[Fwd: Code review request: 7072353 JNDI libraries do not build with javac -Xlint:all -Werror]

[David Holmes]

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.

> 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.

Cheers,
David

> -Joe
>