如何在Java中访问超类中的子类？

class SuperClass {
    protected int num;

    public SuperClass(int num) {
        this.num = num;
    }

    public void display() {
        System.out.println("SuperClass: " + num);
    }
}

class SubClass extends SuperClass {
    private int subNum;

    public SubClass(int num, int subNum) {
        super(num);
        this.subNum = subNum;
    }

    public void display() {
        super.display();
        System.out.println("SubClass: " + subNum);
    }
}

public class Main {
    public static void main(String[] args) {
        SubClass sub = new SubClass(10, 20);
        sub.display();
    }
}

SuperClass: 10
SubClass: 20