多态

http://www.cnblogs.com/200325074/articles/1206116.html#undefined

http://www.cnblogs.com/jhxk/articles/1644018.html

http://www.cnblogs.com/zhangkai2237/archive/2012/12/20/2826734.html

http://blog.csdn.net/huwei2003/article/details/18982039