guava 学习笔记 使用瓜娃（guava）的选择和预判断使代码变得简洁

1，本文翻译自 https://eclipsesource.com/blogs/2012/06/06/cleaner-code-with-guava-optionals-and-preconditions/，有说的不对的地方，欢迎斧正。1，本文翻译自 https://eclipsesource.com/blogs/2012/06