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https://issues.apache.org/jira/browse/HDFS-12653?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16204273#comment-16204273
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Manoj Govindassamy commented on HDFS-12653:
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[~daryn],
Currently ReadOnlyList is predominantly used by the Directory and Snapshot
subsystems for storing their children inodes / snapshots in a _sorted_ order.
I see it as a SortedList and many a times the users of this list make use of
the sorted nature of the elements for searching -
{{ReadOnlyList#Util#binarySearch(ReadOnlyList<E>, K key)}}. On top of this
sorted benefits, {{ReadOnlyList#Util#asList()}} gives a {{List}} where
{{toArray()}} differs significantly from the Collections toArray -- the
returned array is more of a _view_ of the backing read only list, without
copying any elements.
I believe we can make use of ReadOnlyList for enhancing the performance of
{{INodeAttributesProvider#getAttributes()}} by converting byte[][]
bPathComponents to ReadOnlyList<String> sPathComponents only one time and
getting the _view_ of the string path components using toArray() or
subArray(start, end). Collections doesn't have subArray() concept, theres only
subList().
> Implement toArray() and subArray() for ReadOnlyList
> ---------------------------------------------------
>
> Key: HDFS-12653
> URL: https://issues.apache.org/jira/browse/HDFS-12653
> Project: Hadoop HDFS
> Issue Type: Improvement
> Reporter: Manoj Govindassamy
> Assignee: Manoj Govindassamy
>
> {{ReadOnlyList}} today gives an unmodifiable view of the backing List. This
> list supports following Util methods for easy construction of read only views
> of any given list.
> {noformat}
> public static <E> ReadOnlyList<E> asReadOnlyList(final List<E> list)
> public static <E> List<E> asList(final ReadOnlyList<E> list)
> {noformat}
> {{asList}} above additionally overrides {{Object[] toArray()}} of the
> {{java.util.List}} interface. Unlike the {{java.util.List}}, the above one
> returns an array of Objects referring to the backing list and avoid any
> copying of objects. Given that we have many usages of read only lists,
> 1. Lets have a light-weight / shared-view {{toArray()}} implementation for
> {{ReadOnlyList}} as well.
> 2. Additionally, similar to {{java.util.List#subList(fromIndex, toIndex)}},
> lets have {{ReadOnlyList#subArray(fromIndex, toIndex)}}
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