成员内部类的对象访问外部类的对象报错：java: 不是封闭类: hw03.Outer，请问这是什么原理，如何解决？

Question

package hw03;

public class Demo{
public static void main(String[] args){
Outer.Inner obj = new Outer().new Inner();
obj.show();

    System.out.println();

    System.out.println(obj.number);
    System.out.println(Outer.this.number);
}

}

Answer 1

class Outer{
public int number = 10;
class Inner{
public int number = 20;
public void show(){
int number =30;
System.out.println(number);
System.out.println(this.number);
System.out.println(Outer.this.number);
}
}
}