Classes, specializations, and statics

[Palo Marton]

Another option is dissalow using T in static field declaration, but allow
use of T.erased, like this:

class Collection<any T> {
   private __SS Collection<T.erased> emptyCollection = …

   private __SS Collection<T> emptyCollection() {
      return (Collection<T>)emptyCollection; // warning
   }
}

On Wed, Feb 17, 2016 at 6:48 PM, Brian Goetz <brian.goetz at oracle.com> wrote:

> Nice catch.  Yes, this would cause heap pollution.  As would the following
> Java 5 code:
>
>     private Collection<?> collection = new ArrayList<?>();
>     public <T> Collection<T> c() { return (Collection<T>) collection; }
>
> The trouble is, the unchecked warning is in the library implementation,
> not the user code.  In the case of an immutable collection, we happily
> suppress the warning knowing that everything is safe.  If we were returning
> a mutable collection, it would be unsafe to suppress the warning, and doing
> so would be a library bug.
>
> We have several ways out of this hole; one is to restrict invocation, as
> you suggest.  Another is to add some unchecked warnings.  (Another possible
> path is to treat signatures of erased __SS members, when accessed from
> outside, as if they contained capture variables.)
>
>
>
>
>
> On 2/17/2016 12:03 PM, Peter Levart wrote:
>
>> Hi Brian,
>>
>> On 02/15/2016 07:11 PM, Brian Goetz wrote:
>>
>>> Example:
>>>
>>> class Collection<any T> {
>>>    private __SS Collection<T> emptyCollection = …
>>>    // ACC_SS field emptyCollection : ParamType[Collection, TypeVar[T]]
>>>
>>>    private __SS Collection<T> emptyCollection() { return
>>> emptyCollection; }
>>>    ACC_SS emptyCollection()ParamType[Collection, TypeVar[T]] {
>>>        getstatic ParamType[Collection, TypeVar[T]].emptyCollection :
>>> ParamType[Collection, TypeVar[T]]]
>>>        areturn
>>>    }
>>>
>>> When we specialize Collection<int>, the field type, method return type,
>>> etc, will all collapse to Collection<int> by the existing mechanisms.
>>>
>>
>> This would work if the emptyCollection was actually empty and immutable,
>> but could you do the following:
>>
>> class Collection<any T> {
>>    private __SS Collection<T>collection = new ArrayList<T>();
>>
>>    public __SS Collection<T> collection() { return collection; }
>> }
>>
>>
>> And then in code:
>>
>> Collection<String> cs = Collection<String>.collection();
>> Collection<Number> cn = Collection<Number>.collection();
>>
>> cs.add("abc");
>> Number n = cn.iterator().next();
>>
>> If cs and cn hold the same instance, we have introduced heap corruption
>> without compilation warnings.
>>
>> So I suppose in language you could only access the _SS members in the
>> following way:
>>
>> Collection<?>.collection();
>> Collection<int>.collection();
>> Collection<long>.collection();
>> Collection<ValueType>.collection();
>> ...
>>
>> but not:
>>
>> Collection<Object>.collection();
>> Collection<String>.collection();
>> ...
>>
>>
>> Like .class literals in the prototype.
>>
>> Regards, Peter
>>
>>
>>
>