Question

Suppose I have a superclass Item and a subclass MovingItem.

If I create an array of items and then try to cast one of the already created items into a MovingItem and store it in to a vector, does it mean I use a reference or I create a new object, for instance.

Item[] itms = new Item[2];
itms[0] = new Item();
itms[1] = new Item();

Vector<MovingItem> movingItms = new Vector<MovingItem>();
movingItms.add((MovingItem) itms[0]);

What happens when I cast the object of type Itm found in array itms at index 0 and then store it in to the vector? Do I store a reference or casting creates a new object of type MovingItem and then adds it to the vector.

Thanks.

Answer 1

Your example will throw a ClassCastException. You can cast or assign object only to its actual type or any of its supertypes, but not to its subtype.

Item i = new MovingItem(); // ok, because a MovingItem is an Item
                           // a cast is possible, but redundant in this place
MovingItem mi = new Item(); // won't compile, an Item isn't a MovingItem
MovingItem mi = (MovingItem) new Item(); // throws ClassCastException
                                         // an Item isn't a MovingItem

Of course, casting an object doesn’t create a new object, it only creates a new reference to it (if the cast doesn’t fail with a ClassCastException as above).

Object o = new Integer(5); // assign an Integer instance to an Object variable
Integer i = (Integer) o; // cast and assign to a new Integer variable
                         // still points the the same actual instance
assert i == o; // true, these two variables point to the same Integer instance