ClassCastException during iteration

[Joe Harry]

Guys,

Why should the code below throw a ClassCastException??

        for (Object element : (List<Long>) reader.getData()) {
            myList.add( (Long) element);
        }

While the below one works

        for (Object element : (List<MyObject>) reader.getData()) {
            myList.add( (MyObject) element);
        }

the getData returns an Object.

[Rob Spoor]

Obviously the actual contents of the List are instances of MyObject, not Long. That means that the cast to Long won't work. Just because you cast the Object to List<Long> doesn't turn every element into a Long. That's also why you got a compiler warning about this cast; the JVM cannot check at this time if the Object is really a List<Long>; all it can check is if it's a List. The warning's there because you may get unexpected ClassCastExceptions later on -- like you're getting now.

[Seetharaman Venkatasamy]

*instanceof* operator avoid ClassCastException and NullPointerException(some time)