Java中在从父类中继承的成员变量在子类改变，怎么父类（super.num）的成员也一起改变？

class Father {
protected int num = 10;

public void setNum(int num) {
this.num = num;
}
public int getNum() {
return num;
}
}
class Child extends Father {
public void show() {
System.out.println(num);			// 20
System.out.println(this.num);		// 20
System.out.println(super.num);		// 20
}
}

public class TestExtends01 {
public static void main(String[] args) {
Child child = new Child();
//child.show();
child.setNum(20);
child.show();
}
}