> have to take some numbers  from xml  as an input  and
> must do
> calculation in java  and must produce it back  as in
> an another xml
> file.

You can use the classes and methods in Apache Xalan
(freely downloadable) or in Saxon to do this.
To read the xml file you can use the SaxReader and to
write an xml output, you can either use java.io or
DomWriter class.

> Please help  me, Since i couldnt get required
> information  in the net.

The documentation for both these is available as API
and is freely downloadable.

You *will* have to read a bit about XML and the
methods in Sax and Dom before you can attempt this.