NSComparator - sorting objects in a NSMutableArray

I have an NSMutableArray populated with objects. I want do sort them
based on a double type property. sortUsingComparator seems to be the
way to go. this means i need to implement a "compare" function in the
object.

 this is the first time i'm tackling this technique and i'm finding
the javadocs too terse to understand. does anyone know where i can
find an example of this?

thanks,
matt