8143850: retrofit ArrayDeque to implement List

Stuart Marks stuart.marks at oracle.com
Sun Jul 29 00:11:11 UTC 2018

Hi, I finally got some time to get my head around this.

Conceptually, I like the idea of a List that's stored in a circular array, like 
ArrayDeque. The best way to manifest this in the API isn't obvious though. I 
filed the bug as "retrofit ArrayDeque to implement List" but of course it 
doesn't have to be this way.

I also think we want to reuse an existing implementation as much as possible. 
There's already too much duplication between ArrayList and ArrayDeque; we don't 
want a third similar implementation that will need to be maintained in parallel.

To Martin's points:

* Adding List-like methods to ArrayDeque without it implementing List is indeed 
odd, but not necessarily fatal. It does seem to raise the question "Isn't there 
a better way?" though.

* I don't want to have to add null support if at all possible, for the reasons 
Martin mentioned. Also, it's an implementation and maintenance headache. Of 
course the implementations like ArrayList and HashMap are proof that it can be 
done. But each time HashMap has been enhanced significantly, there's been a bug 
tail where null checking was missed in certain cases.

* Growth policy. The shared null array setup for ArrayList was added when we 
observed that a significant number of ArrayLists are created with the default 
constructor and then never populated. Allocating the backing array lazily 
resulted in a considerable space savings. I think this would be a good idea to 
do for ArrayDeque, but this is somewhat orthogonal to the current "retrofit 
List" discussion.

* Regarding nestmates, I don't think the use of nestmates has any advantage or 
disadvantage over package-level access between top-level classes in the same 
package. I think we should decide what we want the API to look like first, and 
then figure out how to factor things so that we can get the code sharing and 
coupling that we want. We might not be forced into contorting the API in order 
to get better sharing/coupling, but let's wait to cross that bridge if we come 
to it.


Alex, I'm not sure where your current proposal stands. Have you sent an update 
since the head of the thread? If you had included something as an attachment, it 
has likely been stripped.



On 7/23/18 8:46 PM, Martin Buchholz wrote:
> (As usual, I don't have enough time to devote to this at the moment)
> I sort of like the idea of adding all of those List methods to ArrayDeque, but 
> it's __weird__ for java code to do that, and remove(int) is a problem when you 
> have ArrayDeque<Integer>, so it will probably end up rejected.
> ---
> Similarly, the idea of an ArrayDeque subclass that accepts nulls will be 
> unpopular - null elements have fallen out of favor.
> While|Deque|implementations are not strictly required to prohibit the insertion 
> of null elements, they are strongly encouraged to do so. Users of 
> any|Deque|implementations that do allow null elements are strongly 
> encouraged/not/to take advantage of the ability to insert nulls. This is so 
> because|null|is used as a special return value by various methods to indicate 
> that the deque is empty.
> ---
> It makes some sense for ArrayDeque's growth policy to be very similar to 
> ArrayList's - that should be done as an independent change.  Are there any 
> lessons to be learned from ArrayList's experience?  Is the world filled with 
> empty ArrayDeques that could share a common backing array?
> ---
> I do like the idea of having ArrayDeque's List-implementing subclass be a 
> nestmate of ArrayDeque, to share implementation details and to help discovery 
> and naming.  I'm not thrilled with reusing "List" in ArrayDeque.List but I don't 
> have a great alternative to suggest.
> On Mon, Jul 16, 2018 at 10:36 AM, Alex Foster <alexfoster at hotmail.ca 
> <mailto:alexfoster at hotmail.ca>> wrote:
>     Hi again,
>     I updated ArrayDeque with your last idea (adding all list methods but not
>     implementing List) and added a subclass ArrayDeque.List which overrides
>     equals and hashcode and implements List. There is also a subclass
>     ArrayDeque.WithNulls that accepts null elements. ArrayDeque has removeAt(int
>     index) instead of remove(index) to avoid overloading remove(Object).
>     I also added shared empty arrays similar to Arraylist, and a reallocate
>     method which can do the same things as trimToSize and ensure capacity in
>     Arraylist. It also allows you to trim to a specific capacity other than the
>     size or skip trimming if the capacity is within a specified distance of the
>     target capacity.
>     Also the bulk add methods call collection.toArray, then check the array for
>     illegal elements, then add the array, which means that a collection could
>     keep the array it returns from toArray and modify it from another thread
>     after it has been checked but before it has been added which could lead to
>     illegal elements being added to the ArrayDeque. We could maybe avoid this by
>     cloning the array or checking the elements after adding them but I'm not
>     sure if it's worth it...
>     What do you think?
>     I also changed the WhiteBox test a bit:
>     --- a/test/jdk/java/util/ArrayDeque/WhiteBox.java
>     +++ b/test/jdk/java/util/ArrayDeque/WhiteBox.java
>     @@ -88,7 +88,10 @@
>           @Test
>           public void defaultConstructor() {
>     -        checkCapacity(new ArrayDeque(), 16);
>     +        ArrayDeque d = new ArrayDeque();
>     +        d.add(new Object());
>     +        d.clear();
>     +        checkCapacity(d, 16);
>           }
>           @Test
>     @@ -131,7 +134,7 @@
>                   if (rnd.nextBoolean()) d.add(99);
>                   ArrayDeque clone = serialClone(d);
>                   assertInvariants(clone);
>     -            assertNotSame(elements(d), elements(clone));
>     +            assertTrue(d.isEmpty() || elements(d) != elements(clone));
>                   assertEquals(d, clone);
>               }
>           }
>     Alex

More information about the core-libs-dev mailing list