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class="vector-toc-numb">2</span> <span>Description</span> </div> </a> <ul id="toc-Description-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Example" class="vector-toc-list-item vector-toc-level-1 vector-toc-list-item-expanded"> <a class="vector-toc-link" href="#Example"> <div class="vector-toc-text"> <span class="vector-toc-numb">3</span> <span>Example</span> </div> </a> <ul id="toc-Example-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Relation_to_other_patterns" class="vector-toc-list-item vector-toc-level-1 vector-toc-list-item-expanded"> <a class="vector-toc-link" href="#Relation_to_other_patterns"> <div class="vector-toc-text"> <span class="vector-toc-numb">4</span> <span>Relation to other patterns</span> </div> </a> <ul id="toc-Relation_to_other_patterns-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Alternatives" class="vector-toc-list-item vector-toc-level-1 vector-toc-list-item-expanded"> <a class="vector-toc-link" href="#Alternatives"> <div 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class="interlanguage-link-target"><span>Български</span></a></li><li class="interlanguage-link interwiki-cs mw-list-item"><a href="https://cs.wikipedia.org/wiki/Null_Object" title="Null Object – Czech" lang="cs" hreflang="cs" data-title="Null Object" data-language-autonym="Čeština" data-language-local-name="Czech" class="interlanguage-link-target"><span>Čeština</span></a></li><li class="interlanguage-link interwiki-de mw-list-item"><a href="https://de.wikipedia.org/wiki/Nullobjekt_(Entwurfsmuster)" title="Nullobjekt (Entwurfsmuster) – German" lang="de" hreflang="de" data-title="Nullobjekt (Entwurfsmuster)" data-language-autonym="Deutsch" data-language-local-name="German" class="interlanguage-link-target"><span>Deutsch</span></a></li><li class="interlanguage-link interwiki-fr mw-list-item"><a href="https://fr.wikipedia.org/wiki/Objet_null_(patron_de_conception)" title="Objet null (patron de conception) – French" lang="fr" hreflang="fr" data-title="Objet null (patron de conception)" data-language-autonym="Français" data-language-local-name="French" class="interlanguage-link-target"><span>Français</span></a></li><li class="interlanguage-link interwiki-hu mw-list-item"><a href="https://hu.wikipedia.org/wiki/%C3%9Cres_objektum_programtervez%C3%A9si_minta" title="Üres objektum programtervezési minta – Hungarian" lang="hu" hreflang="hu" data-title="Üres objektum programtervezési minta" data-language-autonym="Magyar" data-language-local-name="Hungarian" class="interlanguage-link-target"><span>Magyar</span></a></li><li class="interlanguage-link interwiki-pl mw-list-item"><a href="https://pl.wikipedia.org/wiki/Pusty_obiekt_(wzorzec_projektowy)" title="Pusty obiekt (wzorzec projektowy) – Polish" lang="pl" hreflang="pl" data-title="Pusty obiekt (wzorzec projektowy)" data-language-autonym="Polski" data-language-local-name="Polish" class="interlanguage-link-target"><span>Polski</span></a></li><li class="interlanguage-link interwiki-ru mw-list-item"><a href="https://ru.wikipedia.org/wiki/Null_object_(%D1%88%D0%B0%D0%B1%D0%BB%D0%BE%D0%BD_%D0%BF%D1%80%D0%BE%D0%B5%D0%BA%D1%82%D0%B8%D1%80%D0%BE%D0%B2%D0%B0%D0%BD%D0%B8%D1%8F)" title="Null object (шаблон проектирования) – Russian" lang="ru" hreflang="ru" data-title="Null object (шаблон проектирования)" data-language-autonym="Русский" data-language-local-name="Russian" class="interlanguage-link-target"><span>Русский</span></a></li><li class="interlanguage-link interwiki-uk mw-list-item"><a href="https://uk.wikipedia.org/wiki/Null_object_(%D1%88%D0%B0%D0%B1%D0%BB%D0%BE%D0%BD_%D0%BF%D1%80%D0%BE%D1%94%D0%BA%D1%82%D1%83%D0%B2%D0%B0%D0%BD%D0%BD%D1%8F)" title="Null object (шаблон проєктування) – Ukrainian" lang="uk" hreflang="uk" data-title="Null object (шаблон проєктування)" data-language-autonym="Українська" data-language-local-name="Ukrainian" class="interlanguage-link-target"><span>Українська</span></a></li><li class="interlanguage-link interwiki-zh mw-list-item"><a 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<div class="mw-indicators"> </div> <div id="siteSub" class="noprint">From Wikipedia, the free encyclopedia</div> </div> <div id="contentSub"><div id="mw-content-subtitle"><span class="mw-redirectedfrom">(Redirected from <a href="/w/index.php?title=Null_object&redirect=no" class="mw-redirect" title="Null object">Null object</a>)</span></div></div> <div id="mw-content-text" class="mw-body-content"><div class="mw-content-ltr mw-parser-output" lang="en" dir="ltr"><div class="shortdescription nomobile noexcerpt noprint searchaux" style="display:none">Object with no referenced value or with defined neutral ("null") behavior</div> <style data-mw-deduplicate="TemplateStyles:r1236090951">.mw-parser-output .hatnote{font-style:italic}.mw-parser-output div.hatnote{padding-left:1.6em;margin-bottom:0.5em}.mw-parser-output .hatnote i{font-style:normal}.mw-parser-output .hatnote+link+.hatnote{margin-top:-0.5em}@media print{body.ns-0 .mw-parser-output .hatnote{display:none!important}}</style><div role="note" class="hatnote navigation-not-searchable">"Null object" redirects here. For the concept in category theory, see <a href="/wiki/Initial_object" class="mw-redirect" title="Initial object">Initial object</a>.</div> <p>In <a href="/wiki/Object-oriented" class="mw-redirect" title="Object-oriented">object-oriented</a> <a href="/wiki/Computer_programming" title="Computer programming">computer programming</a>, a <b>null object</b> is an <a href="/wiki/Object_(computer_science)" title="Object (computer science)">object</a> with no referenced value or with defined neutral (<i>null</i>) behavior. The null object <a href="/wiki/Design_pattern" title="Design pattern">design pattern</a>, which describes the uses of such objects and their behavior (or lack thereof), was first published as "Void Value"<sup id="cite_ref-1" class="reference"><a href="#cite_note-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup> and later in the <a href="/wiki/Pattern_Languages_of_Programs#Publications" title="Pattern Languages of Programs"><i>Pattern Languages of Program Design</i> book series</a> as "Null Object".<sup id="cite_ref-2" class="reference"><a href="#cite_note-2"><span class="cite-bracket">[</span>2<span class="cite-bracket">]</span></a></sup> </p> <meta property="mw:PageProp/toc" /> <div class="mw-heading mw-heading2"><h2 id="Motivation">Motivation</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Null_object_pattern&action=edit&section=1" title="Edit section: Motivation"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>In most object-oriented languages, such as <a href="/wiki/Java_(programming_language)" title="Java (programming language)">Java</a> or <a href="/wiki/C_Sharp_(programming_language)" title="C Sharp (programming language)">C#</a>, <a href="/wiki/Reference_(computer_science)" title="Reference (computer science)">references</a> may be <a href="/wiki/Null_pointer" title="Null pointer">null</a>. These references need to be checked to ensure they are not null before invoking any <a href="/wiki/Method_(computer_science)" class="mw-redirect" title="Method (computer science)">methods</a>, because methods typically cannot be invoked on null references. </p><p>The <a href="/wiki/Objective-C" title="Objective-C">Objective-C language</a> takes another approach to this problem and does nothing when sending a message to <code>nil</code>; if a return value is expected, <code>nil</code> (for objects), 0 (for numeric values), <code>NO</code> (for <code>BOOL</code> values), or a struct (for struct types) with all its members initialised to <code>null</code>/0/<code>NO</code>/zero-initialised struct is returned.<sup id="cite_ref-3" class="reference"><a href="#cite_note-3"><span class="cite-bracket">[</span>3<span class="cite-bracket">]</span></a></sup> </p> <div class="mw-heading mw-heading2"><h2 id="Description">Description</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Null_object_pattern&action=edit&section=2" title="Edit section: Description"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <style data-mw-deduplicate="TemplateStyles:r1251242444">.mw-parser-output .ambox{border:1px solid #a2a9b1;border-left:10px solid #36c;background-color:#fbfbfb;box-sizing:border-box}.mw-parser-output .ambox+link+.ambox,.mw-parser-output .ambox+link+style+.ambox,.mw-parser-output .ambox+link+link+.ambox,.mw-parser-output .ambox+.mw-empty-elt+link+.ambox,.mw-parser-output .ambox+.mw-empty-elt+link+style+.ambox,.mw-parser-output .ambox+.mw-empty-elt+link+link+.ambox{margin-top:-1px}html body.mediawiki .mw-parser-output .ambox.mbox-small-left{margin:4px 1em 4px 0;overflow:hidden;width:238px;border-collapse:collapse;font-size:88%;line-height:1.25em}.mw-parser-output .ambox-speedy{border-left:10px solid #b32424;background-color:#fee7e6}.mw-parser-output .ambox-delete{border-left:10px solid #b32424}.mw-parser-output .ambox-content{border-left:10px solid #f28500}.mw-parser-output .ambox-style{border-left:10px solid #fc3}.mw-parser-output .ambox-move{border-left:10px solid #9932cc}.mw-parser-output .ambox-protection{border-left:10px solid #a2a9b1}.mw-parser-output .ambox .mbox-text{border:none;padding:0.25em 0.5em;width:100%}.mw-parser-output .ambox .mbox-image{border:none;padding:2px 0 2px 0.5em;text-align:center}.mw-parser-output .ambox .mbox-imageright{border:none;padding:2px 0.5em 2px 0;text-align:center}.mw-parser-output .ambox .mbox-empty-cell{border:none;padding:0;width:1px}.mw-parser-output .ambox .mbox-image-div{width:52px}@media(min-width:720px){.mw-parser-output .ambox{margin:0 10%}}@media print{body.ns-0 .mw-parser-output .ambox{display:none!important}}</style><table class="box-Unreferenced_section plainlinks metadata ambox ambox-content ambox-Unreferenced" role="presentation"><tbody><tr><td class="mbox-image"><div class="mbox-image-div"><span typeof="mw:File"><a href="/wiki/File:Question_book-new.svg" class="mw-file-description"><img alt="" src="//upload.wikimedia.org/wikipedia/en/thumb/9/99/Question_book-new.svg/50px-Question_book-new.svg.png" decoding="async" width="50" height="39" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/en/thumb/9/99/Question_book-new.svg/75px-Question_book-new.svg.png 1.5x, //upload.wikimedia.org/wikipedia/en/thumb/9/99/Question_book-new.svg/100px-Question_book-new.svg.png 2x" data-file-width="512" data-file-height="399" /></a></span></div></td><td class="mbox-text"><div class="mbox-text-span">This section <b>does not <a href="/wiki/Wikipedia:Citing_sources" title="Wikipedia:Citing sources">cite</a> any <a href="/wiki/Wikipedia:Verifiability" title="Wikipedia:Verifiability">sources</a></b>.<span class="hide-when-compact"> Please help <a href="/wiki/Special:EditPage/Null_object_pattern" title="Special:EditPage/Null object pattern">improve this section</a> by <a href="/wiki/Help:Referencing_for_beginners" title="Help:Referencing for beginners">adding citations to reliable sources</a>. Unsourced material may be challenged and <a href="/wiki/Wikipedia:Verifiability#Burden_of_evidence" title="Wikipedia:Verifiability">removed</a>.</span> <span class="date-container"><i>(<span class="date">June 2023</span>)</i></span><span class="hide-when-compact"><i> (<small><a href="/wiki/Help:Maintenance_template_removal" title="Help:Maintenance template removal">Learn how and when to remove this message</a></small>)</i></span></div></td></tr></tbody></table> <p>Instead of using a <a href="/wiki/Null_reference" class="mw-redirect" title="Null reference">null reference</a> to convey the absence of an object (for instance, a non-existent customer), one uses an object which implements the expected <a href="/wiki/Interface_(computing)#In_object-oriented_languages" title="Interface (computing)"> interface</a>, but whose method body is empty. A key purpose of using a null object is to avoid conditionals of different kinds, resulting in code that is more focused, and quicker to read and follow –  i.e. improved readability. One advantage of this approach over a working default implementation is that a null object is very predictable and has no side effects: it does <i>nothing</i>. </p><p>For example, a function may retrieve a list of files in a folder and perform some action on each. In the case of an empty folder, one response may be to throw an exception or return a null reference rather than a list. Thus, the code expecting a list must verify that it in fact has one before continuing, which can complicate the design. </p><p>By returning a null object (i.e., an empty list) instead, there is no need to verify that the return value is in fact a list. The calling function may simply iterate the list as normal, effectively doing nothing. It is, however, still possible to check whether the return value is a null object (an empty list) and react differently if desired. </p><p>The null object pattern can also be used to act as a stub for testing, if a certain feature such as a database is not available for testing. </p> <div class="mw-heading mw-heading2"><h2 id="Example">Example</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Null_object_pattern&action=edit&section=3" title="Edit section: Example"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>Given a <a href="/wiki/Binary_tree" title="Binary tree">binary tree</a>, with this node structure: </p> <pre><b>class</b> node { node left node right } </pre> <p>One may implement a tree size procedure recursively: </p> <pre><b>function</b> tree_size(node) { return 1 + tree_size(node.left) + tree_size(node.right) } </pre> <p>Since the child nodes may not exist, one must modify the procedure by adding non-existence or null checks: </p> <pre><b>function</b> tree_size(node) { set sum = 1 <b>if</b> node.left exists { sum = sum + tree_size(node.left) } <b>if</b> node.right exists { sum = sum + tree_size(node.right) } return sum } </pre> <p>This, however, makes the procedure more complicated by mixing boundary checks with normal logic, and it becomes harder to read. Using the null object pattern, one can create a special version of the procedure but only for null nodes: </p> <pre><b>function</b> tree_size(node) { return 1 + tree_size(node.left) + tree_size(node.right) } </pre> <pre><b>function</b> tree_size(null_node) { return 0 } </pre> <p>This separates normal logic from special case handling and makes the code easier to understand. </p> <div class="mw-heading mw-heading2"><h2 id="Relation_to_other_patterns">Relation to other patterns</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Null_object_pattern&action=edit&section=4" title="Edit section: Relation to other patterns"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>It can be regarded as a special case of the <a href="/wiki/State_pattern" title="State pattern">State pattern</a> and the <a href="/wiki/Strategy_pattern" title="Strategy pattern">Strategy pattern</a>. </p><p>It is not a pattern from <i><a href="/wiki/Design_Patterns" title="Design Patterns">Design Patterns</a></i>, but is mentioned in <a href="/wiki/Martin_Fowler_(software_engineer)" title="Martin Fowler (software engineer)">Martin Fowler's</a> <i>Refactoring</i><sup id="cite_ref-4" class="reference"><a href="#cite_note-4"><span class="cite-bracket">[</span>4<span class="cite-bracket">]</span></a></sup> and Joshua Kerievsky's Refactoring To Patterns<sup id="cite_ref-5" class="reference"><a href="#cite_note-5"><span class="cite-bracket">[</span>5<span class="cite-bracket">]</span></a></sup> as the <i>Insert Null Object</i> <a href="/wiki/Refactoring" class="mw-redirect" title="Refactoring">refactoring</a>. </p><p>Chapter 17 of <a href="/wiki/Robert_Cecil_Martin" class="mw-redirect" title="Robert Cecil Martin">Robert Cecil Martin's</a> <i>Agile Software Development: Principles, Patterns and Practices</i><sup id="cite_ref-6" class="reference"><a href="#cite_note-6"><span class="cite-bracket">[</span>6<span class="cite-bracket">]</span></a></sup> is dedicated to the pattern. </p> <div class="mw-heading mw-heading2"><h2 id="Alternatives">Alternatives</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Null_object_pattern&action=edit&section=5" title="Edit section: Alternatives"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>From C# 6.0 it is possible to use the "?." operator (aka <a href="/wiki/Safe_navigation_operator#C.23" title="Safe navigation operator">null-conditional operator</a>), which will simply evaluate to null if its left operand is null. </p> <div class="mw-highlight mw-highlight-lang-csharp mw-content-ltr" dir="ltr"><pre><span></span><span class="c1">// compile as Console Application, requires C# 6.0 or higher</span> <span class="k">using</span><span class="w"> </span><span class="nn">System</span><span class="p">;</span> <span class="k">namespace</span><span class="w"> </span><span class="nn">ConsoleApplication2</span> <span class="p">{</span> <span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">Program</span> <span class="w"> </span><span class="p">{</span> <span class="w"> </span><span class="k">static</span><span class="w"> </span><span class="k">void</span><span class="w"> </span><span class="nf">Main</span><span class="p">(</span><span class="kt">string</span><span class="p">[]</span><span class="w"> </span><span class="n">args</span><span class="p">)</span> <span class="w"> </span><span class="p">{</span> <span class="w"> </span><span class="kt">string</span><span class="w"> </span><span class="n">str</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="s">"test"</span><span class="p">;</span><span class="w"> </span> <span class="w"> </span><span class="n">Console</span><span class="p">.</span><span class="n">WriteLine</span><span class="p">(</span><span class="n">str</span><span class="o">?.</span><span class="n">Length</span><span class="p">);</span> <span class="w"> </span><span class="n">Console</span><span class="p">.</span><span class="n">ReadKey</span><span class="p">();</span> <span class="w"> </span><span class="p">}</span> <span class="w"> </span><span class="p">}</span> <span class="p">}</span> <span class="c1">// The output will be:</span> <span class="c1">// 4</span> </pre></div> <div class="mw-heading mw-heading3"><h3 id="Extension_methods_and_Null_coalescing">Extension methods and Null coalescing</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Null_object_pattern&action=edit&section=6" title="Edit section: Extension methods and Null coalescing"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1251242444"><table class="box-Unreferenced_section plainlinks metadata ambox ambox-content ambox-Unreferenced" role="presentation"><tbody><tr><td class="mbox-image"><div class="mbox-image-div"><span typeof="mw:File"><a href="/wiki/File:Question_book-new.svg" class="mw-file-description"><img alt="" src="//upload.wikimedia.org/wikipedia/en/thumb/9/99/Question_book-new.svg/50px-Question_book-new.svg.png" decoding="async" width="50" height="39" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/en/thumb/9/99/Question_book-new.svg/75px-Question_book-new.svg.png 1.5x, //upload.wikimedia.org/wikipedia/en/thumb/9/99/Question_book-new.svg/100px-Question_book-new.svg.png 2x" data-file-width="512" data-file-height="399" /></a></span></div></td><td class="mbox-text"><div class="mbox-text-span">This section <b>does not <a href="/wiki/Wikipedia:Citing_sources" title="Wikipedia:Citing sources">cite</a> any <a href="/wiki/Wikipedia:Verifiability" title="Wikipedia:Verifiability">sources</a></b>.<span class="hide-when-compact"> Please help <a href="/wiki/Special:EditPage/Null_object_pattern" title="Special:EditPage/Null object pattern">improve this section</a> by <a href="/wiki/Help:Referencing_for_beginners" title="Help:Referencing for beginners">adding citations to reliable sources</a>. Unsourced material may be challenged and <a href="/wiki/Wikipedia:Verifiability#Burden_of_evidence" title="Wikipedia:Verifiability">removed</a>.</span> <span class="date-container"><i>(<span class="date">June 2023</span>)</i></span><span class="hide-when-compact"><i> (<small><a href="/wiki/Help:Maintenance_template_removal" title="Help:Maintenance template removal">Learn how and when to remove this message</a></small>)</i></span></div></td></tr></tbody></table> <p>In some <a href="/wiki/Microsoft_.NET" class="mw-redirect" title="Microsoft .NET">Microsoft .NET</a> languages, <a href="/wiki/Extension_method" title="Extension method">Extension methods</a> can be used to perform what is called 'null coalescing'. This is because extension methods can be called on null values as if it concerns an 'instance method invocation' while in fact extension methods are static. Extension methods can be made to check for null values, thereby freeing code that uses them from ever having to do so. Note that the example below uses the <a href="/wiki/C_Sharp_(programming_language)" title="C Sharp (programming language)">C#</a> <a href="/wiki/Null_coalescing_operator" title="Null coalescing operator">Null coalescing operator</a> to guarantee error free invocation, where it could also have used a more mundane if...then...else. The following example only works when you do not care the existence of null, or you treat null and empty string the same. The assumption may not hold in other applications. </p> <div class="mw-highlight mw-highlight-lang-csharp mw-content-ltr" dir="ltr"><pre><span></span><span class="c1">// compile as Console Application, requires C# 3.0 or higher</span> <span class="k">using</span><span class="w"> </span><span class="nn">System</span><span class="p">;</span> <span class="k">using</span><span class="w"> </span><span class="nn">System.Linq</span><span class="p">;</span> <span class="k">namespace</span><span class="w"> </span><span class="nn">MyExtensionWithExample</span><span class="w"> </span><span class="p">{</span> <span class="w"> </span><span class="k">public</span><span class="w"> </span><span class="k">static</span><span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">StringExtensions</span><span class="w"> </span><span class="p">{</span><span class="w"> </span> <span class="w"> </span><span class="k">public</span><span class="w"> </span><span class="k">static</span><span class="w"> </span><span class="kt">int</span><span class="w"> </span><span class="nf">SafeGetLength</span><span class="p">(</span><span class="k">this</span><span class="w"> </span><span class="kt">string</span><span class="w"> </span><span class="n">valueOrNull</span><span class="p">)</span><span class="w"> </span><span class="p">{</span><span class="w"> </span> <span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="p">(</span><span class="n">valueOrNull</span><span class="w"> </span><span class="o">??</span><span class="w"> </span><span class="kt">string</span><span class="p">.</span><span class="n">Empty</span><span class="p">).</span><span class="n">Length</span><span class="p">;</span><span class="w"> </span> <span class="w"> </span><span class="p">}</span> <span class="w"> </span><span class="p">}</span> <span class="w"> </span><span class="k">public</span><span class="w"> </span><span class="k">static</span><span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">Program</span><span class="w"> </span><span class="p">{</span> <span class="w"> </span><span class="c1">// define some strings</span> <span class="w"> </span><span class="k">static</span><span class="w"> </span><span class="k">readonly</span><span class="w"> </span><span class="kt">string</span><span class="p">[]</span><span class="w"> </span><span class="n">strings</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="k">new</span><span class="w"> </span><span class="p">[]</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="s">"Mr X."</span><span class="p">,</span><span class="w"> </span><span class="s">"Katrien Duck"</span><span class="p">,</span><span class="w"> </span><span class="k">null</span><span class="p">,</span><span class="w"> </span><span class="s">"Q"</span><span class="w"> </span><span class="p">};</span> <span class="w"> </span><span class="c1">// write the total length of all the strings in the array</span> <span class="w"> </span><span class="k">public</span><span class="w"> </span><span class="k">static</span><span class="w"> </span><span class="k">void</span><span class="w"> </span><span class="nf">Main</span><span class="p">(</span><span class="kt">string</span><span class="p">[]</span><span class="w"> </span><span class="n">args</span><span class="p">)</span><span class="w"> </span><span class="p">{</span> <span class="w"> </span><span class="kt">var</span><span class="w"> </span><span class="n">query</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="k">from</span><span class="w"> </span><span class="n">text</span><span class="w"> </span><span class="k">in</span><span class="w"> </span><span class="n">strings</span><span class="w"> </span><span class="k">select</span><span class="w"> </span><span class="n">text</span><span class="p">.</span><span class="n">SafeGetLength</span><span class="p">();</span><span class="w"> </span><span class="c1">// no need to do any checks here</span> <span class="w"> </span><span class="n">Console</span><span class="p">.</span><span class="n">WriteLine</span><span class="p">(</span><span class="n">query</span><span class="p">.</span><span class="n">Sum</span><span class="p">());</span> <span class="w"> </span><span class="p">}</span> <span class="w"> </span><span class="p">}</span> <span class="p">}</span> <span class="c1">// The output will be:</span> <span class="c1">// 18</span> </pre></div> <div class="mw-heading mw-heading2"><h2 id="In_various_languages">In various languages</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Null_object_pattern&action=edit&section=7" title="Edit section: In various languages"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1251242444"><table class="box-Unreferenced_section plainlinks metadata ambox ambox-content ambox-Unreferenced" role="presentation"><tbody><tr><td class="mbox-image"><div class="mbox-image-div"><span typeof="mw:File"><a href="/wiki/File:Question_book-new.svg" class="mw-file-description"><img alt="" src="//upload.wikimedia.org/wikipedia/en/thumb/9/99/Question_book-new.svg/50px-Question_book-new.svg.png" decoding="async" width="50" height="39" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/en/thumb/9/99/Question_book-new.svg/75px-Question_book-new.svg.png 1.5x, //upload.wikimedia.org/wikipedia/en/thumb/9/99/Question_book-new.svg/100px-Question_book-new.svg.png 2x" data-file-width="512" data-file-height="399" /></a></span></div></td><td class="mbox-text"><div class="mbox-text-span">This section <b>does not <a href="/wiki/Wikipedia:Citing_sources" title="Wikipedia:Citing sources">cite</a> any <a href="/wiki/Wikipedia:Verifiability" title="Wikipedia:Verifiability">sources</a></b>.<span class="hide-when-compact"> Please help <a href="/wiki/Special:EditPage/Null_object_pattern" title="Special:EditPage/Null object pattern">improve this section</a> by <a href="/wiki/Help:Referencing_for_beginners" title="Help:Referencing for beginners">adding citations to reliable sources</a>. Unsourced material may be challenged and <a href="/wiki/Wikipedia:Verifiability#Burden_of_evidence" title="Wikipedia:Verifiability">removed</a>.</span> <span class="date-container"><i>(<span class="date">June 2013</span>)</i></span><span class="hide-when-compact"><i> (<small><a href="/wiki/Help:Maintenance_template_removal" title="Help:Maintenance template removal">Learn how and when to remove this message</a></small>)</i></span></div></td></tr></tbody></table> <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1251242444"><table class="box-Original_research plainlinks metadata ambox ambox-content ambox-Original_research" role="presentation"><tbody><tr><td class="mbox-image"><div class="mbox-image-div"><span typeof="mw:File"><span><img alt="" src="//upload.wikimedia.org/wikipedia/en/thumb/b/b4/Ambox_important.svg/40px-Ambox_important.svg.png" decoding="async" width="40" height="40" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/en/thumb/b/b4/Ambox_important.svg/60px-Ambox_important.svg.png 1.5x, //upload.wikimedia.org/wikipedia/en/thumb/b/b4/Ambox_important.svg/80px-Ambox_important.svg.png 2x" data-file-width="40" data-file-height="40" /></span></span></div></td><td class="mbox-text"><div class="mbox-text-span">This article <b>possibly contains <a href="/wiki/Wikipedia:No_original_research" title="Wikipedia:No original research">original research</a></b>.<span class="hide-when-compact"> Please <a class="external text" href="https://en.wikipedia.org/w/index.php?title=Null_object_pattern&action=edit">improve it</a> by <a href="/wiki/Wikipedia:Verifiability" title="Wikipedia:Verifiability">verifying</a> the claims made and adding <a href="/wiki/Wikipedia:Citing_sources#Inline_citations" title="Wikipedia:Citing sources">inline citations</a>. Statements consisting only of original research should be removed.</span> <span class="date-container"><i>(<span class="date">June 2013</span>)</i></span><span class="hide-when-compact"><i> (<small><a href="/wiki/Help:Maintenance_template_removal" title="Help:Maintenance template removal">Learn how and when to remove this message</a></small>)</i></span></div></td></tr></tbody></table> <div class="mw-heading mw-heading3"><h3 id="C++"><span id="C.2B.2B"></span>C++</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Null_object_pattern&action=edit&section=8" title="Edit section: C++"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>A language with statically typed references to objects illustrates how the null object becomes a more complicated pattern: </p> <div class="mw-highlight mw-highlight-lang-cpp mw-content-ltr" dir="ltr"><pre><span></span><span class="k">import</span><span class="w"> </span><span class="n">std</span><span class="p">;</span> <span class="k">class</span><span class="w"> </span><span class="nc">Animal</span><span class="w"> </span><span class="p">{</span> <span class="w"> </span><span class="k">public</span><span class="o">:</span> <span class="w"> </span><span class="k">virtual</span><span class="w"> </span><span class="o">~</span><span class="n">Animal</span><span class="p">()</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="k">default</span><span class="p">;</span> <span class="w"> </span><span class="k">virtual</span><span class="w"> </span><span class="kt">void</span><span class="w"> </span><span class="nf">MakeSound</span><span class="p">()</span><span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mi">0</span><span class="p">;</span> <span class="p">};</span> <span class="k">class</span><span class="w"> </span><span class="nc">Dog</span><span class="o">:</span><span class="w"> </span><span class="k">public</span><span class="w"> </span><span class="n">Animal</span><span class="w"> </span><span class="p">{</span> <span class="w"> </span><span class="k">public</span><span class="o">:</span> <span class="w"> </span><span class="k">virtual</span><span class="w"> </span><span class="kt">void</span><span class="w"> </span><span class="n">MakeSound</span><span class="p">()</span><span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="k">override</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">println</span><span class="p">(</span><span class="s">"woof!"</span><span class="p">);</span><span class="w"> </span><span class="p">}</span> <span class="p">};</span> <span class="k">class</span><span class="w"> </span><span class="nc">NullAnimal</span><span class="o">:</span><span class="w"> </span><span class="k">public</span><span class="w"> </span><span class="n">Animal</span><span class="w"> </span><span class="p">{</span> <span class="w"> </span><span class="k">public</span><span class="o">:</span> <span class="w"> </span><span class="k">virtual</span><span class="w"> </span><span class="kt">void</span><span class="w"> </span><span class="n">MakeSound</span><span class="p">()</span><span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="k">override</span><span class="w"> </span><span class="p">{}</span> <span class="p">};</span> </pre></div> <p>Here, the idea is that there are situations where a pointer or reference to an <code>Animal</code> object is required, but there is no appropriate object available. A null reference is impossible in standard-conforming C++. A null <code>Animal*</code> pointer is possible, and could be useful as a place-holder, but may not be used for direct dispatch: <code>a->MakeSound()</code> is undefined behavior if <code>a</code> is a null pointer. </p><p>The null object pattern solves this problem by providing a special <code>NullAnimal</code> class which can be instantiated bound to an <code>Animal</code> pointer or reference. </p><p>The special null class must be created for each class hierarchy that is to have a null object, since a <code>NullAnimal</code> is of no use when what is needed is a null object with regard to some <code>Widget</code> base class that is not related to the <code>Animal</code> hierarchy. </p><p>Note that NOT having a null class at all is an important feature, in contrast to languages where "anything is a reference" (e.g., Java and C#). In C++, the design of a function or method may explicitly state whether null is allowed or not. </p> <div class="mw-highlight mw-highlight-lang-cpp mw-content-ltr" dir="ltr"><pre><span></span><span class="c1">// Function which requires an |Animal| instance, and will not accept null.</span> <span class="kt">void</span><span class="w"> </span><span class="nf">DoSomething</span><span class="p">(</span><span class="k">const</span><span class="w"> </span><span class="n">Animal</span><span class="o">&</span><span class="w"> </span><span class="n">animal</span><span class="p">)</span><span class="w"> </span><span class="p">{</span> <span class="w"> </span><span class="c1">// |animal| may never be null here.</span> <span class="p">}</span> <span class="c1">// Function which may accept an |Animal| instance or null.</span> <span class="kt">void</span><span class="w"> </span><span class="nf">DoSomething</span><span class="p">(</span><span class="k">const</span><span class="w"> </span><span class="n">Animal</span><span class="o">*</span><span class="w"> </span><span class="n">animal</span><span class="p">)</span><span class="w"> </span><span class="p">{</span> <span class="w"> </span><span class="c1">// |animal| may be null.</span> <span class="p">}</span> </pre></div> <div class="mw-heading mw-heading3"><h3 id="C#"><span id="C.23"></span>C#</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Null_object_pattern&action=edit&section=9" title="Edit section: C#"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>C# is a language in which the null object pattern can be properly implemented. This example shows animal objects that display sounds and a NullAnimal instance used in place of the C# null keyword. The null object provides consistent behaviour and prevents a runtime null reference exception that would occur if the C# null keyword were used instead. </p> <div class="mw-highlight mw-highlight-lang-csharp mw-content-ltr" dir="ltr"><pre><span></span><span class="cm">/* Null object pattern implementation:</span> <span class="cm"> */</span> <span class="k">using</span><span class="w"> </span><span class="nn">System</span><span class="p">;</span> <span class="c1">// Animal interface is the key to compatibility for Animal implementations below.</span> <span class="k">interface</span><span class="w"> </span><span class="n">IAnimal</span> <span class="p">{</span> <span class="w"> </span><span class="k">void</span><span class="w"> </span><span class="nf">MakeSound</span><span class="p">();</span> <span class="p">}</span> <span class="c1">// Animal is the base case.</span> <span class="k">abstract</span><span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">Animal</span><span class="w"> </span><span class="p">:</span><span class="w"> </span><span class="n">IAnimal</span> <span class="p">{</span> <span class="w"> </span><span class="c1">// A shared instance that can be used for comparisons</span> <span class="w"> </span><span class="k">public</span><span class="w"> </span><span class="k">static</span><span class="w"> </span><span class="k">readonly</span><span class="w"> </span><span class="n">IAnimal</span><span class="w"> </span><span class="n">Null</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="k">new</span><span class="w"> </span><span class="n">NullAnimal</span><span class="p">();</span> <span class="w"> </span> <span class="w"> </span><span class="c1">// The Null Case: this NullAnimal class should be used in place of C# null keyword.</span> <span class="w"> </span><span class="k">private</span><span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">NullAnimal</span><span class="w"> </span><span class="p">:</span><span class="w"> </span><span class="n">Animal</span> <span class="w"> </span><span class="p">{</span> <span class="w"> </span><span class="k">public</span><span class="w"> </span><span class="k">override</span><span class="w"> </span><span class="k">void</span><span class="w"> </span><span class="nf">MakeSound</span><span class="p">()</span> <span class="w"> </span><span class="p">{</span> <span class="w"> </span><span class="c1">// Purposefully provides no behaviour.</span> <span class="w"> </span><span class="p">}</span> <span class="w"> </span><span class="p">}</span> <span class="w"> </span><span class="k">public</span><span class="w"> </span><span class="k">abstract</span><span class="w"> </span><span class="k">void</span><span class="w"> </span><span class="nf">MakeSound</span><span class="p">();</span> <span class="p">}</span> <span class="c1">// Dog is a real animal.</span> <span class="k">class</span><span class="w"> </span><span class="nc">Dog</span><span class="w"> </span><span class="p">:</span><span class="w"> </span><span class="n">Animal</span> <span class="p">{</span> <span class="w"> </span><span class="k">public</span><span class="w"> </span><span class="k">override</span><span class="w"> </span><span class="k">void</span><span class="w"> </span><span class="nf">MakeSound</span><span class="p">()</span> <span class="w"> </span><span class="p">{</span> <span class="w"> </span><span class="n">Console</span><span class="p">.</span><span class="n">WriteLine</span><span class="p">(</span><span class="s">"Woof!"</span><span class="p">);</span> <span class="w"> </span><span class="p">}</span> <span class="p">}</span> <span class="cm">/* =========================</span> <span class="cm"> * Simplistic usage example in a Main entry point.</span> <span class="cm"> */</span> <span class="k">static</span><span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">Program</span> <span class="p">{</span> <span class="w"> </span><span class="k">static</span><span class="w"> </span><span class="k">void</span><span class="w"> </span><span class="nf">Main</span><span class="p">()</span> <span class="w"> </span><span class="p">{</span> <span class="w"> </span><span class="n">IAnimal</span><span class="w"> </span><span class="n">dog</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="k">new</span><span class="w"> </span><span class="n">Dog</span><span class="p">();</span> <span class="w"> </span><span class="n">dog</span><span class="p">.</span><span class="n">MakeSound</span><span class="p">();</span><span class="w"> </span><span class="c1">// outputs "Woof!"</span> <span class="w"> </span><span class="cm">/* Instead of using C# null, use the Animal.Null instance.</span> <span class="cm"> * This example is simplistic but conveys the idea that if the Animal.Null instance is used then the program</span> <span class="cm"> * will never experience a .NET System.NullReferenceException at runtime, unlike if C# null were used.</span> <span class="cm"> */</span> <span class="w"> </span><span class="n">IAnimal</span><span class="w"> </span><span class="n">unknown</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">Animal</span><span class="p">.</span><span class="n">Null</span><span class="p">;</span><span class="w"> </span><span class="c1">//<< replaces: IAnimal unknown = null;</span> <span class="w"> </span><span class="n">unknown</span><span class="p">.</span><span class="n">MakeSound</span><span class="p">();</span><span class="w"> </span><span class="c1">// outputs nothing, but does not throw a runtime exception </span> <span class="w"> </span><span class="p">}</span> <span class="p">}</span> </pre></div> <div class="mw-heading mw-heading3"><h3 id="Smalltalk">Smalltalk</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Null_object_pattern&action=edit&section=10" title="Edit section: Smalltalk"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>Following the Smalltalk principle, <i>everything is an object</i>, the absence of an object is itself modeled by an object, called <code>nil</code>. In the GNU Smalltalk for example, the class of <code>nil</code> is <code>UndefinedObject</code>, a direct descendant of <code>Object</code>. </p><p>Any operation that fails to return a sensible object for its purpose may return <code>nil</code> instead, thus avoiding the special case of returning "no object" unsupported by Smalltalk designers. This method has the advantage of simplicity (no need for a special case) over the classical "null" or "no object" or "null reference" approach. Especially useful messages to be used with <code>nil</code> are <code>isNil</code>, <code>ifNil:</code> or <code>ifNotNil:</code>, which make it practical and safe to deal with possible references to <code>nil</code> in Smalltalk programs. </p> <div class="mw-heading mw-heading3"><h3 id="Common_Lisp">Common Lisp</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Null_object_pattern&action=edit&section=11" title="Edit section: Common Lisp"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>In Lisp, functions can gracefully accept the special object <code>nil</code>, which reduces the amount of special case testing in application code. For instance, although <code>nil</code> is an atom and does not have any fields, the functions <code>car</code> and <code>cdr</code> accept <code>nil</code> and just return it, which is very useful and results in shorter code. </p><p>Since <code>nil</code> <b>is</b> the empty list in Lisp, the situation described in the introduction above doesn't exist. Code which returns <code>nil</code> is returning what is in fact the empty list (and not anything resembling a null reference to a list type), so the caller does not need to test the value to see whether or not it has a list. </p><p>The null object pattern is also supported in multiple value processing. If the program attempts to extract a value from an expression which returns no values, the behavior is that the null object <code>nil</code> is substituted. Thus <code>(list (values))</code> returns <code>(nil)</code> (a one-element list containing nil). The <code>(values)</code> expression returns no values at all, but since the function call to <code>list</code> needs to reduce its argument expression to a value, the null object is automatically substituted. </p> <div class="mw-heading mw-heading3"><h3 id="CLOS">CLOS</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Null_object_pattern&action=edit&section=12" title="Edit section: CLOS"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>In Common Lisp, the object <code>nil</code> is the one and only instance of the special class <code>null</code>. What this means is that a method can be specialized to the <code>null</code> class, thereby implementing the null design pattern. Which is to say, it is essentially built into the object system: </p> <div class="mw-highlight mw-highlight-lang-lisp mw-content-ltr" dir="ltr"><pre><span></span><span class="c1">;; empty dog class</span> <span class="p">(</span><span class="nb">defclass</span><span class="w"> </span><span class="nv">dog</span><span class="w"> </span><span class="p">()</span><span class="w"> </span><span class="p">())</span> <span class="c1">;; a dog object makes a sound by barking: woof! is printed on standard output</span> <span class="c1">;; when (make-sound x) is called, if x is an instance of the dog class.</span> <span class="p">(</span><span class="nb">defmethod</span><span class="w"> </span><span class="nv">make-sound</span><span class="w"> </span><span class="p">((</span><span class="nv">obj</span><span class="w"> </span><span class="nv">dog</span><span class="p">))</span> <span class="w"> </span><span class="p">(</span><span class="nb">format</span><span class="w"> </span><span class="no">t</span><span class="w"> </span><span class="s">"woof!~%"</span><span class="p">))</span> <span class="c1">;; allow (make-sound nil) to work via specialization to null class.</span> <span class="c1">;; innocuous empty body: nil makes no sound.</span> <span class="p">(</span><span class="nb">defmethod</span><span class="w"> </span><span class="nv">make-sound</span><span class="w"> </span><span class="p">((</span><span class="nv">obj</span><span class="w"> </span><span class="nb">null</span><span class="p">)))</span> </pre></div> <p>The class <code>null</code> is a subclass of the <code>symbol</code> class, because <code>nil</code> is a symbol. Since <code>nil</code> also represents the empty list, <code>null</code> is a subclass of the <code>list</code> class, too. Methods parameters specialized to <code>symbol</code> or <code>list</code> will thus take a <code>nil</code> argument. Of course, a <code>null</code> specialization can still be defined which is a more specific match for <code>nil</code>. </p> <div class="mw-heading mw-heading3"><h3 id="Scheme">Scheme</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Null_object_pattern&action=edit&section=13" title="Edit section: Scheme"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>Unlike Common Lisp, and many dialects of Lisp, the Scheme dialect does not have a nil value which works this way; the functions <code>car</code> and <code>cdr</code> may not be applied to an empty list; Scheme application code therefore has to use the <code>empty?</code> or <code>pair?</code> predicate functions to sidestep this situation, even in situations where very similar Lisp would not need to distinguish the empty and non-empty cases thanks to the behavior of <code>nil</code>. </p> <div class="mw-heading mw-heading3"><h3 id="Ruby">Ruby</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Null_object_pattern&action=edit&section=14" title="Edit section: Ruby"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>In <a href="/wiki/Duck_typing" title="Duck typing">duck-typed</a> languages like <a href="/wiki/Ruby_(programming_language)" title="Ruby (programming language)">Ruby</a>, language inheritance is not necessary to provide expected behavior. </p> <div class="mw-highlight mw-highlight-lang-ruby mw-content-ltr" dir="ltr"><pre><span></span><span class="k">class</span><span class="w"> </span><span class="nc">Dog</span> <span class="w"> </span><span class="k">def</span><span class="w"> </span><span class="nf">sound</span> <span class="w"> </span><span class="s2">"bark"</span> <span class="w"> </span><span class="k">end</span> <span class="k">end</span> <span class="w"> </span> <span class="k">class</span><span class="w"> </span><span class="nc">NilAnimal</span> <span class="w"> </span><span class="k">def</span><span class="w"> </span><span class="nf">sound</span><span class="p">(</span><span class="o">*</span><span class="p">);</span><span class="w"> </span><span class="k">end</span> <span class="k">end</span> <span class="k">def</span><span class="w"> </span><span class="nf">get_animal</span><span class="p">(</span><span class="n">animal</span><span class="o">=</span><span class="no">NilAnimal</span><span class="o">.</span><span class="n">new</span><span class="p">)</span> <span class="w"> </span><span class="n">animal</span> <span class="k">end</span> <span class="n">get_animal</span><span class="p">(</span><span class="no">Dog</span><span class="o">.</span><span class="n">new</span><span class="p">)</span><span class="o">.</span><span class="n">sound</span> <span class="w"> </span><span class="o">=></span><span class="w"> </span><span class="s2">"bark"</span> <span class="n">get_animal</span><span class="o">.</span><span class="n">sound</span> <span class="w"> </span><span class="o">=></span><span class="w"> </span><span class="kp">nil</span> </pre></div> <p>Attempts to directly <a href="/wiki/Monkey-patch" class="mw-redirect" title="Monkey-patch">monkey-patch</a> NilClass instead of providing explicit implementations give more unexpected side effects than benefits. </p> <div class="mw-heading mw-heading3"><h3 id="JavaScript">JavaScript</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Null_object_pattern&action=edit&section=15" title="Edit section: JavaScript"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>In <a href="/wiki/Duck_typing" title="Duck typing">duck-typed</a> languages like <a href="/wiki/JavaScript" title="JavaScript">JavaScript</a>, language inheritance is not necessary to provide expected behavior. </p> <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre><span></span><span class="kd">class</span><span class="w"> </span><span class="nx">Dog</span><span class="w"> </span><span class="p">{</span> <span class="w"> </span><span class="nx">sound</span><span class="p">()</span><span class="w"> </span><span class="p">{</span> <span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="s1">'bark'</span><span class="p">;</span> <span class="w"> </span><span class="p">}</span> <span class="p">}</span> <span class="kd">class</span><span class="w"> </span><span class="nx">NullAnimal</span><span class="w"> </span><span class="p">{</span> <span class="w"> </span><span class="nx">sound</span><span class="p">()</span><span class="w"> </span><span class="p">{</span> <span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="kc">null</span><span class="p">;</span> <span class="w"> </span><span class="p">}</span> <span class="p">}</span> <span class="kd">function</span><span class="w"> </span><span class="nx">getAnimal</span><span class="p">(</span><span class="nx">type</span><span class="p">)</span><span class="w"> </span><span class="p">{</span> <span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="nx">type</span><span class="w"> </span><span class="o">===</span><span class="w"> </span><span class="s1">'dog'</span><span class="w"> </span><span class="o">?</span><span class="w"> </span><span class="ow">new</span><span class="w"> </span><span class="nx">Dog</span><span class="p">()</span><span class="w"> </span><span class="o">:</span><span class="w"> </span><span class="ow">new</span><span class="w"> </span><span class="nx">NullAnimal</span><span class="p">();</span> <span class="p">}</span> <span class="p">[</span><span class="s1">'dog'</span><span class="p">,</span><span class="w"> </span><span class="kc">null</span><span class="p">].</span><span class="nx">map</span><span class="p">((</span><span class="nx">animal</span><span class="p">)</span><span class="w"> </span><span class="p">=></span><span class="w"> </span><span class="nx">getAnimal</span><span class="p">(</span><span class="nx">animal</span><span class="p">).</span><span class="nx">sound</span><span class="p">());</span> <span class="c1">// Returns ["bark", null]</span> </pre></div> <div class="mw-heading mw-heading3"><h3 id="Java">Java</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Null_object_pattern&action=edit&section=16" title="Edit section: Java"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <div class="mw-highlight mw-highlight-lang-java mw-content-ltr" dir="ltr"><pre><span></span><span class="kd">public</span><span class="w"> </span><span class="kd">interface</span> <span class="nc">Animal</span><span class="w"> </span><span class="p">{</span> <span class="w"> </span><span class="kt">void</span><span class="w"> </span><span class="nf">makeSound</span><span class="p">();</span> <span class="p">}</span> <span class="kd">public</span><span class="w"> </span><span class="kd">class</span> <span class="nc">Dog</span><span class="w"> </span><span class="kd">implements</span><span class="w"> </span><span class="n">Animal</span><span class="w"> </span><span class="p">{</span> <span class="w"> </span><span class="kd">public</span><span class="w"> </span><span class="kt">void</span><span class="w"> </span><span class="nf">makeSound</span><span class="p">()</span><span class="w"> </span><span class="p">{</span> <span class="w"> </span><span class="n">System</span><span class="p">.</span><span class="na">out</span><span class="p">.</span><span class="na">println</span><span class="p">(</span><span class="s">"woof!"</span><span class="p">);</span> <span class="w"> </span><span class="p">}</span> <span class="p">}</span> <span class="kd">public</span><span class="w"> </span><span class="kd">class</span> <span class="nc">NullAnimal</span><span class="w"> </span><span class="kd">implements</span><span class="w"> </span><span class="n">Animal</span><span class="w"> </span><span class="p">{</span> <span class="w"> </span><span class="kd">public</span><span class="w"> </span><span class="kt">void</span><span class="w"> </span><span class="nf">makeSound</span><span class="p">()</span><span class="w"> </span><span class="p">{</span> <span class="w"> </span><span class="c1">// silence...</span> <span class="w"> </span><span class="p">}</span> <span class="p">}</span> </pre></div> <p>This code illustrates a variation of the C++ example, above, using the Java language. As with C++, a null class can be instantiated in situations where a reference to an <code>Animal</code> object is required, but there is no appropriate object available. A null <code>Animal</code> object is possible (<code>Animal myAnimal = null;</code>) and could be useful as a place-holder, but may not be used for calling a method. In this example, <code>myAnimal.makeSound();</code> will throw a NullPointerException. Therefore, additional code may be necessary to test for null objects. </p><p>The null object pattern solves this problem by providing a special <code>NullAnimal</code> class which can be instantiated as an object of type <code>Animal</code>. As with C++ and related languages, that special null class must be created for each class hierarchy that needs a null object, since a <code>NullAnimal</code> is of no use when what is needed is a null object that does not implement the <code>Animal</code> interface. </p> <div class="mw-heading mw-heading3"><h3 id="PHP">PHP</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Null_object_pattern&action=edit&section=17" title="Edit section: PHP"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <div class="mw-highlight mw-highlight-lang-php mw-content-ltr" dir="ltr"><pre><span></span><span class="k">interface</span> <span class="nx">Animal</span> <span class="p">{</span> <span class="k">public</span> <span class="k">function</span> <span class="nf">makeSound</span><span class="p">();</span> <span class="p">}</span> <span class="k">class</span> <span class="nc">Dog</span> <span class="k">implements</span> <span class="nx">Animal</span> <span class="p">{</span> <span class="k">public</span> <span class="k">function</span> <span class="nf">makeSound</span><span class="p">()</span> <span class="p">{</span> <span class="k">echo</span> <span class="s2">"Woof...</span><span class="se">\n</span><span class="s2">"</span><span class="p">;</span> <span class="p">}</span> <span class="p">}</span> <span class="k">class</span> <span class="nc">Cat</span> <span class="k">implements</span> <span class="nx">Animal</span> <span class="p">{</span> <span class="k">public</span> <span class="k">function</span> <span class="nf">makeSound</span><span class="p">()</span> <span class="p">{</span> <span class="k">echo</span> <span class="s2">"Meowww...</span><span class="se">\n</span><span class="s2">"</span><span class="p">;</span> <span class="p">}</span> <span class="p">}</span> <span class="k">class</span> <span class="nc">NullAnimal</span> <span class="k">implements</span> <span class="nx">Animal</span> <span class="p">{</span> <span class="k">public</span> <span class="k">function</span> <span class="nf">makeSound</span><span class="p">()</span> <span class="p">{</span> <span class="c1">// silence...</span> <span class="p">}</span> <span class="p">}</span> <span class="nv">$animalType</span> <span class="o">=</span> <span class="s1">'elephant'</span><span class="p">;</span> <span class="k">function</span> <span class="nf">makeAnimalFromAnimalType</span><span class="p">(</span><span class="nx">string</span> <span class="nv">$animalType</span><span class="p">)</span><span class="o">:</span> <span class="nx">Animal</span> <span class="p">{</span> <span class="k">switch</span> <span class="p">(</span><span class="nv">$animalType</span><span class="p">)</span> <span class="p">{</span> <span class="k">case</span> <span class="s1">'dog'</span><span class="o">:</span> <span class="k">return</span> <span class="k">new</span> <span class="nx">Dog</span><span class="p">();</span> <span class="k">case</span> <span class="s1">'cat'</span><span class="o">:</span> <span class="k">return</span> <span class="k">new</span> <span class="nx">Cat</span><span class="p">();</span> <span class="k">default</span><span class="o">:</span> <span class="k">return</span> <span class="k">new</span> <span class="nx">NullAnimal</span><span class="p">();</span> <span class="p">}</span> <span class="p">}</span> <span class="nx">makeAnimalFromAnimalType</span><span class="p">(</span><span class="nv">$animalType</span><span class="p">)</span><span class="o">-></span><span class="na">makeSound</span><span class="p">();</span> <span class="c1">// ..the null animal makes no sound</span> <span class="k">function</span> <span class="nf">animalMakeSound</span><span class="p">(</span><span class="nx">Animal</span> <span class="nv">$animal</span><span class="p">)</span><span class="o">:</span> <span class="nx">void</span> <span class="p">{</span> <span class="nv">$animal</span><span class="o">-></span><span class="na">makeSound</span><span class="p">();</span> <span class="p">}</span> <span class="k">foreach</span> <span class="p">([</span> <span class="nx">makeAnimalFromAnimalType</span><span class="p">(</span><span class="s1">'dog'</span><span class="p">),</span> <span class="nx">makeAnimalFromAnimalType</span><span class="p">(</span><span class="s1">'NullAnimal'</span><span class="p">),</span> <span class="nx">makeAnimalFromAnimalType</span><span class="p">(</span><span class="s1">'cat'</span><span class="p">),</span> <span class="p">]</span> <span class="k">as</span> <span class="nv">$animal</span><span class="p">)</span> <span class="p">{</span> <span class="c1">// That's also reduce null handling code</span> <span class="nx">animalMakeSound</span><span class="p">(</span><span class="nv">$animal</span><span class="p">);</span> <span class="p">}</span> </pre></div> <div class="mw-heading mw-heading3"><h3 id="Visual_Basic_.NET">Visual Basic .NET</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Null_object_pattern&action=edit&section=18" title="Edit section: Visual Basic .NET"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>The following null object pattern implementation demonstrates the concrete class providing its corresponding null object in a static field <code>Empty</code>. This approach is frequently used in the .NET Framework (<code>String.Empty</code>, <code>EventArgs.Empty</code>, <code>Guid.Empty</code>, etc.). </p> <div class="mw-highlight mw-highlight-lang-vbnet mw-content-ltr" dir="ltr"><pre><span></span><span class="k">Public</span><span class="w"> </span><span class="k">Class</span><span class="w"> </span><span class="nc">Animal</span> <span class="w"> </span><span class="k">Public</span><span class="w"> </span><span class="k">Shared</span><span class="w"> </span><span class="k">ReadOnly</span><span class="w"> </span><span class="n">Empty</span><span class="w"> </span><span class="ow">As</span><span class="w"> </span><span class="n">Animal</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="k">New</span><span class="w"> </span><span class="n">AnimalEmpty</span><span class="p">()</span> <span class="w"> </span><span class="k">Public</span><span class="w"> </span><span class="k">Overridable</span><span class="w"> </span><span class="k">Sub</span><span class="w"> </span><span class="nf">MakeSound</span><span class="p">()</span> <span class="w"> </span><span class="n">Console</span><span class="p">.</span><span class="n">WriteLine</span><span class="p">(</span><span class="s">"Woof!"</span><span class="p">)</span> <span class="w"> </span><span class="k">End</span><span class="w"> </span><span class="k">Sub</span> <span class="k">End</span><span class="w"> </span><span class="k">Class</span> <span class="k">Friend</span><span class="w"> </span><span class="k">NotInheritable</span><span class="w"> </span><span class="k">Class</span><span class="w"> </span><span class="nc">AnimalEmpty</span> <span class="w"> </span><span class="k">Inherits</span><span class="w"> </span><span class="n">Animal</span> <span class="w"> </span><span class="k">Public</span><span class="w"> </span><span class="k">Overrides</span><span class="w"> </span><span class="k">Sub</span><span class="w"> </span><span class="nf">MakeSound</span><span class="p">()</span> <span class="w"> </span><span class="c">' </span> <span class="w"> </span><span class="k">End</span><span class="w"> </span><span class="k">Sub</span> <span class="k">End</span><span class="w"> </span><span class="k">Class</span> </pre></div> <div class="mw-heading mw-heading2"><h2 id="Criticism">Criticism</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Null_object_pattern&action=edit&section=19" title="Edit section: Criticism"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>This pattern should be used carefully, as it can make errors/bugs appear as normal program execution.<sup id="cite_ref-7" class="reference"><a href="#cite_note-7"><span class="cite-bracket">[</span>7<span class="cite-bracket">]</span></a></sup> </p><p>Care should be taken not to implement this pattern just to avoid null checks and make code more readable, since the harder-to-read code may just move to another place and be less standard—such as when different logic must execute in case the object provided is indeed the null object. The common pattern in most languages with <a href="/wiki/Reference_type" class="mw-redirect" title="Reference type">reference types</a> is to compare a reference to a single value referred to as null or nil. Also, there is additional need for testing that no code anywhere ever assigns null instead of the null object, because in most cases and languages with static typing, this is not a compiler error if the null object is of a reference type, although it would certainly lead to errors at run time in parts of the code where the pattern was used to avoid null checks. On top of that, in most languages and assuming there can be many null objects (i.e., the null object is a reference type but doesn't implement the <a href="/wiki/Singleton_pattern" title="Singleton pattern">singleton pattern</a> in one or another way), checking for the null object instead of for the null or nil value introduces overhead, as does the singleton pattern likely itself upon obtaining the singleton reference. </p> <div class="mw-heading mw-heading2"><h2 id="See_also">See also</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Null_object_pattern&action=edit&section=20" title="Edit section: See also"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <ul><li><a href="/wiki/Nullable_type" title="Nullable type">Nullable type</a></li> <li><a href="/wiki/Option_type" title="Option type">Option type</a></li></ul> <div class="mw-heading mw-heading2"><h2 id="References">References</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Null_object_pattern&action=edit&section=21" title="Edit section: References"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <style data-mw-deduplicate="TemplateStyles:r1239543626">.mw-parser-output .reflist{margin-bottom:0.5em;list-style-type:decimal}@media screen{.mw-parser-output .reflist{font-size:90%}}.mw-parser-output .reflist .references{font-size:100%;margin-bottom:0;list-style-type:inherit}.mw-parser-output .reflist-columns-2{column-width:30em}.mw-parser-output .reflist-columns-3{column-width:25em}.mw-parser-output .reflist-columns{margin-top:0.3em}.mw-parser-output .reflist-columns ol{margin-top:0}.mw-parser-output .reflist-columns li{page-break-inside:avoid;break-inside:avoid-column}.mw-parser-output .reflist-upper-alpha{list-style-type:upper-alpha}.mw-parser-output .reflist-upper-roman{list-style-type:upper-roman}.mw-parser-output .reflist-lower-alpha{list-style-type:lower-alpha}.mw-parser-output .reflist-lower-greek{list-style-type:lower-greek}.mw-parser-output .reflist-lower-roman{list-style-type:lower-roman}</style><div class="reflist"> <div class="mw-references-wrap"><ol class="references"> <li id="cite_note-1"><span class="mw-cite-backlink"><b><a href="#cite_ref-1">^</a></b></span> <span class="reference-text"><style data-mw-deduplicate="TemplateStyles:r1238218222">.mw-parser-output cite.citation{font-style:inherit;word-wrap:break-word}.mw-parser-output .citation q{quotes:"\"""\"""'""'"}.mw-parser-output .citation:target{background-color:rgba(0,127,255,0.133)}.mw-parser-output .id-lock-free.id-lock-free a{background:url("//upload.wikimedia.org/wikipedia/commons/6/65/Lock-green.svg")right 0.1em center/9px no-repeat}.mw-parser-output .id-lock-limited.id-lock-limited a,.mw-parser-output .id-lock-registration.id-lock-registration a{background:url("//upload.wikimedia.org/wikipedia/commons/d/d6/Lock-gray-alt-2.svg")right 0.1em center/9px no-repeat}.mw-parser-output .id-lock-subscription.id-lock-subscription a{background:url("//upload.wikimedia.org/wikipedia/commons/a/aa/Lock-red-alt-2.svg")right 0.1em center/9px no-repeat}.mw-parser-output .cs1-ws-icon a{background:url("//upload.wikimedia.org/wikipedia/commons/4/4c/Wikisource-logo.svg")right 0.1em center/12px no-repeat}body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .id-lock-free a,body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .id-lock-limited a,body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .id-lock-registration a,body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .id-lock-subscription a,body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .cs1-ws-icon a{background-size:contain;padding:0 1em 0 0}.mw-parser-output .cs1-code{color:inherit;background:inherit;border:none;padding:inherit}.mw-parser-output .cs1-hidden-error{display:none;color:var(--color-error,#d33)}.mw-parser-output .cs1-visible-error{color:var(--color-error,#d33)}.mw-parser-output .cs1-maint{display:none;color:#085;margin-left:0.3em}.mw-parser-output .cs1-kern-left{padding-left:0.2em}.mw-parser-output .cs1-kern-right{padding-right:0.2em}.mw-parser-output .citation .mw-selflink{font-weight:inherit}@media screen{.mw-parser-output .cs1-format{font-size:95%}html.skin-theme-clientpref-night .mw-parser-output .cs1-maint{color:#18911f}}@media screen and (prefers-color-scheme:dark){html.skin-theme-clientpref-os .mw-parser-output .cs1-maint{color:#18911f}}</style><cite id="CITEREFKühne1996" class="citation conference cs1">Kühne, Thomas (1996). "Void Value". <i>Proceedings of the First International Conference on Object-Oriented Technology, White Object-Oriented Nights 1996 (WOON'96), St. Petersburg, Russia</i>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=conference&rft.atitle=Void+Value&rft.btitle=Proceedings+of+the+First+International+Conference+on+Object-Oriented+Technology%2C+White+Object-Oriented+Nights+1996+%28WOON%2796%29%2C+St.+Petersburg%2C+Russia&rft.date=1996&rft.aulast=K%C3%BChne&rft.aufirst=Thomas&rfr_id=info%3Asid%2Fen.wikipedia.org%3ANull+object+pattern" class="Z3988"></span></span> </li> <li id="cite_note-2"><span class="mw-cite-backlink"><b><a href="#cite_ref-2">^</a></b></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite id="CITEREFWoolf1998" class="citation book cs1">Woolf, Bobby (1998). "Null Object". In Martin, Robert; Riehle, Dirk; Buschmann, Frank (eds.). <i>Pattern Languages of Program Design 3</i>. Addison-Wesley.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=bookitem&rft.atitle=Null+Object&rft.btitle=Pattern+Languages+of+Program+Design+3&rft.pub=Addison-Wesley&rft.date=1998&rft.aulast=Woolf&rft.aufirst=Bobby&rfr_id=info%3Asid%2Fen.wikipedia.org%3ANull+object+pattern" class="Z3988"></span></span> </li> <li id="cite_note-3"><span class="mw-cite-backlink"><b><a href="#cite_ref-3">^</a></b></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite class="citation web cs1"><a rel="nofollow" class="external text" href="https://developer.apple.com/library/ios/documentation/Cocoa/Conceptual/ProgrammingWithObjectiveC/WorkingwithObjects/WorkingwithObjects.html#//apple_ref/doc/uid/TP40011210-CH4-SW22">"Working with Objects (Working with nil)"</a>. <i>iOS Developer Library</i>. Apple, Inc. 2012-12-13<span class="reference-accessdate">. Retrieved <span class="nowrap">2014-05-19</span></span>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&rft.genre=unknown&rft.jtitle=iOS+Developer+Library&rft.atitle=Working+with+Objects+%28Working+with+nil%29&rft.date=2012-12-13&rft_id=https%3A%2F%2Fdeveloper.apple.com%2Flibrary%2Fios%2Fdocumentation%2FCocoa%2FConceptual%2FProgrammingWithObjectiveC%2FWorkingwithObjects%2FWorkingwithObjects.html%23%2F%2Fapple_ref%2Fdoc%2Fuid%2FTP40011210-CH4-SW22&rfr_id=info%3Asid%2Fen.wikipedia.org%3ANull+object+pattern" class="Z3988"></span></span> </li> <li id="cite_note-4"><span class="mw-cite-backlink"><b><a href="#cite_ref-4">^</a></b></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite id="CITEREFFowler1999" class="citation book cs1"><a href="/wiki/Martin_Fowler_(software_engineer)" title="Martin Fowler (software engineer)">Fowler, Martin</a> (1999). <span class="id-lock-registration" title="Free registration required"><a rel="nofollow" class="external text" href="https://archive.org/details/isbn_9780201485677"><i>Refactoring. Improving the Design of Existing Code</i></a></span>. Addison-Wesley. <a href="/wiki/ISBN_(identifier)" class="mw-redirect" title="ISBN (identifier)">ISBN</a> <a href="/wiki/Special:BookSources/0-201-48567-2" title="Special:BookSources/0-201-48567-2"><bdi>0-201-48567-2</bdi></a>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=book&rft.btitle=Refactoring.+Improving+the+Design+of+Existing+Code&rft.pub=Addison-Wesley&rft.date=1999&rft.isbn=0-201-48567-2&rft.aulast=Fowler&rft.aufirst=Martin&rft_id=https%3A%2F%2Farchive.org%2Fdetails%2Fisbn_9780201485677&rfr_id=info%3Asid%2Fen.wikipedia.org%3ANull+object+pattern" class="Z3988"></span></span> </li> <li id="cite_note-5"><span class="mw-cite-backlink"><b><a href="#cite_ref-5">^</a></b></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite id="CITEREFKerievsky2004" class="citation book cs1"><a href="/w/index.php?title=Joshua_Kerievsky&action=edit&redlink=1" class="new" title="Joshua Kerievsky (page does not exist)">Kerievsky, Joshua</a> (2004). <i>Refactoring To Patterns</i>. Addison-Wesley. <a href="/wiki/ISBN_(identifier)" class="mw-redirect" title="ISBN (identifier)">ISBN</a> <a href="/wiki/Special:BookSources/0-321-21335-1" title="Special:BookSources/0-321-21335-1"><bdi>0-321-21335-1</bdi></a>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=book&rft.btitle=Refactoring+To+Patterns&rft.pub=Addison-Wesley&rft.date=2004&rft.isbn=0-321-21335-1&rft.aulast=Kerievsky&rft.aufirst=Joshua&rfr_id=info%3Asid%2Fen.wikipedia.org%3ANull+object+pattern" class="Z3988"></span></span> </li> <li id="cite_note-6"><span class="mw-cite-backlink"><b><a href="#cite_ref-6">^</a></b></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite id="CITEREFMartin2002" class="citation book cs1"><a href="/wiki/Robert_Cecil_Martin" class="mw-redirect" title="Robert Cecil Martin">Martin, Robert</a> (2002). <span class="id-lock-registration" title="Free registration required"><a rel="nofollow" class="external text" href="https://archive.org/details/agilesoftwaredev00robe"><i>Agile Software Development: Principles, Patterns and Practices</i></a></span>. Pearson Education. <a href="/wiki/ISBN_(identifier)" class="mw-redirect" title="ISBN (identifier)">ISBN</a> <a href="/wiki/Special:BookSources/0-13-597444-5" title="Special:BookSources/0-13-597444-5"><bdi>0-13-597444-5</bdi></a>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=book&rft.btitle=Agile+Software+Development%3A+Principles%2C+Patterns+and+Practices&rft.pub=Pearson+Education&rft.date=2002&rft.isbn=0-13-597444-5&rft.aulast=Martin&rft.aufirst=Robert&rft_id=https%3A%2F%2Farchive.org%2Fdetails%2Fagilesoftwaredev00robe&rfr_id=info%3Asid%2Fen.wikipedia.org%3ANull+object+pattern" class="Z3988"></span></span> </li> <li id="cite_note-7"><span class="mw-cite-backlink"><b><a href="#cite_ref-7">^</a></b></span> <span class="reference-text">Fowler, Martin (1999). Refactoring, p. 216.</span> </li> </ol></div></div> <div class="mw-heading mw-heading2"><h2 id="External_links">External links</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Null_object_pattern&action=edit&section=22" title="Edit section: External links"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <ul><li><a rel="nofollow" class="external text" href="http://www.cs.oberlin.edu/~jwalker/nullObjPattern/">Jeffery Walker's account of the Null Object Pattern</a></li> <li><a rel="nofollow" class="external text" href="http://martinfowler.com/eaaCatalog/specialCase.html">Martin Fowler's description of Special Case, a slightly more general pattern</a></li> <li><a rel="nofollow" class="external text" href="http://www.owlnet.rice.edu/~comp212/00-spring/handouts/week06/null_object_revisited.htm">Null Object Pattern Revisited</a></li> <li><a rel="nofollow" class="external text" href="http://refactoring.com/catalog/introduceNullObject.html">Introduce Null Object refactoring</a></li> <li><a rel="nofollow" class="external text" href="http://sourcemaking.com/design_patterns/null_object">SourceMaking Tutorial</a></li> <li><a rel="nofollow" class="external text" href="https://medium.com/@eofster/null-object-pattern-in-swift-1b96e03b2756">Null Object Pattern in Swift</a></li></ul> <div class="navbox-styles"><style data-mw-deduplicate="TemplateStyles:r1129693374">.mw-parser-output .hlist dl,.mw-parser-output .hlist ol,.mw-parser-output .hlist ul{margin:0;padding:0}.mw-parser-output .hlist dd,.mw-parser-output .hlist dt,.mw-parser-output .hlist li{margin:0;display:inline}.mw-parser-output .hlist.inline,.mw-parser-output .hlist.inline dl,.mw-parser-output .hlist.inline ol,.mw-parser-output .hlist.inline ul,.mw-parser-output .hlist dl dl,.mw-parser-output .hlist dl ol,.mw-parser-output .hlist dl ul,.mw-parser-output .hlist ol dl,.mw-parser-output .hlist ol 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a>span,.mw-parser-output .navbar a>abbr{text-decoration:inherit}.mw-parser-output .navbar-mini abbr{font-variant:small-caps;border-bottom:none;text-decoration:none;cursor:inherit}.mw-parser-output .navbar-ct-full{font-size:114%;margin:0 7em}.mw-parser-output .navbar-ct-mini{font-size:114%;margin:0 4em}html.skin-theme-clientpref-night .mw-parser-output .navbar li a abbr{color:var(--color-base)!important}@media(prefers-color-scheme:dark){html.skin-theme-clientpref-os .mw-parser-output .navbar li a abbr{color:var(--color-base)!important}}@media print{.mw-parser-output .navbar{display:none!important}}</style><div class="navbar plainlinks hlist navbar-mini"><ul><li class="nv-view"><a href="/wiki/Template:Nulls" title="Template:Nulls"><abbr title="View this template">v</abbr></a></li><li class="nv-talk"><a href="/wiki/Template_talk:Nulls" title="Template talk:Nulls"><abbr title="Discuss this template">t</abbr></a></li><li class="nv-edit"><a href="/wiki/Special:EditPage/Template:Nulls" title="Special:EditPage/Template:Nulls"><abbr title="Edit this template">e</abbr></a></li></ul></div><div id="Nulls_in_computing108" style="font-size:114%;margin:0 4em">Nulls in <a href="/wiki/Computing" title="Computing">computing</a></div></th></tr><tr><td colspan="2" class="navbox-list navbox-odd hlist" style="width:100%;padding:0"><div style="padding:0 0.25em"> <ul><li><a href="/wiki/Null_character" title="Null character">Null character</a></li> <li><a href="/wiki/Null_device" title="Null device">Null device</a></li> <li><a href="/wiki/Null_function" title="Null function">Null function</a></li> <li><a href="/wiki/Null_modem" title="Null modem">Null modem</a></li> <li><a class="mw-selflink selflink">Null object pattern</a></li> <li><a href="/wiki/Null_pointer" title="Null pointer">Null pointer</a></li> <li><a href="/wiki/Null_(SQL)" title="Null (SQL)">Null in SQL</a></li> <li><a href="/wiki/Empty_string" title="Empty string">Null string</a></li> <li><a href="/wiki/Null_coalescing_operator" title="Null coalescing operator">Null coalescing operator</a></li></ul> </div></td></tr><tr><td class="navbox-abovebelow hlist" colspan="2"><div> <ul><li><i>See also:</i> <a href="/wiki/Null_coalescing_operator" title="Null coalescing operator">Null coalescing operator</a></li> <li><a href="/wiki/Nullable_type" title="Nullable type">Nullable type</a></li> <li><a href="/wiki/Undefined_value" title="Undefined value">Undefined value</a></li></ul> </div></td></tr></tbody></table></div> <div class="navbox-styles"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1129693374"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1236075235"></div><div role="navigation" class="navbox" aria-labelledby="Software_design_patterns225" style="padding:3px"><table class="nowraplinks mw-collapsible autocollapse navbox-inner" style="border-spacing:0;background:transparent;color:inherit"><tbody><tr><th scope="col" class="navbox-title" colspan="2"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1129693374"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1239400231"><div class="navbar plainlinks hlist navbar-mini"><ul><li class="nv-view"><a href="/wiki/Template:Design_patterns" title="Template:Design patterns"><abbr title="View this template">v</abbr></a></li><li class="nv-talk"><a href="/wiki/Template_talk:Design_patterns" title="Template talk:Design patterns"><abbr title="Discuss this template">t</abbr></a></li><li class="nv-edit"><a href="/wiki/Special:EditPage/Template:Design_patterns" title="Special:EditPage/Template:Design patterns"><abbr title="Edit this template">e</abbr></a></li></ul></div><div id="Software_design_patterns225" style="font-size:114%;margin:0 4em"><a href="/wiki/Software_design_pattern" title="Software design pattern">Software design patterns</a></div></th></tr><tr><th scope="row" class="navbox-group" style="width:1%"><a href="/wiki/Design_Patterns" title="Design Patterns">Gang of Four<br />patterns</a></th><td class="navbox-list-with-group navbox-list navbox-odd hlist" style="width:100%;padding:0"><div style="padding:0 0.25em"></div><table class="nowraplinks navbox-subgroup" style="border-spacing:0"><tbody><tr><th scope="row" class="navbox-group" style="width:1%"><a href="/wiki/Creational_pattern" title="Creational pattern">Creational</a></th><td class="navbox-list-with-group navbox-list navbox-odd" style="width:100%;padding:0"><div style="padding:0 0.25em"> <ul><li><a href="/wiki/Abstract_factory_pattern" title="Abstract factory pattern">Abstract factory</a></li> <li><a href="/wiki/Builder_pattern" title="Builder pattern">Builder</a></li> <li><a href="/wiki/Factory_method_pattern" title="Factory method pattern">Factory method</a></li> <li><a href="/wiki/Prototype_pattern" title="Prototype pattern">Prototype</a></li> <li><a href="/wiki/Singleton_pattern" title="Singleton pattern">Singleton</a></li></ul> </div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%"><a href="/wiki/Structural_pattern" title="Structural pattern">Structural</a></th><td class="navbox-list-with-group navbox-list navbox-even" style="width:100%;padding:0"><div style="padding:0 0.25em"> <ul><li><a href="/wiki/Adapter_pattern" title="Adapter pattern">Adapter</a></li> <li><a href="/wiki/Bridge_pattern" title="Bridge pattern">Bridge</a></li> <li><a href="/wiki/Composite_pattern" title="Composite pattern">Composite</a></li> <li><a href="/wiki/Decorator_pattern" title="Decorator pattern">Decorator</a></li> <li><a href="/wiki/Facade_pattern" title="Facade pattern">Facade</a></li> <li><a href="/wiki/Flyweight_pattern" title="Flyweight pattern">Flyweight</a></li> <li><a href="/wiki/Proxy_pattern" title="Proxy pattern">Proxy</a></li></ul> </div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%"><a href="/wiki/Behavioral_pattern" title="Behavioral pattern">Behavioral</a></th><td class="navbox-list-with-group navbox-list navbox-odd" style="width:100%;padding:0"><div style="padding:0 0.25em"> <ul><li><a href="/wiki/Chain-of-responsibility_pattern" title="Chain-of-responsibility pattern">Chain of responsibility</a></li> <li><a href="/wiki/Command_pattern" title="Command pattern">Command</a></li> <li><a href="/wiki/Interpreter_pattern" title="Interpreter pattern">Interpreter</a></li> <li><a href="/wiki/Iterator_pattern" title="Iterator pattern">Iterator</a></li> <li><a href="/wiki/Mediator_pattern" title="Mediator pattern">Mediator</a></li> <li><a href="/wiki/Memento_pattern" title="Memento pattern">Memento</a></li> <li><a href="/wiki/Observer_pattern" title="Observer pattern">Observer</a></li> <li><a href="/wiki/State_pattern" title="State pattern">State</a></li> <li><a href="/wiki/Strategy_pattern" title="Strategy pattern">Strategy</a></li> <li><a href="/wiki/Template_method_pattern" title="Template method pattern">Template method</a></li> <li><a href="/wiki/Visitor_pattern" title="Visitor pattern">Visitor</a></li></ul> </div></td></tr></tbody></table><div></div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%"><a href="/wiki/Concurrency_pattern" title="Concurrency pattern">Concurrency<br />patterns</a></th><td class="navbox-list-with-group navbox-list navbox-even hlist" style="width:100%;padding:0"><div style="padding:0 0.25em"> <ul><li><a href="/wiki/Active_object" title="Active object">Active object</a></li> <li><a href="/wiki/Balking_pattern" title="Balking pattern">Balking</a></li> <li><a href="/wiki/Binding_properties_pattern" title="Binding properties pattern">Binding properties</a></li> <li><a href="/wiki/Double-checked_locking" title="Double-checked locking">Double-checked locking</a></li> <li><a href="/wiki/Asynchronous_method_invocation" title="Asynchronous method invocation">Event-based asynchronous</a></li> <li><a href="/wiki/Guarded_suspension" title="Guarded suspension">Guarded suspension</a></li> <li><a href="/wiki/Join-pattern" title="Join-pattern">Join</a></li> <li><a href="/wiki/Lock_(computer_science)" title="Lock (computer science)">Lock</a></li> <li><a href="/wiki/Monitor_(synchronization)" title="Monitor (synchronization)">Monitor</a></li> <li><a href="/wiki/Proactor_pattern" title="Proactor pattern">Proactor</a></li> <li><a href="/wiki/Reactor_pattern" title="Reactor pattern">Reactor</a></li> <li><a href="/wiki/Readers%E2%80%93writer_lock" title="Readers–writer lock">Read–write lock</a></li> <li><a href="/wiki/Scheduler_pattern" class="mw-redirect" title="Scheduler pattern">Scheduler</a></li> <li><a href="/wiki/Scheduled-task_pattern" title="Scheduled-task pattern">Scheduled-task pattern</a></li> <li><a href="/wiki/Semaphore_(programming)" title="Semaphore (programming)">Semaphore</a></li> <li><a href="/wiki/Thread_pool" title="Thread pool">Thread pool</a></li> <li><a href="/wiki/Thread-local_storage" title="Thread-local storage">Thread-local storage</a></li></ul> </div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%"><a href="/wiki/Architectural_pattern" title="Architectural pattern">Architectural<br />patterns</a></th><td class="navbox-list-with-group navbox-list navbox-odd hlist" style="width:100%;padding:0"><div style="padding:0 0.25em"> <ul><li><a href="/wiki/Front_controller" title="Front controller">Front controller</a></li> <li><a href="/wiki/Interceptor_pattern" title="Interceptor pattern">Interceptor</a></li> <li><a href="/wiki/Model%E2%80%93view%E2%80%93controller" title="Model–view–controller">MVC</a> <ul><li><a href="/wiki/Model%E2%80%93view%E2%80%93presenter" title="Model–view–presenter">MVP</a></li> <li><a href="/wiki/Model%E2%80%93view%E2%80%93viewmodel" title="Model–view–viewmodel">MVVM</a></li></ul></li> <li><a href="/wiki/Action%E2%80%93domain%E2%80%93responder" title="Action–domain–responder">ADR</a></li> <li><a href="/wiki/Entity_component_system" title="Entity component system">ECS</a></li> <li><a href="/wiki/Multitier_architecture" title="Multitier architecture"><i>n</i>-tier</a></li> <li><a href="/wiki/Specification_pattern" title="Specification pattern">Specification</a></li> <li><a href="/wiki/Publish%E2%80%93subscribe_pattern" title="Publish–subscribe pattern">Publish–subscribe</a></li> <li><a href="/wiki/Naked_objects" title="Naked objects">Naked objects</a></li> <li><a href="/wiki/Service_locator_pattern" title="Service locator pattern">Service locator</a></li> <li><a href="/wiki/Active_record_pattern" title="Active record pattern">Active record</a></li> <li><a href="/wiki/Identity_map_pattern" title="Identity map pattern">Identity map</a></li> <li><a href="/wiki/Data_access_object" title="Data access object">Data access object</a></li> <li><a href="/wiki/Data_transfer_object" title="Data transfer object">Data transfer object</a></li> <li><a href="/wiki/Inversion_of_control" title="Inversion of control">Inversion of control</a></li> <li><a href="/wiki/JSP_model_2_architecture" title="JSP model 2 architecture">Model 2</a></li> <li><a href="/wiki/Broker_pattern" title="Broker pattern">Broker</a></li></ul> </div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">Other<br />patterns</th><td class="navbox-list-with-group navbox-list navbox-even hlist" style="width:100%;padding:0"><div style="padding:0 0.25em"> <ul><li><a href="/wiki/Blackboard_design_pattern" class="mw-redirect" title="Blackboard design pattern">Blackboard</a></li> <li><a href="/wiki/Business_delegate_pattern" title="Business delegate pattern">Business delegate</a></li> <li><a href="/wiki/Composite_entity_pattern" title="Composite entity pattern">Composite entity</a></li> <li><a href="/wiki/Dependency_injection" title="Dependency injection">Dependency injection</a></li> <li><a href="/wiki/Intercepting_filter_pattern" title="Intercepting filter pattern">Intercepting filter</a></li> <li><a href="/wiki/Lazy_loading" title="Lazy loading">Lazy loading</a></li> <li><a href="/wiki/Mock_object" title="Mock object">Mock object</a></li> <li><a class="mw-selflink selflink">Null object</a></li> <li><a href="/wiki/Object_pool_pattern" title="Object pool pattern">Object pool</a></li> <li><a href="/wiki/Servant_(design_pattern)" title="Servant (design pattern)">Servant</a></li> <li><a href="/wiki/Twin_pattern" title="Twin pattern">Twin</a></li> <li><a href="/wiki/Type_Tunnel_pattern" class="mw-redirect" title="Type Tunnel pattern">Type tunnel</a></li> <li><a href="/wiki/Method_chaining" title="Method chaining">Method chaining</a></li> <li><a href="/wiki/Delegation_pattern" title="Delegation pattern">Delegation</a></li></ul> </div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">Books</th><td class="navbox-list-with-group navbox-list navbox-odd hlist" style="width:100%;padding:0"><div style="padding:0 0.25em"> <ul><li><i><a href="/wiki/Design_Patterns" title="Design Patterns">Design Patterns</a></i></li> <li><i><a href="/wiki/Enterprise_Integration_Patterns" title="Enterprise Integration Patterns">Enterprise Integration Patterns</a></i></li></ul> </div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">People</th><td class="navbox-list-with-group navbox-list navbox-even hlist" style="width:100%;padding:0"><div style="padding:0 0.25em"> <ul><li><a href="/wiki/Christopher_Alexander" title="Christopher Alexander">Christopher Alexander</a></li> <li><a href="/wiki/Erich_Gamma" title="Erich Gamma">Erich Gamma</a></li> <li><a href="/wiki/Ralph_Johnson_(computer_scientist)" title="Ralph Johnson (computer scientist)">Ralph Johnson</a></li> <li><a href="/wiki/John_Vlissides" title="John Vlissides">John Vlissides</a></li> <li><a href="/wiki/Grady_Booch" title="Grady Booch">Grady Booch</a></li> <li><a href="/wiki/Kent_Beck" title="Kent Beck">Kent Beck</a></li> <li><a href="/wiki/Ward_Cunningham" title="Ward Cunningham">Ward Cunningham</a></li> <li><a href="/wiki/Martin_Fowler_(software_engineer)" title="Martin Fowler (software engineer)">Martin Fowler</a></li> <li><a href="/wiki/Robert_C._Martin" title="Robert C. Martin">Robert Martin</a></li> <li><a href="/wiki/Jim_Coplien" title="Jim Coplien">Jim Coplien</a></li> <li><a href="/wiki/Douglas_C._Schmidt" title="Douglas C. Schmidt">Douglas Schmidt</a></li> <li><a href="/wiki/Linda_Rising" title="Linda Rising">Linda Rising</a></li></ul> </div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">Communities</th><td class="navbox-list-with-group navbox-list navbox-odd hlist" style="width:100%;padding:0"><div style="padding:0 0.25em"> <ul><li><a href="/wiki/The_Hillside_Group" title="The Hillside Group">The Hillside Group</a></li> <li><a href="/wiki/Portland_Pattern_Repository" title="Portland Pattern Repository">Portland Pattern Repository</a></li></ul> </div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">See also</th><td class="navbox-list-with-group navbox-list navbox-even hlist" style="width:100%;padding:0"><div style="padding:0 0.25em"> <ul><li><a href="/wiki/Anti-pattern" title="Anti-pattern">Anti-pattern</a></li> <li><a href="/wiki/Architectural_pattern" title="Architectural pattern">Architectural pattern</a></li></ul> </div></td></tr></tbody></table></div>    </div><noscript><img src="https://login.wikimedia.org/wiki/Special:CentralAutoLogin/start?useformat=desktop&type=1x1&usesul3=0" alt="" width="1" height="1" style="border: none; position: absolute;"></noscript> <div class="printfooter" data-nosnippet="">Retrieved from "<a dir="ltr" href="https://en.wikipedia.org/w/index.php?title=Null_object_pattern&oldid=1276258505">https://en.wikipedia.org/w/index.php?title=Null_object_pattern&oldid=1276258505</a>"</div></div> <div id="catlinks" class="catlinks" data-mw="interface"><div id="mw-normal-catlinks" class="mw-normal-catlinks"><a href="/wiki/Help:Category" title="Help:Category">Category</a>: <ul><li><a href="/wiki/Category:Software_design_patterns" title="Category:Software design patterns">Software design patterns</a></li></ul></div><div id="mw-hidden-catlinks" class="mw-hidden-catlinks mw-hidden-cats-hidden">Hidden categories: <ul><li><a href="/wiki/Category:Articles_with_short_description" title="Category:Articles with short description">Articles with short description</a></li><li><a href="/wiki/Category:Short_description_matches_Wikidata" title="Category:Short description matches Wikidata">Short description matches Wikidata</a></li><li><a href="/wiki/Category:Articles_needing_additional_references_from_June_2023" title="Category:Articles needing additional references from June 2023">Articles needing additional references from June 2023</a></li><li><a href="/wiki/Category:All_articles_needing_additional_references" title="Category:All articles needing additional references">All articles needing additional references</a></li><li><a href="/wiki/Category:Articles_needing_additional_references_from_June_2013" title="Category:Articles needing additional references from June 2013">Articles needing additional references from June 2013</a></li><li><a href="/wiki/Category:Articles_that_may_contain_original_research_from_June_2013" title="Category:Articles that may contain original research from June 2013">Articles that may contain original research from June 2013</a></li><li><a href="/wiki/Category:All_articles_that_may_contain_original_research" title="Category:All articles that may contain original research">All articles that may contain original research</a></li><li><a href="/wiki/Category:Articles_with_example_Java_code" title="Category:Articles with example Java code">Articles with example Java code</a></li></ul></div></div> </div> </main> </div> <div class="mw-footer-container"> <footer id="footer" class="mw-footer" > <ul id="footer-info"> <li id="footer-info-lastmod"> This page was last edited on 17 February 2025, at 20:44<span class="anonymous-show"> (UTC)</span>.</li> <li id="footer-info-copyright">Text is available under the <a href="/wiki/Wikipedia:Text_of_the_Creative_Commons_Attribution-ShareAlike_4.0_International_License" title="Wikipedia:Text of the Creative Commons Attribution-ShareAlike 4.0 International License">Creative Commons Attribution-ShareAlike 4.0 License</a>; additional terms may apply. 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