Generics and ClassCastException

Question

Does anyone know why in the following code map.get("one").getClass() is the only line which throws ClassCastException.
        Map<Integer, Integer> map = new HashMap<Integer, Integer>();
        map.put(1, 1);
        System.out.println("map.get(1) =" + map.get(1));
        Object obj = map;
        Map<String, Number> map2 = (Map<String, Number>) obj;
        map2.put("one", 1L);
        System.out.println("map2.get(1) =" + map2.get(1));
        System.out.println("map2.get(one) =" + map2.get("one"));
        Map<String, Integer> map3 = (Map<String, Integer>) obj;
        System.out.println("map3.get(1) =" + map3.get(1));
        // Doesn't throw ClassCastException even though the return value is a Long, not Integer.
        Object objOne = map3.get("one");
        System.out.println("map3.get(one) =" + map3.get("one") + ", " + objOne.getClass());
        try {
            // throws ClassCastException ????.
            System.out.println("map3.get(one).getClass() =" + map3.get("one").getClass());
        } catch (ClassCastException e) {
            System.out.println("map3.get(one).getClass() throws " + e);
   }I have JDK 1.5.0_02

Answer 1

Map<String, Integer> map3 = (Map<String,<String, Integer>) obj;
System.out.println("map3.get(1) =" +
(1) =" + map3.get(1));
// Doesn't throw ClassCastException even
ion even though the return value is a Long, not
Integer.
Object objOne = map3.get("one");
System.out.println("map3.get(one) =" +
ne) =" + map3.get("one") + ", " +
objOne.getClass());
try {
// throws ClassCastException ????.
System.out.println("map3.get(one).getClass()
etClass() =" + map3.get("one").getClass());
} catch (ClassCastException e) {
System.out.println("map3.get(one).getClass()
etClass() throws " + e);
I have JDK 1.5.0_02
Okay, now that I have my X-Ray Specs on, I see the longs. At first, I was surprised
by the class cast, but what I think is happening is that the expression:
map3.get("one")Has an implicit cast in it, thanks to generics:
((Integer)map3.get("one"))And it's that cast that's generating the runtime exception. Now, how do folks
dump out class files in readable format? I'd like to see to myself...