java的List中使用filter过滤出符合特定条件的元素List
http://blog.csdn.net/zhangchao19890805/article/details/53871219
Java 8 中的 Streams API 详解
https://www.ibm.com/developerworks/cn/java/j-lo-java8streamapi/
https://www.ibm.com/developerworks/cn/java/j-java-streams-2-brian-goetz/index.html
http://www.adam-bien.com/roller/abien/entry/java_8_groupingby_example