因为您的类被命名为 String ， String city 中的非限定类型引用被视为对您自己的类的引用。

Since your class is named String, unqualified type reference in String city is taken as reference to your own class.

将类重命名为一些其他名称，或者你必须写 java.lang.String ，无论你在哪里引用内置Java String class。

Either rename the class to some other name, or you'll have to write java.lang.String wherever you reference the "built-in" Java String class.