Arrays.asList returned List - Its modifying/write-through semantics (original) (raw)

Ivan Gerasimov ivan.gerasimov at oracle.com
Wed Aug 15 07:50:14 UTC 2018

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

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.

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

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 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 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 >>>

With kind regards, Ivan Gerasimov

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