hashMap如何工作
http://howtodoinjava.com/2012/10/09/how-hashmap-works-in-java/
hashCode()和equals()方法相关分析
http://howtodoinjava.com/2012/10/09/working-with-hashcode-and-equals-methods-in-java/
如何设计好的hashMap key
http://howtodoinjava.com/2013/05/02/how-to-design-a-good-key-for-hashmap/
如何创建不可变的类
http://howtodoinjava.com/2012/10/28/how-to-make-a-java-class-immutable/
hashmap工作机制（比较深入）
http://java.dzone.com/articles/hashmap-internal