On Jan 7, 2015, at 11:56 AM, Chris Hegarty <chris.hega...@oracle.com> wrote:
> On 07/01/15 10:47, Daniel Fuchs wrote:
>> On 07/01/15 11:31, Paul Sandoz wrote:
>>>
>>> On Jan 7, 2015, at 10:45 AM, Remi Forax <fo...@univ-mlv.fr> wrote:
>>>
>>>> A simple Java question, what this code does ?
>>>>
>>>> ArrayList<String> list = new ArrayList<>();
>>>> list.add("foo");
>>>> list.add("bar");
>>>> for(String s: list) {
>>>> list.remove(s);
>>>> }
>>>>
>>>> :(
>>>>
>>>
>>> We could improve the best-effort basis by which
>>> ConcurrentModificationException is thrown by snapshotting the
>>> collection size on construction of the iterator
>
> This was my thought too.
>
> diff --git a/src/java.base/share/classes/java/util/ArrayList.java
> b/src/java.base/share/classes/java/util/ArrayList.java
> --- a/src/java.base/share/classes/java/util/ArrayList.java
> +++ b/src/java.base/share/classes/java/util/ArrayList.java
> @@ -853,9 +853,10 @@
> int cursor; // index of next element to return
> int lastRet = -1; // index of last element returned; -1 if no such
> int expectedModCount = modCount;
> + int itrSize = size;
>
> public boolean hasNext() {
> - return cursor != size;
> + return cursor != itrSize;
> }
>
> @SuppressWarnings("unchecked")
> @@ -881,6 +882,7 @@
> cursor = lastRet;
> lastRet = -1;
> expectedModCount = modCount;
> + itrSize--;
> } catch (IndexOutOfBoundsException ex) {
> throw new ConcurrentModificationException();
> }
> @@ -909,7 +911,7 @@
> }
>
> final void checkForComodification() {
> - if (modCount != expectedModCount)
> + if (modCount != expectedModCount || size != itrSize)
> throw new ConcurrentModificationException();
> }
> }
>
Under what conditions can the mod count be the same but the size is different?
Would this not indicate there was a bug in some other method that made a
structural change but did not update the mod count? Same apples to this:
Object[] elementData = ArrayList.this.elementData;
if (i >= elementData.length)
throw new ConcurrentModificationException();
(Note as per usual in these discussions i am not referring to concurrent
operation from multiple threads, under such conditions all bets are off.)
Unfortunately the change history in openjdk only goes back so far, and i agree
with Doug that there might be some reason why this change was not made earlier
on (it seems like an "obvious" thing to do).
Paul.
