RFR : 8016446 : (m) Add override forEach/replaceAll to HashMap, Hashtable, IdentityHashMap, WeakHashMap, TreeMap

Remi Forax forax at univ-mlv.fr
Thu Jun 13 21:56:08 UTC 2013

On 06/13/2013 04:47 PM, Paul Sandoz wrote:
> On Jun 13, 2013, at 4:06 PM, Remi Forax <forax at univ-mlv.fr> wrote:
>>>> There is a difference between an Iterator/forEach and a spliterator/stream,
>>>> with a stream you know that the called lambdas will not interfere and mutate the source collection.
>>> You do? I don't think there is any conceptual difference between the following w.r.t. interference:
>>>    ArrayList l = ...
>>>    l.stream().filter(...).forEach(e -> l.add(e));
>>>    l.spliterator().forEachRemaining(e -> l.add(e));
>>> and:
>>>    ArrayList l = ...
>>>    l.forEach(e -> l.add(e));
>>>    l.iterator().forEachRemaining(e -> l.add(e));
>>> Of course we have (or will have) strong wording saying don't implement interfering lambdas, but we still have to check for co-modification in the traversal methods of ArrayList spliterator.
>> Isn't it because if you remove an element from an ArrayList while iterating you can see a stale value ?
>> While with a HashMap, if you have only one thread, you can not see a stale entry ?
> Assuming just one thread do you agree that in all of the above examples the only way the list can be interfered with is by the Consumer instance e -> l.add(s) ?

yes, as I said to Mike, what is important IMO is that the semantics of 
forEach and the semantics of for(:) should be the same.

>> So a spliterator on HashMap can only check the modCount at the end unlike the one on ArrayList that need to check at each step.
> The ArrayList.spliterator.forEachRemaining implementation also checks at the end.

Given that a spliterator is something new which is weaker than an 
iterator, the semantics can be relaxed.
For ArrayList, because you can see a stale entry if you mutate during a 
you have no choice but checking at each step. And because the semantics 
is not tight to the iterator one,
I agree that you can also perform a check at the end.
For HashMap.spliterator.forEachRemaining,  because you can not see stale 
entry (without concurrency),
you can only perform a check at the end. It's IMO also a valid semantics.

> Paul.


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