java中downcast向下转型到底有什么用
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.
马克- to-win:马克 java社区:防盗版实名手机尾号: 73203。
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
/* 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();
}
}
the result is:
父类中a =5
子类中a = 10