RFR: 8035584: (s) ArrayList(c) should avoid inflation if c is empty

[Martin Buchholz]

I'm hoping y'all have evidence that empty ArrayLists are common in the wild.


It is curious that empty lists grow immediately to 10, while ArrayList
with capacity 1 grows to 2, then 3...  Some people might think that a
bug.


There are more   changes that need to be reverted.

Else looks good.

-     * More formally, returns the lowest index <tt>i</tt> such that
-     * <tt>(o==null ? get(i)==null : o.equals(get(i)))</tt>,
+     * More formally, returns the lowest index {@code i} such that
+     * {@code (o==null ? get(i)==null : o.equals(get(i)))},




On Tue, Mar 11, 2014 at 5:20 PM, Mike Duigou <mike.duigou at oracle.com> wrote:

> I've actually always used scp. :-)
>
> Since I accepted all of your changes as suggested and had no other changes
> I was just going to go ahead and push once testing was done.
>
> I've now prepared a revised webrev and can still accept feedback.
>
> http://cr.openjdk.java.net/~mduigou/JDK-8035584/1/webrev/
>
> (Note: The webrev also contains
> https://bugs.openjdk.java.net/browse/JDK-8037097 since I am
> testing/pushing the two issues together.)
> <https://bugs.openjdk.java.net/browse/JDK-8037097>
> Mike
>
> On Mar 11 2014, at 16:42 , Martin Buchholz <martinrb at google.com> wrote:
>
> I don't see the updated webrev.  Maybe you also fell victim to "rsync to
> cr no longer working"?
>
>
> On Tue, Mar 11, 2014 at 4:34 PM, Mike Duigou <mike.duigou at oracle.com>wrote:
>
>>
>> On Feb 21 2014, at 14:56 , Martin Buchholz <martinrb at google.com> wrote:
>>
>> You should do <tt> -> code conversion separately, and do it pervasively
>> across the entire JDK.
>>
>>
>> From your lips to God's ears.... I keep suggesting this along with a
>> restyle to official style every time we create new repos. Seems unlikely
>> unfortunately as it makes backports harder.
>>
>> This is not right.
>> +     * {@code
>> (o==null ? get(i)==null : o.equals(get(i)))}
>>
>>
>> Corrected.
>>
>> You accidentally deleted a stray space here?
>>
>> -        this.elementData = EMPTY_ELEMENTDATA;
>> +       this.elementData = EMPTY_ELEMENTDATA;
>>
>>
>> Corrected.
>>
>>      public ArrayList(int initialCapacity) {
>> -        super();
>>          if (initialCapacity < 0)
>>              throw new IllegalArgumentException("Illegal Capacity: "+
>>                                                 initialCapacity);
>> -        this.elementData = new Object[initialCapacity];
>> +        this.elementData = (initialCapacity > 0)
>> +                ? new Object[initialCapacity]
>> +                : EMPTY_ELEMENTDATA;
>>      }
>>
>> When optimizing for special cases, we should try very hard minimize
>> overhead in the common case.  In the above, we now have two branches in the
>> common case.  Instead,
>>
>> if (initialCapacity > 0) this.elementData = new Object[initialCapacity];
>> else if (initialCapacity == 0) this.elementData = EMPTY_ELEMENTDATA;
>> else barf
>>
>>
>> Corrected.
>>
>> Thanks as always for the feedback.
>>
>> Mike
>>
>>
>>
>>
>> On Fri, Feb 21, 2014 at 2:41 PM, Mike Duigou <mike.duigou at oracle.com>wrote:
>>
>>> Hello all;
>>>
>>> This changeset consists of two small performance improvements for
>>> ArrayList. Both are related to the lazy initialization introduced in
>>> JDK-8011200.
>>>
>>> The first change is in the ArrayList(int capacity) constructor and
>>> forces lazy initialization if the requested capacity is zero. It's been
>>> observed that in cases where zero capacity is requested that it is very
>>> likely that the list never receives any elements. For these cases we
>>> permanently avoid the allocation of an element array.
>>>
>>> The second change, noticed by Gustav Åkesson, involves the
>>> ArrayList(Collection c) constructor. If c is an empty collection then there
>>> is no reason to inflate the backing array for the ArrayList.
>>>
>>> http://cr.openjdk.java.net/~mduigou/JDK-8035584/0/webrev/
>>>
>>> I also took the opportunity to the <tt></tt> -> {@code } conversion for
>>> the javadoc.
>>>
>>> Enjoy!
>>>
>>> Mike
>>
>>
>>
>>
>
>