code: private void addGreeting(String guestbookName, String user, String message)
        throws DatastoreException {
      Entity.Builder greeting = Entity.newBuilder();
      greeting.setKey(makeKey(GUESTBOOK_KIND, guestbookName, GREETING_KIND));
      greeting.getMutableProperties().put(USER_PROPERTY, makeValue(user).build());
      greeting.getMutableProperties().put(MESSAGE_PROPERTY, makeValue(message).build());
      greeting.getMutableProperties().put(DATE_PROPERTY, makeValue(new Date()).build());
      Key greetingKey = insert(greeting.build());
      System.out.println("greeting key is: " + greetingKey);
    }
docs: Add a greeting to the specified guestbook.