ClassCastException in Java foreach loop

Answer 1

I can't reproduce it either, but I spot the following mistakes that must be corrected:

• make getObjects() return List<Integer>, rather than a raw type
• Don't expect List<String>, but a List<Integer> instead
• When iterating, loop for (Integer o : list)

Answer 2

The problem is the method static List getObjects() { returns a generic (non parameterised) List. And you assign it to List<String>. This line should give a compiler warning, and that should have signalled a problem.

Answer 3

This part :

List<String> list = getObjects();
for (Object o : list) { // ClassCastException?
    System.out.println(o);
}

Will be simplified as

List<String> list = getObjects();
for (Iterator<String> iterator = list.iterator(); iterator.hasNext();) {
    Object o = iterator.next();
    System.out.println(o);
}

But the implementation of Iterator will try to cast when calling the next() method the content send by the Iterator in a String.

That's why you have a CCE.

Solutions :

Either use generics everywhere or don't use them, but it's really important to be consistant. If you had returned a List<Integer> or a List<? super Integer> you would have seen this problem at compilation time.