Question: What is the result of running the following program?
1 import java.util.*;
2 class A {}
3 class B extends A {}
4 class C extends A {}
5 public class Boxes {
6 private static int countBoxes(List<? extends A> list) { return list.size();}
7 private static int countBoxes2(List<? super A> list) { return list.size();}
8 private static int countBoxes3(List<?> list) { return list.size();}
9 private static int countBoxes4(List<A> list) { return list.size();}
10 public static void main(String[] args) {
11 List<B> b = new ArrayList<>();
12 b.add(new B());
13 List<C> cs = new ArrayList<>();
14 cs.add(new C());
15 List<A> a = new ArrayList<>();
16 a.addAll(b);
17 a.addAll(cs);
18 System.out.print(countBoxes(b));
19 System.out.print(countBoxes(a));
20 System.out.print(countBoxes2(a));
21 System.out.print(countBoxes3(a));
22 System.out.print(countBoxes3(b));
23 System.out.print(countBoxes4(a));
24 }}
Choice:
A. 121121
B. 122212
C. 211121
D. 133313
E. code throw exceptions at runtime
F. compiler error at line 7
G. compiler error at line 18
H. compiler error at other lines
