Sorting custom class array-list string using Collections.sort

Question

I am trying to sort my custom class array-list using Collections.sort by declaring my own anonymous comparator. But the sort is not working as expected.

My code is

Collections.sort(arrlstContacts, new Comparator<Contacts>() {

        public int compare(Contacts lhs, Contacts rhs) {

            int result = lhs.Name.compareTo(rhs.Name);

            if(result > 0)
            {
                return 1;

            }
            else if (result < 0)
            {
                return -1;
            }
            else
            {
                return 0;
            }
        }
    });

The result is not in sorted order.

You know you can just use "return lhs.Name.compareTo(rhs.Name)" ? — Adam
– Adam, Commented Apr 10, 2012 at 14:58
Under which conditions is this not working? Try stepping through with the debugger and/or writing some unit tests. — Eric Levine
– Eric Levine, Commented Apr 10, 2012 at 15:00

Nick Holt · Accepted Answer · 2012-04-10 15:44:00Z

10

Like Adam says, simply do:

Collections.sort(
  arrlstContacts, 
  new Comparator<Contacts>() 
  {
    public int compare(Contacts lhs, Contacts rhs) 
    {
      return lhs.Name.compareTo(rhs.Name);
    }
  }
);

The method String.compareTo performs a lexicographical comparison which your original code is negating. For example the strings number1 and number123 when compared would produce -2 and 2 respectively.

By simply returning 1, 0 or -1 there's a chance (as is happening for you) that the merge part of the merge sort used Collections.sort method is unable to differentiate sufficiently between the strings in the list resulting in a list that isn't alphabetically sorted.

edited Apr 10, 2012 at 15:44

answered Apr 10, 2012 at 15:36

Nick Holt

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Comments

Mark Pazon · Accepted Answer · 2012-04-10 15:29:42Z

7

As indicated by Adam, you can use return (lhs.Name.compareTo(rhs.Name)); likeso:

Collections.sort(arrlstContacts, new Comparator<Contacts>() {
     public int compare(Contacts lhs, Contacts rhs) {
         return (lhs.Name.compareTo(rhs.Name));
     }
});

edited Apr 10, 2012 at 15:29

answered Apr 10, 2012 at 15:12

Mark Pazon

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2 Comments

Paul Bellora Over a year ago

Might want to cite Adam's comment.

Mark Pazon Over a year ago

Didn't see that it has been answered already in the comments. Editing... Thanks!

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