转自：http://blog.csdn.net/ld422586546/article/details/9707997
Java中父类强制转换成子类的原则：父类型的引用指向的是哪个子类的实例，就能转换成哪个子类的引用。
例：
public class Test {
public static void main(String[] args) {
Person person = new Boy();
Boy boy = (Boy) person;
boy.eat();
}
}
class Person {
public void eat() {
System.out.println("The people were eating");
}
}
class Boy extends Person {
public void eat() {
System.out.println("The boy were eating");
}
}
打印结果：The boy were eating
原因：当Boy实例化后将引用地址返回传给person，这时person引用实际指向的是Boy，所以将person转换成Boy能成功。
再定义一个类：
class Girl extends Person {
public void eat() {
System.out.println("The girl were eating");
}
}
main方法中添加：
Person p = new Girl();
Boy b = (Boy)p;
b.eat();
运行时提示：Girl cannot be cast to Boy（不能将女孩转换成男孩）
原因：当Girl实例化后将引用地址返回传给p，这时p引用实际指向的是Girl，将p转换成Boy也就是说将Girl转换成Boy，肯定不能成功。
上面的例子换句话来说，男孩和女孩都是人这肯定是对的，但你要说女孩是男孩肯定是不对的。