Question on layer/peeling

Maurizio Cimadamore maurizio.cimadamore at oracle.com
Tue Jan 6 22:37:38 UTC 2015


On 06/01/15 20:38, Michael Barker wrote:
>> The trick can also be coaxed to give semi-plausible semantics to seemingly
>> bogus expressions like "x == null" or "x == (Object)y" (for any x).  If "x"
>> is a primitive, since autobox conversions never produce "null", then "x ==
>> null" and "x == (Object)y" are complicated ways of saying "false".  But
>> this doesn't help with nulling assignments like "a[i] = null"; there you
>> need "T.null" (my bikeshed preference to "default(T)").
>>
> I think allowing x == null (always false) and a[i] = T.null for value types
> would certainly make the coding simpler as it would avoid layering for a
> number of very common cases.  In the null comparison case I would assume
> that Hotspot could easily throw away the condition altogether in the
> specialised class.
I think we should distinguish between default value and null value; I 
believe null only makes sense for references - while default value makes 
sense in a broader sense. You don't want end up mixing the two concepts 
- as you would end up that i.e. int.null is 0 (zero), which happens to 
be quite a common value...

Maurizio


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