[jira] [Closed] (COLLECTIONS-544) Undocumented performance issue in the retainAll method in CollectionUtils

["Thomas Neidhart (JIRA)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/COLLECTIONS-544?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Thomas Neidhart closed COLLECTIONS-544.
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> Undocumented performance issue in the retainAll method in CollectionUtils 
> --------------------------------------------------------------------------
>
>                 Key: COLLECTIONS-544
>                 URL: https://issues.apache.org/jira/browse/COLLECTIONS-544
>             Project: Commons Collections
>          Issue Type: Bug
>          Components: Collection
>    Affects Versions: 4.1
>         Environment: Ubuntu 14.04
>            Reporter: Oswaldo Olivo
>            Priority: Trivial
>              Labels: Collections, documentaion, performance
>             Fix For: 4.1
>
>
> The method {{retainAll}} in {{CollectionUtils}} is inefficient when the parameter collection has a slow containment method.
> The following is the current implementation with its documentation:
> {noformat}
>  /**
>      * Returns a collection containing all the elements in <code>collection</code>
>      * that are also in <code>retain</code>. The cardinality of an element <code>e</code>
>      * in the returned collection is the same as the cardinality of <code>e</code>
>      * in <code>collection</code> unless <code>retain</code> does not contain <code>e</code>, in which
>      * case the cardinality is zero. This method is useful if you do not wish to modify
>      * the collection <code>c</code> and thus cannot call <code>c.retainAll(retain);</code>.
>      *
>      * @param <C>  the type of object the {@link Collection} contains
>      * @param collection  the collection whose contents are the target of the #retailAll operation
>      * @param retain  the collection containing the elements to be retained in the returned collection
>      * @return a <code>Collection</code> containing all the elements of <code>collection</code>
>      * that occur at least once in <code>retain</code>.
>      * @throws NullPointerException if either parameter is null
>      * @since 3.2
>      */
>     public static <C> Collection<C> retainAll(final Collection<C> collection, final Collection<?> retain) {
>         return ListUtils.retainAll(collection, retain);
>     }
> {noformat}
> We can notice the inefficiency by looking at the {{retainAll}} method in {{ListUtils}}.
> The {{retainAll}} method from {{ListUtils}} is implemented and documented as follows:
> {noformat}
>   /**
>      * Returns a List containing all the elements in <code>collection</code>
>      * that are also in <code>retain</code>. The cardinality of an element <code>e</code>
>      * in the returned list is the same as the cardinality of <code>e</code>
>      * in <code>collection</code> unless <code>retain</code> does not contain <code>e</code>, in which
>      * case the cardinality is zero. This method is useful if you do not wish to modify
>      * the collection <code>c</code> and thus cannot call <code>collection.retainAll(retain);</code>.
>      * <p>
>      * This implementation iterates over <code>collection</code>, checking each element in
>      * turn to see if it's contained in <code>retain</code>. If it's contained, it's added
>      * to the returned list. As a consequence, it is advised to use a collection type for
>      * <code>retain</code> that provides a fast (e.g. O(1)) implementation of
>      * {@link Collection#contains(Object)}.
>      *
>      * @param <E>  the element type
>      * @param collection  the collection whose contents are the target of the #retailAll operation
>      * @param retain  the collection containing the elements to be retained in the returned collection
>      * @return a <code>List</code> containing all the elements of <code>c</code>
>      * that occur at least once in <code>retain</code>.
>      * @throws NullPointerException if either parameter is null
>      * @since 3.2
>      */
>     public static <E> List<E> retainAll(final Collection<E> collection, final Collection<?> retain) {
>         final List<E> list = new ArrayList<E>(Math.min(collection.size(), retain.size()));
>         for (final E obj : collection) {
>             if (retain.contains(obj)) {
>                 list.add(obj);
>             }
>         }
>         return list;
>     }
> {noformat}
> In the case of {{ListUtils#retainAll}}, the inefficiency is properly documented.
> Perhaps the disclaimer about potential inefficiencies depending on the type 
> of the parameter collection in ListUtils:retainAll should also be included in {{CollectionUtils#retainAll}}.



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