Java Collections.sort doesn't call compare method

Question

I've read all answers etc, but non of them solved my problem.

I have basic function to sort list of custom object array by String InvoiceNumber.

List<Invoice> invoiceList = new ArrayList<>();

...
// invoiceList filled here
...

Collections.sort(invoiceList, new Comparator<Invoice>()
{
    @Override
    public int compare(Invoice o1, Invoice o2)
    {
           return o1.InvoiceNumber.compareTo(o2.InvoiceNumber);
    }
});

Some part of Invoice class.

public class Invoice extends AttributeContainer
implements KvmSerializable,android.os.Parcelable, Comparable
{

public String CustomerId;

public String InvoiceNumber;

However, it never calls compare method.

I'm using android studio with latest libraries. Is there anything that I'm missing?

Post the class / method which utilizes this comparator. We cannot determine why it's not being called (or if it's not even being called for that matter) with the little snippet of code you've posted. — Jake Miller
– Jake Miller, Commented Apr 25, 2017 at 20:01
It's confusing to see InvoiceNumber spelled like a type name but used like a member variable. Which is it? — Lew Bloch
– Lew Bloch, Commented Apr 25, 2017 at 20:02
Note that Invoice shouldn't implement Comparable, but rather Comparable<Invoice>. And you don't really need to implement Comparable<Invoice> (or Comparable) if you're providing a Comparator; but you can, if you so desire. — Andy Turner
– Andy Turner, Commented Apr 25, 2017 at 20:13

Gray · Accepted Answer · 2017-04-25 22:08:15Z

However, it never calls compare method.

The only time that the compare(...) method would not be called is if your list has 0 or 1 items in it. Probably this is a debugging problem with Android and the compare(...) is being called but maybe the breakpoint is being skipped. Putting some sort of log messages inside of the method will confirm this.

I suspect that your problem is here:

public String InvoiceNumber;

First of all the field name should be invoiceNumber with a starting lowercase letter. Also it probably should not be public for data hiding reasons.

But I suspect that real problem is with String comparisons of numbers. For example if you have the following invoice-number strings in your entities:

Then these will sort like:

That's how the String.compare(...) method would sort it because the string "3" is greater than the string "201". If you want them to be sorted as numbers then you should make the invoiceNumber field be an int or long.

Hope this helps.

htafoya · Accepted Answer · 2018-11-24 10:11:50Z

0

I had the same problem when creating anonymous class for the sort method. Try to extract your Comparator class and you'll be able to debug.

answered Nov 24, 2018 at 10:11

htafoya

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