检查字符串 s 不是 null 。 String＃charAt返回的字符是原始的 char 类型，永远不会是 null ：

Check that the String s is not null before doing any character checks. The characters returned by String#charAt are primitive char types and will never be null:

if (s != null) {
  ...

如果您尝试一次处理一个 String 中的字符，您可以使用：

If you're trying to process characters from String one at a time, you can use:

for (char c: s.toCharArray()) {
   // do stuff with char c  
}

（与 C 不同， NULL 终止符检查不是用Java完成的。）

(Unlike C, NULL terminator checking is not done in Java.)