成员类java中的错误

Question

我试图理解成员类在java中是如何工作的，但是当我尝试这段代码时

public class Class1 {
    public static void main(String[] args) {
        Class2 c = new Class2();
        c.show();
    }

    class Class2 {
        public void show() {
            System.out.println("helloworld");
        }
    }

}

我收到此错误No enclosing instance of type Class1 is accessible. Must qualify the allocation with an enclosing instance of type Class1 (e.g. x.new A() where x is an instance of Class1)，我无法理解原因！