Locale sensitive String sorting in Java

So, the day after I get made a Microsoft MVP I do two posts about Java - go figure.  Anyway, today I had one of those moments where you thought you understood something and then realize you didn't and probably a lot of your code that you've written over the past 10 years doesn't work as well as you thought...  All this with the humble String.compareTo method.

Take the following strings:-

• charlotte
• Chloé
• Raoul
• Real
• Réal
• Rico

In .NET, if you want to perform a standard case insensitive, dictionary based comparison between two strings then you can use the String.Compare method.  This does a culture based, case insensitive comparison.

In Java, if you were to do use the Comparable interface which makes use of the standard String.compareTo method to sort a list, you would end up with:-

• Chloé
• Raoul
• Real
• Rico
• Réal
• charlotte

That is because compareTo looks at the unicode value of the character and sorts on that - which for those of us that tend to live in the ASCII range tends to work ok (only that lowercase letters come after the uppercase ones) - however if you have a language that uses one of the many other characters it doesn't work so well.  If you had a language where M comes before A in the alphabet you are totally screwed.

This is were you should be using the java.text.Collator class in Java.  The Collator class does locale sensitive string comparisons - i.e. allowing you to do a dictionary base sort of a set of strings.

Dope.  One of those classes I should have been using for a while...  I thought I was just being dumb, but then a couple of other people I mentioned this to were not aware of the issue so I thought it worth a blog post.