I use  jExcelAPI on Windows XP and I get an error when I try to
read an excel file
that is already open --> my guess is that you would also with
POI, but it is just a guess.

Then you will /have/ to talk program-to-program.  You won't be able to get the
data you want by reading a file since there's a fair chance that the user
hasn't saved the file yet, and anyway the "current cursor position" is unlikely
to be part of the saved file (but with MS Office stuff, one never knows).

You /might/ be able to get an effect similar to what you are looking for by
having the user select and copy the cell to the Windows clipboard, whence (I
think, but I've never looked into it) you can retrieve it from Java.  I have no
idea what format the data would turn out to be.

Otherwise, you'll probably have to use COM or DDE just as you would if you were
writing a C++ or VB app.  There are Java<->COM bridges available (both free and
commercial); I'd guess that there are Java<->DDE bridges too, but I haven't
come across one myself (but then, I haven't looked).

   -- chris

It's easier then you might think.
The way to do it is by using automation.
To do this in JAVA, you must have a JAVA-COM bridge library.
Fortunately, there are some out there.
The one I used is free and open-source: JACOB.
http://danadler.com/jacob/
You can download the latest from Sourceforge:
http://sourceforge.net/projects/jacob-project/
Here is a little example I made to extract the value of one cell in an
already running Excel sheet:

import com.jacob.activeX.ActiveXComponent;
import com.jacob.com.ComThread;
import com.jacob.com.Dispatch;

public class ReadOpenedExcel
{

   public static void main(String[] args)
   throws Exception
   {
       new ReadOpenedExcel().exec();
   }

   void exec()
   throws Exception
   {
       try
       {
           ComThread.InitMTA();
           ActiveXComponent component =
ActiveXComponent.connectToActiveInstance("Excel.Application");

           if (component != null)
           {
               Dispatch comExcel = component.getObject();
               Dispatch comRange = Dispatch.get(comExcel,
"ActiveCell").toDispatch();
               double value = Dispatch.get(comRange,
"Value").toDouble();

               System.out.println("value=" + value);
           }
       }
       finally
       {
           ComThread.Release();
       }
   }
   
}

Regards