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:
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