ClassCastException when casting object Array to Long array

Question

I get this exception when i try to cast an Object array to a Long array.

Exception in thread \"main\" java.lang.ClassCastException: [Ljava.lang.O

Answer 1

change

returnValues = (Long[]) hotelRooms.keySet().toArray();

to

returnValues = hotelRooms.keySet().toArray(new Long[hotelRooms.size()]);

and let me know if it works :-)

Answer 2

It's because Object[] Set.toArray() returns an object array. You can't downcast the array to a more specific type. Use <T> T[]Set.toArray(T[] a) instead. If the generic type method didn't exist you'd have to loop through each of the Objects in the return object array and cast each individually into a new Long array.