Java learning - molecular methods class area of three of the same name

/ *
Local variables: write directly to the member variable name
member variables of this class: this member variable name.
Member variables of the parent class:. Super member variable name
* /

public class Demo01ExtendsField {

    public static void main(String[] args) {
        Zi zi = new Zi();

        zi.method();
    }

}

public class Fu {
    int num = 10;
}

public class Zi extends Fu {

    int num = 20;

    public void method() {
        int num = 30;
        System.out.println(num); // 30，局部变量
        System.out.println(this.num); // 20，本类的成员变量
        System.out.println(super.num); // 10，父类的成员变量
    }

}