java中downcast向下转型到底有什么用？ 举例说明！

What is the point of downcast? 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---

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

/* 我们不能去掉下面的话，因为SuperClassM_t_w没有printA方法。马 克-to-wi n：we can not comment the following statement,because SuperClassM_t_w does not have the method of printA, report error */

        SubClass sc = (SubClass) s1;

        sc.printA();

    }

}

更多请见：https://blog.csdn.net/qq_44639795/article/details/103117168