No enclosing instance of type Stu is accessible

No enclosing instance of type Stu is accessible. Must qualify the allocation with an enclosing instance of type Stu (e.g. x.new A() where x is an instance of Stu).

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根据提示，没有可以访问的实例 SxtStu，必须分配一个合适的外部类实例以访问内部类。
正确的方式可以是：