基类指针指向派生类，我们已经很熟了。（视频下载） （全部书籍）假如我们想用派生类反过来指向基类，就需要有两个要求：1）马克-to-win：基类指针开始时指向派生类，2）我们还需要清清楚楚的转型一下。
if you want to use derived class pointer
point to base class, there are two requirements:
1) base class pointer is initially the type of the derived class like
Animal a2 = new Dog();
2) 马克-to-win：we still need to explicitly cast it like
Dog d = (Dog)a2;
What is the point of downcast? （视频下载） （全部书籍）当一个方法只有子类才有，马克-to-win：不是说基类和子类都有，开始时又是基类指针指向派生类，这时就需要downcast,
see the following example. after you cast with SubClass,sc is pure SubClass type.
例1.9.1---本章源码
class SuperClassM_t_w {
int a;
SuperClassM_t_w() {
a = 5;
}
public void printAsuper() {
System.out.println("父类中a =" + a);
}
}
class SubClass extends SuperClassM_t_w {
int a;
SubClass(int a) {
this.a = a;
}
public void printA() {
System.out.println("子类中a = " + a);
}
}
public class Test {
public static void main(String args[]) {
/* note that new SubClass(10) will call SuperClassM_t_w(), default constructor. */
SuperClassM_t_w s1 = new SubClass(10);
s1.printAsuper();//基类指针指向派生类时，马克-to-win： 可以用基类指针调用基类仅有的方法， 但不能调用子类仅有的方法。必须向下强转一下。
// s1.printA();错误
。。。。。。。。。。
详情请见：http://www.mark-to-win.com/index.html?content=JavaBeginner/javaUrl.html&chapter=JavaBeginner/JavaBeginner3_web.html#HowToUseDowncasting
原文地址：https://www.cnblogs.com/mark-to-win/p/9693479.html