optional是如何解决NPE的

https://blog.csdn.net/aitangyong/article/details/54564100

https://juejin.im/post/5d33bdeb51882557d67fe513

https://www.cnblogs.com/peida/archive/2013/06/14/guava_optional.html

https://www.runoob.com/java/java8-optional-class.html