No enclosing instance of type Demo5 is accessible. Must qualify the allocation ...

So Baidu Google a bit of relevant information. Originally I wrote the inner class is dynamic, that is, the beginning of the public class start. and the main program is public static class main. In Java, a static method in a class cannot invoke a dynamic method directly. Only an inner class is decorated as a static class, and then the member variables and member methods of the class can be called in a static class. So in the case of no other changes , the simplest solution is to change the public class to public static class.

No enclosing instance of type Demo5 is accessible. Must qualify the allocation ...