何使用派生类指针指向基类，即downcast向下转型？

Posted 2021-01-08 mark-to-win

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

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详情请见：http://www.mark-to-win.com/index.html?content=JavaBeginner/javaUrl.html&chapter=JavaBeginner/JavaBeginner3_web.html#HowToUseDowncasting