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| </div><div id='content' class='page-1'><div class='row'><div class='row-fluid'><div class='col-lg-3'><ul class='nav-sidebar'><li><a href='./'>Blog index</a></li><li class='active'><a href='#doc'>Groovy and Sequenced Collections (JEP-431)</a></li><li><a href='#_sequenced_collections_summary' class='anchor-link'>Sequenced Collections Summary</a></li><li><a href='#_accessing_the_first_and_last_element' class='anchor-link'>Accessing the first and last element</a></li><li><a href='#_removing_first_or_last_elements' class='anchor-link'>Removing first or last elements</a></li><li><a href='#_adding_elements_to_the_frontend' class='anchor-link'>Adding elements to the front/end</a></li><li><a href='#_working_with_reversed_collections' class='anchor-link'>Working with reversed collections</a></li><li><a href='#_additional_references' class='anchor-link'>Additional References</a></li><li><a href='#_conclusion' class='anchor-link'>Conclusion</a></li></ul><br/><ul class='nav-sidebar'><li style='padding: 0.35em 0.625em; background-color: #eee'><span>Related posts</span></li><li><a href='./wordle-checker'>Checking Wordle with Groovy</a></li><li><a href='./groovy-null-processing'>Groovy Processing Nulls In Lists</a></li><li><a href='./groovy-list-processing-cheat-sheet'>Groovy List Processing Cheat Sheet</a></li><li><a href='./zipping-collections-with-groovy'>Zipping Collections with Groovy</a></li></ul></div><div class='col-lg-8 col-lg-pull-0'><a name='doc'></a><h1>Groovy and Sequenced Collections (JEP-431)</h1><p><span>Author: <i>Paul King</i></span><br/><span>Published: 2023-04-29 09:00AM</span></p><hr/><div id="preamble"> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>An exciting feature coming in JDK21 is |
| <a href="https://openjdk.org/jeps/431">Sequenced Collections</a> |
| which provide improved processing for collections which have |
| a defined encounter order. Additional details about the new |
| functionality can be found in the <a href="#_additional_references">Additional References</a> section later.</p> |
| </div> |
| <div class="paragraph"> |
| <p>Since Groovy is designed to work very closely with the JDK libraries, |
| once JDK21 is released, Groovy will get the new functionality "for free". |
| Groovy however has its own solutions to some of the problems which JEP-431 |
| addresses, so this post looks at the existing functionality (which you can use with older JDKs) |
| and the new functionality you can use once JDK21 is generally available, |
| and you upgrade to that JDK version.</p> |
| </div> |
| <div class="paragraph"> |
| <p>The examples in this post use JDK21ea Build 20 (2023/4/27) and Groovy 4.0.11. |
| While EA builds come with numerous disclaimers warning that changes or removal |
| of functionality might occur before general release, |
| we’d expect the examples to work for subsequent releases. |
| We’ll post an update if anything changes.</p> |
| </div> |
| </div> |
| </div> |
| <div class="sect1"> |
| <h2 id="_sequenced_collections_summary">Sequenced Collections Summary</h2> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>Sequenced Collections adds three new interfaces: <code>SequencedSet</code>, |
| <code>SequencedCollection</code>, and <code>SequencedMap</code>. Each interface adds |
| some new methods which we’ll encounter in later examples. |
| In the rest of this post, we’ll explain the new sequenced collections functionality |
| by looking at various scenarios you might face when processing collections.</p> |
| </div> |
| </div> |
| </div> |
| <div class="sect1"> |
| <h2 id="_accessing_the_first_and_last_element">Accessing the first and last element</h2> |
| <div class="sectionbody"> |
| <div class="sect2"> |
| <h3 id="_jdk_before_jep_431">JDK before JEP-431</h3> |
| <div class="paragraph"> |
| <p>Something as simple as accessing the first and last elements |
| for various collection types isn’t consistent or as easy as might be expected (hence JEP-431). |
| Here are some examples of the JDK API calls you would use for this scenario:</p> |
| </div> |
| <table class="tableblock frame-all grid-all stretch"> |
| <colgroup> |
| <col style="width: 33.3333%;"> |
| <col style="width: 33.3333%;"> |
| <col style="width: 33.3334%;"> |
| </colgroup> |
| <thead> |
| <tr> |
| <th class="tableblock halign-left valign-top">Collection Type</th> |
| <th class="tableblock halign-left valign-top">First element</th> |
| <th class="tableblock halign-left valign-top">Last element</th> |
| </tr> |
| </thead> |
| <tbody> |
| <tr> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><code>List</code></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><code>list.get(0)</code></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><code>list.get(list.size()-1)</code></p></td> |
| </tr> |
| <tr> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><code>Deque</code></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><code>deque.getFirst()</code></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><code>deque.getLast()</code></p></td> |
| </tr> |
| <tr> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><code>Set</code></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><code>set.iterator().next()</code> or<br> |
| <code>set.stream().findFirst().get()</code></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock">requires iterating through the set</p></td> |
| </tr> |
| <tr> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><code>SortedSet</code></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><code>set.first()</code></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><code>set.last()</code></p></td> |
| </tr> |
| </tbody> |
| </table> |
| </div> |
| <div class="sect2"> |
| <h3 id="_jdk_after_jep_431">JDK after JEP-431</h3> |
| <div class="paragraph"> |
| <p>After JEP-431, this improves greatly:</p> |
| </div> |
| <table class="tableblock frame-all grid-all stretch"> |
| <colgroup> |
| <col style="width: 33.3333%;"> |
| <col style="width: 33.3333%;"> |
| <col style="width: 33.3334%;"> |
| </colgroup> |
| <thead> |
| <tr> |
| <th class="tableblock halign-left valign-top">Collection Type</th> |
| <th class="tableblock halign-left valign-top">First element</th> |
| <th class="tableblock halign-left valign-top">Last element</th> |
| </tr> |
| </thead> |
| <tbody> |
| <tr> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><code>List</code>, <code>Deque</code>, <code>Set</code></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><code>collection.getFirst()</code></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><code>collection.getLast()</code></p></td> |
| </tr> |
| </tbody> |
| </table> |
| </div> |
| <div class="sect2"> |
| <h3 id="_groovy_before_jep_431">Groovy before JEP-431</h3> |
| <div class="paragraph"> |
| <p>Groovy provides extension methods, <code>first()</code> and <code>last()</code>, for arrays and any <code>Iterable</code>, with |
| various optimised versions when it makes sense. The <em>subscript</em> operator |
| is provided for any class having a <code>getAt</code> method. There are built-in implementations |
| for collections, arrays and numerous other classes.</p> |
| </div> |
| <table class="tableblock frame-all grid-all stretch"> |
| <colgroup> |
| <col style="width: 33.3333%;"> |
| <col style="width: 33.3333%;"> |
| <col style="width: 33.3334%;"> |
| </colgroup> |
| <thead> |
| <tr> |
| <th class="tableblock halign-left valign-top">Aggregate Type</th> |
| <th class="tableblock halign-left valign-top">First element</th> |
| <th class="tableblock halign-left valign-top">Last element</th> |
| </tr> |
| </thead> |
| <tbody> |
| <tr> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><code>List</code>, <code>Deque</code>, <code>Set</code>, array</p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><code>aggregate[0]</code> or<br> |
| <code>aggregate.first()</code></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><code>aggregate[-1]</code> or<br> |
| <code>aggregate.last()</code></p></td> |
| </tr> |
| </tbody> |
| </table> |
| <div class="paragraph"> |
| <p>Groovy also provides <em>take</em> extension methods which could be used here. You could use |
| <code>list.take(1)</code> to get a list containing just the first element of the original list, |
| and <code>takeRight(1)</code> for a list containing just the last element. These work across |
| all the different aggregate types too.</p> |
| </div> |
| </div> |
| <div class="sect2"> |
| <h3 id="_groovy_after_jep_431">Groovy after JEP-431</h3> |
| <div class="paragraph"> |
| <p>No special support is yet added for JEP-431. |
| So Groovy functionality will be existing functionality |
| plus the new JDK functionality.</p> |
| </div> |
| <table class="tableblock frame-all grid-all stretch"> |
| <colgroup> |
| <col style="width: 33.3333%;"> |
| <col style="width: 33.3333%;"> |
| <col style="width: 33.3334%;"> |
| </colgroup> |
| <thead> |
| <tr> |
| <th class="tableblock halign-left valign-top">Aggregate Type</th> |
| <th class="tableblock halign-left valign-top">First element</th> |
| <th class="tableblock halign-left valign-top">Last element</th> |
| </tr> |
| </thead> |
| <tbody> |
| <tr> |
| <td class="tableblock halign-left valign-top"><p class="tableblock">array</p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><code>array[0]</code><br> |
| <code>array.first()</code><br></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><code>array[-1]</code><br> |
| <code>array.last()</code></p></td> |
| </tr> |
| <tr> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><code>List</code>, <code>Deque</code>, <code>Set</code></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><code>collection[0]</code><br> |
| <code>collection.first()</code><br> |
| <code>collection.getFirst()</code><br> |
| <code>collection.first</code></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><code>collection[-1]</code><br> |
| <code>collection.last()</code><br> |
| <code>collection.getLast()</code><br> |
| <code>collection.last</code></p></td> |
| </tr> |
| </tbody> |
| </table> |
| <div class="paragraph"> |
| <p>For now, Groovy’s approach provides uniformity also across arrays (and potentially other classes). |
| Folks should not feel any urgent pressure to use JEP-431 functionality for this scenario |
| with an important caveat. Over time, additional collection types may emerge which might |
| provide more efficient implementations for <code>getFirst()</code> or <code>getLast()</code> in which case it would |
| be beneficial to use those methods.</p> |
| </div> |
| <div class="paragraph"> |
| <p>Future Groovy versions may provide specialised sequenced collections support. |
| The <code>first()</code> and <code>last()</code> extension methods may be implemented in terms |
| of <code>getFirst()</code> and <code>getLast()</code>. |
| We might also extend sequenced collection methods to arrays (and maybe other classes), |
| though it isn’t a high priority for now.</p> |
| </div> |
| </div> |
| <div class="sect2"> |
| <h3 id="_examples">Examples</h3> |
| <div class="listingblock"> |
| <div class="content"> |
| <pre class="prettyprint highlight"><code data-lang="groovy">List list = [1, 2, 3] |
| assert list.get(0) == 1 |
| assert list[0] == 1 |
| assert list.first() == 1 |
| assert list.getFirst() == 1 // NEW |
| assert list.first == 1 // NEW |
| |
| assert list.get(list.size() - 1) == 3 |
| assert list[-1] == 3 |
| assert list.last() == 3 |
| assert list.getLast() == 3 // NEW |
| assert list.last == 3 // NEW |
| |
| LinkedList deque = [1, 2, 3] |
| assert deque[0] == 1 |
| assert deque.first() == 1 |
| assert deque.getFirst() == 1 // NEW |
| assert deque.first == 1 // NEW |
| |
| assert deque[-1] == 3 |
| assert deque.last() == 3 |
| assert deque.getLast() == 3 // NEW |
| assert deque.last == 3 // NEW |
| |
| LinkedHashSet set = [1, 2, 3] |
| assert set.iterator().next() == 1 |
| assert set[0] == 1 |
| assert set.first() == 1 |
| assert set.getFirst() == 1 // NEW |
| assert set.first == 1 // NEW |
| |
| assert set[-1] == 3 |
| assert list.last() == 3 |
| assert set.getLast() == 3 // NEW |
| assert set.last == 3 // NEW |
| |
| TreeSet sortedSet = [2, 4, 1, 3] |
| assert sortedSet[0] == 1 |
| assert sortedSet.first() == 1 |
| assert sortedSet.getFirst() == 1 // NEW |
| assert sortedSet.first == 1 // NEW |
| |
| assert sortedSet[-1] == 4 |
| assert sortedSet.last() == 4 |
| assert sortedSet.getLast() == 4 // NEW |
| assert sortedSet.last == 4 // NEW |
| |
| Integer[] array = [1, 2, 3] |
| assert array[0] == 1 |
| assert array.first() == 1 |
| assert array[-1] == 3 |
| assert array.last() == 3</code></pre> |
| </div> |
| </div> |
| </div> |
| </div> |
| </div> |
| <div class="sect1"> |
| <h2 id="_removing_first_or_last_elements">Removing first or last elements</h2> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>If you need to mutate a collection, removing the first or last element, |
| Groovy doesn’t offer consistent extension methods across all the aggregate types. |
| You can use the JDK <code>remove(0)</code> method from <code>List</code> to remove the first element from the list (and Groovy also provides a nice <code>removeAt(0)</code> alias). |
| Groovy also provides <code>removeLast()</code> for lists. |
| Given this, the <code>removeFirst()</code> and <code>removeLast()</code> |
| methods from <code>SequencedCollection</code> are a nice addition.</p> |
| </div> |
| <div class="paragraph"> |
| <p>If you want to create a new aggregate which is the same as the original |
| but with the first (or last) element removed, Groovy provides |
| <code>tail()</code> and <code>drop(1)</code> (or <code>init()</code> and <code>dropRight(1)</code>).</p> |
| </div> |
| </div> |
| </div> |
| <div class="sect1"> |
| <h2 id="_adding_elements_to_the_frontend">Adding elements to the front/end</h2> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>If you need to mutate a collection, adding elements at the front or end, |
| Groovy doesn’t offer consistent extension methods across all the aggregate types. |
| You’d normally use <code>add(element)</code> or <code>add(0, element)</code> for lists. |
| So the <code>addFirst()</code> and <code>addLast()</code> |
| methods from <code>SequencedCollection</code> are a nice addition. |
| Groovy does offer the <code>leftShift</code> operator (<code><<</code>) as another way to append to the end of a list.</p> |
| </div> |
| </div> |
| </div> |
| <div class="sect1"> |
| <h2 id="_working_with_reversed_collections">Working with reversed collections</h2> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>Another area tackled by JEP-431 is improved consistency for |
| working with a collection in reverse order. |
| Groovy already offers some enhancements for this scenario |
| with <code>reverse</code>, <code>reverseEach</code> and <code>asReversed</code> extension methods. |
| The functionality isn’t universal however and sometimes catches folks out. |
| The <code>reverse</code> method isn’t available for maps and sets. You need to |
| use e.g. the set’s iterator. Also, the standard <code>reverse</code> produces |
| a new collection (or array) and there is an optional boolean parameter |
| which makes the method a mutating operation - reversing itself in-place. |
| This is in contrast to <code>reversed()</code> from JEP-431 and <code>asReversed()</code> |
| which return a view. |
| Also, the <code>reverseEach</code> and <code>asReversed</code> are only provided for |
| <code>NavigableSet</code> instances.</p> |
| </div> |
| <div class="paragraph"> |
| <p>So, all in all, this functionality provided by JEP-431 is most welcome.</p> |
| </div> |
| <table class="tableblock frame-all grid-all stretch"> |
| <colgroup> |
| <col style="width: 25%;"> |
| <col style="width: 25%;"> |
| <col style="width: 25%;"> |
| <col style="width: 25%;"> |
| </colgroup> |
| <thead> |
| <tr> |
| <th class="tableblock halign-left valign-top">Collection Type</th> |
| <th class="tableblock halign-left valign-top">Before JEP-431</th> |
| <th class="tableblock halign-left valign-top">After JEP-431</th> |
| <th class="tableblock halign-left valign-top">Groovy</th> |
| </tr> |
| </thead> |
| <tbody> |
| <tr> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><code>List</code></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock">use <code>list.listIterator(list.size()).previous()</code></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><code>list.reversed()</code></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><code>list.reverseEach</code><br> |
| <code>list.asReversed()</code></p></td> |
| </tr> |
| <tr> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><code>Deque</code></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock">use <code>deque.descendingIterator()</code></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><code>deque.reversed()</code></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><code>deque.reverseEach</code><br> |
| <code>deque.asReversed()</code></p></td> |
| </tr> |
| <tr> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><code>NavigableSet</code></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock">use <code>set.descendingSet()</code></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><code>set.reversed()</code></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><code>set.reverseEach</code><br> |
| <code>set.asReversed()</code></p></td> |
| </tr> |
| <tr> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><code>Set</code></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock">N/A</p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><code>set.reversed()</code></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock">N/A</p></td> |
| </tr> |
| </tbody> |
| </table> |
| <div class="sect2"> |
| <h3 id="_examples_2">Examples</h3> |
| <div class="listingblock"> |
| <div class="content"> |
| <pre class="prettyprint highlight"><code data-lang="groovy">var result = [] |
| list.reverseEach { result << it } |
| assert result == [3, 2, 1] |
| assert list.asReversed() == [3, 2, 1] |
| assert list.reverse() == [3, 2, 1] |
| assert list.reversed() == [3, 2, 1] // NEW |
| |
| result = [] |
| deque.reverseEach { result << it } |
| assert result == [3, 2, 1] |
| assert deque.asReversed() == [3, 2, 1] |
| assert deque.reverse() == [3, 2, 1] |
| assert deque.reversed() == [3, 2, 1] // NEW |
| |
| result = [] |
| assert set.iterator().reverse().toList() == [3, 2, 1] |
| assert set.reversed() == [3, 2, 1] as Set // NEW |
| |
| result = [] |
| sortedSet.reverseEach { result << it } |
| assert result == [4, 3, 2, 1] |
| assert sortedSet.asReversed() == [4, 3, 2, 1] as Set |
| assert sortedSet.reversed() == [4, 3, 2, 1] as Set // NEW |
| |
| var map = [a: 1, b: 2] |
| result = [] |
| map.reverseEach { k, v -> result << [k, v] } |
| assert result == [['b', 2], ['a', 1]] |
| assert map.reversed() == [b:2, a:1] // NEW</code></pre> |
| </div> |
| </div> |
| </div> |
| </div> |
| </div> |
| <div class="sect1"> |
| <h2 id="_additional_references">Additional References</h2> |
| <div class="sectionbody"> |
| <div class="ulist"> |
| <ul> |
| <li> |
| <p><a href="https://openjdk.org/jeps/431">JEP-431 Proposal</a></p> |
| </li> |
| <li> |
| <p><a href="https://www.infoworld.com/article/3689880/jdk-21-the-new-features-in-java-21.html">Summary of features coming in JDK21</a> (Paul Krill on Infoworld)</p> |
| </li> |
| <li> |
| <p><a href="https://www.youtube.com/watch?v=9G_0el3RWPE">Inside Java Newscast #45</a> (with Nicolai)</p> |
| </li> |
| <li> |
| <p><a href="https://inside.java/2023/04/25/podcast-031/">Inside Java Podcast Episode 31</a> (Ana-Maria Mihalceanu with Stuart Marks)</p> |
| </li> |
| <li> |
| <p><a href="https://www.infoq.com/news/2023/03/collections-framework-makeover/" class="bare">https://www.infoq.com/news/2023/03/collections-framework-makeover/</a> (A N M Bazlur Rahman on InfoQ)</p> |
| </li> |
| <li> |
| <p><a href="https://groovy.apache.org/blog/groovy-list-processing-cheat-sheet">Groovy list processing cheat sheet</a></p> |
| </li> |
| </ul> |
| </div> |
| </div> |
| </div> |
| <div class="sect1"> |
| <h2 id="_conclusion">Conclusion</h2> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>We have had a quick look at using JEP-431 functionality with Groovy. |
| While Groovy already offers some of the functionality which JEP-431 provides, |
| it certainly looks like a nice addition to the JDK.</p> |
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