From ed95a2be0fc31c6a28fdaea6c876eae52f2905d4 Mon Sep 17 00:00:00 2001
From: JerryYanWan <ywanad@connect.ust.hk>
Date: Tue, 4 Jul 2017 21:21:26 +0800
Subject: [PATCH] revise hashCode to avoid null pointer