深入Java集合学习系列:http://zhangshixi.iteye.com/blog/674856
http://blog.csdn.net/shf4715/article/details/47061111
http://blog.csdn.net/u011032983/article/details/52270553
http://www.cnblogs.com/dolphin0520/archive/2012/09/28/2700000.html
LinkedHashMap 保持有序迭代原理:https://my.oschina.net/scjelly/blog/528117