注意：本文档基于google guava 28 wiki
Joiner
连接器
将字符串拼接
Joiner joiner = Joiner.on("; ").skipNulls();
return joiner.join("Harry", null, "Ron", "Hermione");
返回Harry; Ron; Hermione
skipNulls()方法是直接忽略null，使用useForNull(String)方法可以给定某个字符串来替换null，即
Joiner joiner = Joiner.on("; ").useForNull("替换字符串");