code: void addExtent(String classname, ClassDescriptor cld)
      {
          synchronized (extentTable)
          {
              extentTable.put(classname, cld);
          }
      }
docs: Add a pair of extent/classdescriptor to the extentTable to gain speed
  while retrieval of extents.
  @param classname the name of the extent itself
  @param cld the class descriptor, where it belongs to