RFR: 8005051: optimized defaults for Iterator.forEachRemaining

[Peter Levart]

On 04/26/2013 03:06 PM, Remi Forax wrote:
> On 04/26/2013 02:50 PM, Paul Sandoz wrote:
>> On Apr 26, 2013, at 2:06 PM, David Holmes <david.holmes at oracle.com> 
>> wrote:
>>>>>> To avoid blocking the feature, I have filed
>>>>>> https://jbs.oracle.com/bugs/browse/JDK-8013150 to refine the 
>>>>>> behavior of remove and other ListIterator methods after 
>>>>>> forEachRemaining returns.
>>>>> I think the fact that the last element can be removed should be 
>>>>> specified as unspecified,
>>>>> because it's more an artefact of the default implementation than 
>>>>> something which part
>>>>> of the semantics.
>>>>>
>>>> I was wondering about that too. What happens if an implementing 
>>>> class decides later on to override the default implementation? If 
>>>> the overriding implementation does not conform to the same 
>>>> behaviour as the default it could break compatibility.
>>>>
>>>> Plus one could change code from:
>>>>
>>>>    while(it.hasNext() { doSomething(it.next()); }
>>>>    doSomethingAtEnd(it);
>>>>
>>>> to
>>>>
>>>>    it.forEachRemaining(this::doSomething}
>>>>    doSomethingAtEnd(it);
>>> All implementations must obey the contract of the specification, and 
>>> no clients should assume any kind of behaviour beyond what that 
>>> contract says.
>>>
>>> That said I've lost the context of this particular issue - what is 
>>> the problem here?
>>>
>> What is the state of the Iterator after a call to 
>> Iterator.forEachRemaining e.g.:
>>
>>    void doSometingAtEnd(Iterator it) {
>>      it.remove();
>>    }
>>
>> IMO overriding forEachRemaining implementations should place the 
>> iterator in the same state as the default method implementation. We 
>> just need to call this out more clearly in the docs.
>
> I don't like the fact that the default implementation dictates the 
> semantics of forEachRemaining.
> It's cleaner to separate the two and says that you can replace the 
> while loop by forEachRemaining
> only if the iterator is not used after the while loop.
>
> Basically, it means that the semantics of forEachRemaining is more 
> like the semantics of the enhanced for loop.

Another "interesting" usage:

Iterator<?> i = ...;

i.forEachRemaining(e -> {
    ...
    if (...) {
       i.forEachRemaining(dummy -> {}); // drain it
    }
    ...
});

It seems that mixing external and internal iteration in the same object 
is like a box of chocolates. For internal iteration, Iterable.forEach is 
more suitable than Iterator.forEachRemaining unless the later is 
specified to have the common state for both external and internal 
iteration, which means that internal can not be more effective than 
external.

Regards, Peter

>
>>
>> Paul.
>
> Rémi
>