Arrays.asList returned List - Its modifying/write-through semantics

[Jaikiran Pai]

Hi Ivan,


On 15/08/18 1:20 PM, Ivan Gerasimov wrote:
> Hi Jaikiran!
>
> The first part (the documentation clarification) was requested some
> time ago [1], so it may be eventually fixed.
>
> [1] https://bugs.openjdk.java.net/browse/JDK-7033681
Thank you for pointing to that one. I hadn't found it during my search
before sending this mail.

>
> With respect to the second part (throwing UOE from remove(Object)), I
> agree with you that it would be more consistent to always throw it,
> regardless of the presence of the searched item.
> However, it would introduce a change of behavior, so I think it's
> unlikely to happen.
I understand.
>
> It may make sense to update the documentation for all the
> structurally-modifying methods of Collection/List (remove(Object),
> retainAll, removeAll, clear) in the same way it was done for
> List.removeIf() here [2].
>
> [2] https://bugs.openjdk.java.net/browse/JDK-8023339

I looked at that commit and yes, a similar @throws explaining the
UnsupportedOperationException will be helpful.

-Jai
>
> With kind regards,
> Ivan
>
>
> On 8/14/18 10:58 PM, Jaikiran Pai wrote:
>> Consider the following code:
>>
>> import java.util.Arrays;
>> import java.util.List;
>>
>> public class ArraysAsListTest {
>>
>>      public static void main(final String[] args) throws Exception {
>>          final List<String> someList = Arrays.asList(new String[]
>> {"a"});
>>          System.out.println("Removed? " + someList.remove("a"));
>>      }
>> }
>>
>> It uses Arrays.asList to create a java.util.List and then calls a
>> list.remove(...) on it. This currently runs intothe following exception:
>>
>> Exception in thread "main" java.lang.UnsupportedOperationException:
>> remove
>>      at java.base/java.util.Iterator.remove(Iterator.java:102)
>>      at
>> java.base/java.util.AbstractCollection.remove(AbstractCollection.java:299)
>>
>>      at ArraysAsListTest.main(ArraysAsListTest.java:8)
>>
>> The javadoc of Arrays.asList[1] states the following:
>>
>> "Returns a fixed-size list backed by the specified array. (Changes to
>> the returned list "write through" to the array.)..."
>>
>> and has no other mention of how it's supposed to behave with "write"
>> operations. Given that the javadoc states that the returned list is
>> "write through", would that mean a "add/removal" is allowed? Probably
>> not, because it does say it's a fixed size list.So the size altering,
>> write operations (like add(), remove()) aren't allowed, but that isn't
>> clear from the javadoc. So should the javadoc be updated to clarify the
>> semantics of what changes are "write through" and what changes are not
>> supported?
>>
>> Furthermore, the UnsupportedOperationException isn't always thrown from
>> such a returned list. Consider this minor modification to the above
>> code:
>>
>>          final List<String> someList = Arrays.asList(new String[]
>> {"a"});
>>          System.out.println("Removed? " + someList.remove("b"));
>>
>> I add "a" and remove "b". This now runs fine without exceptions and
>> prints "Removed? false". Should the implementation just throw the
>> UnsupportedOperationException irrespective of whether or not the element
>> is contained insuch a returned list? If not, should that be made clear
>> in the javadoc (maybe same for addoperations)?
>>
>> [1]
>> https://docs.oracle.com/javase/10/docs/api/java/util/Arrays.html#asList(T...)
>>
>>
>> -Jaikiran
>>
>>
>>
>