http://wiki.jikexueyuan.com/project/java-collection/hashmap.html,

http://wiki.jikexueyuan.com/project/java-collection/hashset.html,

http://www.codedata.com.tw/java/jdk8-map-default-methods/,

http://irusist.github.io/2016/01/04/Java-8%E4%B9%8BMap%E6%96%B0%E5%A2%9E%E6%96%B9%E6%B3%95/

https://www.google.com/search?site=&source=hp&q=map%E7%9A%84merge%E6%96%B9%E6%B3%95&oq=map%E7%9A%84merge%E6%96%B9%E6%B3%95&gs_l=psy-ab.3...9500.17313.0.17463.11.7.0.0.0.0.387.1011.2-1j2.3.0.iac%2Ccfro%3D1%2Ciazh%3D1...0...1.1.64.psy-ab..8.2.645...0j0i12k1.oF3je9g5Y-Y

map的merge方法和

compute 方法

Java8 hashMap