David Holmes wrote:
Joe Darcy said the following on 08/04/11 02:24:
On 8/3/2011 12:42 AM, David Holmes wrote:
Alexandre Boulgakov said the following on 08/03/11 04:44:
On 8/2/2011 2:19 AM, Xuelei Fan wrote:
3017 Vector<Object> temp = (Vector)extractURLs(res.errorMessage);
You may not need the conversion any more, the return value of
extractURLs() has been updated to
2564 private static Vector<String> extractURLs(String
refString)
The cast is needed to go from Vector<String> to Vector<Object>.
Raw types should be avoided (here and elsewhere there are casts to
raw Vector). I'm surprised (generics continue to surprise me) that
despite all our advances in type-inference etc that the compiler can
not tell that a Vector<T> is-a Vector<Object>. :(
That is because in general a Vector<T> is not a Vector<Object>
because of the way subtyping works. As with arrays, it all looks
fine until you want to change the container; consider
Vector<String> vs = new Vector<>();
...
Vector<Object> vo = vs; // Assume this was okay to alias an object
vector and a string vector
vo.add(new Integer(1)); // Add an Integer to a list of strings, boom!
I see what you are saying but with arrays the boom would be an
ArrayStoreException would it not? So the problem here is that there is
no runtime checking for generics that could do the same. So we have to
disallow this.
Exactly; since arrays are reified so the VM has the information needed
to check for an errant store and when appropriate throw the
ArrayStoreException. Since generics are implemented via erasure, the
analogous information is not available at runtime for collections. Note
that the behavior of arrays is statically unsafe and requires the
runtime check.
Using wildcards makes the subtyping work along the type argument axis.
So what is the right fix here? To declare the underlying Vector as a
Vector<?> and cast it to something concrete when needed? It seems very
wrong to me to be inserting raw type casts all through this code.
It isn't entirely clear to be from a quick inspection of the code what
the actual type usage is. A writable general Vector should be a
Vector<Object> and Vector just meant for reading should be a Vector<?>
(or the equivalent Vector<? extends Object>).
If the type usage is "a sequence of X's and Y's" where X and Y don't
have some useful supertype, I would recommend using a somewhat different
set of data structures, like a list of type-safe heterogeneous
containers or a list of a new package-level XorY class.
-Joe
