Declaring an XSLT Extension Function

import org.w3c.dom.*; 
import java.lang.Double; 
public class NumberUtils 
{  
  public Object Average(NodeList nl) 
  { 
    double nSum = 0; 
  for (int i = nl.getLength() - 1; i >= 0; i--) 
   { 
    nSum +=  
    Double.valueOf(nl.item(i). 
    getNodeValue()).doubleValue(); 
   } 
  return new Double(nSum / nl.getLength()); 
  } 
}