java collections - keyset() vs entrySet() in map

https://stackoverflow.com/questions/8962459/java-collections-keyset-vs-entryset-in-map

http://blog.csdn.net/lwzcjd/article/details/5432430

http://blog.csdn.net/nuoshueihe/article/details/7874069

http://www.cnblogs.com/mimimimimi/p/4094265.html

https://coderanch.com/t/203203/java/entrySet-keySet-Map