Sorting an ArrayList<MyObject> by a member of said MyObject

[Greg Reeder]

Hello everyone.

I have an ArrayList<> which holds objects. These objects contain among other things java.sql.Date() fields. I want to sort the ArrayList by this field. Does anyone know the best way to go about doing this?

Thank you in advance,


Greg

[Marco Masi]

Mmm the easiest way is use Collections utility and one of two:
implements Comparable it that sorting is the natural sorting of you object and use Collections.sort(List<T>)
write a custom Comparator and use Collections.sort(List<T>,Comparator<T>)

A comparator could be something like that (Date is already comparable ^__^)

public class MyObjectDateComparator extends Comparator<MyObject>
{
   public int compare( MyObject o1, MyObject o2 )
   {
      // Pay attention to null dates! It could result in a NullPointerException... in case you should handle it
      o1.getDate().compareTo(o2.getDate);
   }
}

[Greg Reeder]

wouldnt I have to call a swap() somewhere?

[Marco Masi]

You can call swap if you just need to... swap ( ) two known elements. If you need to sort the entire list just use sort (with or without comparator as I wrote before). You could write your own sorting algorithm but I wouldn't do it if not in real special cases, it handle sorting quite well.

[Greg Reeder]

I started to try to build that comparitor as per your code, but i cannot find Comparator in the libraries. Is that a standard class?

[James Sabre]

Greg Reeder wrote:I started to try to build that comparitor as per your code, but i cannot find Comparator in the libraries. Is that a standard class?

java.util.Comparator

[Greg Reeder]

I finally figured it out. Thank you for all of your help. Rather than making a new class as per your example, I was able to just use an interface and override 1 method. The was done right where i needed it upon retrieving the arraylist from the db. Thanks again

Greg

[Rob Spoor]

Can't you make the database do the sorting for you? Simply specify an ORDER BY clause.

[Greg Reeder]

I could. But that would require making another method in my DBHelper object. i already have the list (sorted the way I want it from the DB), and this is just 1 special case. I figured it would be faster, not to mention easier, to just use a sort function with an already existing arraylist. At any rate, this issue is now solved. on to the next beast -- adding a column with calculated values in my JTable.