different things.