多选题 Given:
2. class Bird {
3. public static String s = "";
4. public static void fly() {s += "fly "; }
5. }
6. public class Hummingbird extends Bird {
7. public static void fly() {s += "hover "; }
8. public static void main(String[] args) {
9. Bird b1 = new Bird();
10. Bird b2 = new Hummingbird();
11. Bird b3 = (Hummingbird)b2;
12. Hummingbird b4 = (Hummingbird)b2;
13.
14. b1.fly(); b2.fly(); b3.fly(); b4.fly();
15. System.out.println(s);
16. } }
What is the result?