解释:
http://blog.csdn.net/on_1y/article/details/30109975
http://blog.csdn.net/ztkhhhhhd/article/details/53235384
https://zhuanlan.zhihu.com/p/22293328
Comparison method violates its general contract! 原因分析
https://miaoxinguo.github.io/java/2016/04/04/fix.compare.html