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.
Thanks,
s'marks
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 <alexfos...@hotmail.ca
<mailto:alexfos...@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
