code: public synchronized boolean put(Object key, byte[] value, int len, long expirationTime)
                      throws IOException,
                      EOFException,
                      FileManagerException,
                      ClassNotFoundException,
                      HashtableOnDiskException {
          if (key == null)
              return false;
          //if ( value == null ) return false;  // comment this line to allow the value=NULL
  
          mapPut(key, value, len, expirationTime, -1, null, null, 0, !ALIAS_ID);
  
          return true;
      }
docs: ************************************************************************
  Associates the object with the key and stores it. If an object already
  exists for this key it is replaced and the previous object discard. The
  object is assumed to have been "manually" serialized by the caller and will
  be written up to length "len", directly on disk with no further
  serialization.
  
  @param key The key for the object to store.
  @param value The object to store.
  @param len The maximum number of bytes from "value" to write to disk.
  @param expirationTime The expiration time of this object
  
  @return true if the object is placed into the hashtable
  false oherwise. False only occurs if the key is null.
  
  @exception FileManagerException The underlying file manager has a problem.
  @exception ClassNotFoundException Some key in the hash bucket cannot be
  deserialized while searching to see if the object already exists. The
  underlying file is likely corrupted.
  @exception IOException The underlying file has a problem and is likely
  corrupt.
  @exception EOFxception We were asked to seek beyond the end of the file.
  The file is likely corrupt.
  @exception HashtableOnDiskException The hashtable header is readable but invalid.
  One or more of the following is true: the magic string is invalid, the header
  pointers are null, the header pointers do not point to a recognizable hashtable.
  ***********************************************************************