Extending Pig SUM function

[Koppula, Abhilash Reddy]

Hi All,
The built in Pig SUM eval function returns null if the bag is empty . How can I 
override this(as simple as possible ) to return 0 instead of null .
  public class CustomSum extends SUM{
   // Which methods to override.
}

Or Is there any way I can do it in script to check if SUM returned null then 
make it 0.

Eg : A = FOREACH (COGROUP OUTER) group as url , (1-$d)+$d*SUM(a.values) as 
totalValue .

I am doing a outer COGROUP . So there are some records in the group which 
doesn't have values and in that case I would like to return constant(1-$d) . 
However as SUM returns null , totalValues is becoming null.


Thanks,
Abhilash