[java] Sorting Values of Set

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

Use the Integer wrapper class instead of String because it is doing the hard work for you by implementing Comparable<Integer>. Then java.util.Collections.sort(list); would do the trick.

Strings are sorted lexicographically. The behavior you're seeing is correct.

Define your own comparator to sort the strings however you prefer.

It would also work the way you're expecting (5 as the first element) if you changed your collections to Integer instead of using String.

You need to pass in a Comparator instance to the sort method otherwise the elements will be sorted in their natural order.

For more information check Collections.sort(List, Comparator)

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

You're using the default comparator to sort a Set<String>. In this case, that means lexicographic order. Lexicographically, "12" comes before "15", comes before "5".

Either use a Set<Integer>:

Set<Integer> set=new HashSet<Integer>();
set.add(12);
set.add(15);
set.add(5);

Or use a different comparator:

Collections.sort(list, new Comparator<String>() {
    public int compare(String a, String b) {
        return Integer.parseInt(a) - Integer.parseInt(b);
    }
});