RFR: 8266571: Sequenced Collections

[Stuart Marks]

On Sat, 5 Nov 2022 11:17:04 GMT, ExE Boss <duke at openjdk.org> wrote:

>> @liach 
>> 
>>> Is there a particular reason we define poll (null on empty) in SequencedMap but remove (NSEE on empty) in SequencedCollection?
>>> 
>>> I understand that SequencedCollection doesn't want to be null-ambiguous, and map entries are non-null so poll there is not ambiguous. But I still think using remove for both look more consistent.
>> 
>> Yes, this is definitely an asymmetry. I did it this way to avoid proliferation of new methods, so I just promoted existing ones from NavigableMap into SequencedMap. But I might take another swing at this and see if there's a way to get throwing versions into SequencedMap. The problem is that `firstKey` throws if empty but `firstEntry` returns null if empty. So to make things consistently throwing, we'd need to add `getFirst/LastEntry` and `removeFirst/LastEntry` to SequencedMap (and probably get rid of some of the other methods). This would make SequencedMap and probably LinkedHashMap fairly nice, but it would clutter up NavigableMap.
>
> @stuart-marks
>> The problem is that `firstKey` throws if empty but `firstEntry` returns null if empty.
> 
> That’s because there’s no way for `firstKey` to distinguish `null` as meaning an `Entry` where `getKey()` returns `null`, and `null` meaning no mapping.
> 
> Whereas with `firstEntry`, `null` can only be returned when the map is empty, because a `null` key mapped to some value would return `Entry[key=null, value=…]`.
> 
> --------------------------------------------------------------------------------
> 
> Another way to think about it is that the default implementation of `firstKey()` does:
> 
> default K firstKey() {
> 	return this.firstEntry().getKey();
> }

@ExE-Boss Yes, I'm aware of the problem if `firstKey` were to return null. (Didn't prevent `Map::get` from being added though.) The problem I was referring to was one of consistency. We have an uncomfortable mixture of throwing things and null-returning things. The challenge is to come up with a reasonable set of things that makes internal sense and that also fits with what's there already. This might not be possible. Thus, some tradeoffs are necessary.

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PR Comment: https://git.openjdk.org/jdk/pull/7387#issuecomment-1304678458