The behavior of comparator

Question

Hi
How do the following codes produce different results? (Both are part of a Comparator implementation)
public final int compare ( Object a, Object b )
            return( (Comparable)a ).compareTo( b );
public final int compare ( Object a, Object b )
      // reverse the usual roles of a and b
      return( (Comparable)b ).compareTo( a );
   }Actually how do the caller of the comparator method utilize the return value?
I mean say the list is {5,4,....}
so a = 5, b = 4
In the first case, a<b so the return value is negative. and for the second case b>a so the return value is positive.
If the caller wants to put negative values first and positive values next, then in both cases a should come before b. No where in the comparator I have mentioned that In the first case I want to put negative values first and in the second case positive values first.
So please somebody clarify the things.

Answer 1

amishera2006 wrote:
Actually how do the caller of the comparator method utilize the return value? When you're using Comparable, try to think of compareTo as determining whether something is absolutely greater or less than something else, NOT whether it is before or after something else. Typically, when you use Comparable, it's because two things can be compared in such a way. For instance, it's natural to say that a date in the year 1900 happened before a date in the year 2000. Dates have a natural ordering. It wouldn't be natural to say that two cars have an intrinsic, deferential and natural ordering, so implementing Comparable for a class "Car" wouldn't make much sense.
When you see this, you also see that compareTo should return the same thing, no matter if you are trying to sort forward or in reverse. According to the Javadoc, compareTo() "Returns a negative integer, zero, or a positive integer as this object is less than, equal to, or greater than the specified object" -- not depending on how you choose to sort your objects.
To sort backwards you'd just create a Comparator like in your second example.