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aria-controls="-sidenav-1-8"><span>Class modifiers</span> <span class="material-symbols expander" aria-hidden="true">expand_more</span></button><ul class="nav collapse" id="-sidenav-1-8"><li class="nav-item"><a class="nav-link" href="/language/class-modifiers"><div><span>Overview & usage</span></div></a></li><li class="nav-item"><a class="nav-link" href="/language/class-modifiers-for-apis"><div><span>Class modifiers for API maintainers</span></div></a></li><li class="nav-item"><a class="nav-link" href="/language/modifier-reference"><div><span>Reference</span></div></a></li></ul></li><li class="nav-item"><button class="nav-link collapsible collapsed" data-toggle="collapse" data-target="#-sidenav-1-9" role="button" aria-expanded="false" aria-controls="-sidenav-1-9"><span>Concurrency</span> <span class="material-symbols expander" aria-hidden="true">expand_more</span></button><ul class="nav collapse" id="-sidenav-1-9"><li class="nav-item"><a class="nav-link" 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href="/null-safety/understanding-null-safety"><div><span>Understanding null safety</span></div></a></li><li class="nav-item"><a class="nav-link" href="/null-safety/unsound-null-safety"><div><span>Unsound null safety</span></div></a></li><li class="nav-item"><a class="nav-link" href="/null-safety/faq"><div><span>FAQ</span></div></a></li></ul></li></ul></li><li class="nav-item"><button class="nav-link collapsed collapsible" data-toggle="collapse" data-target="#-sidenav-2" role="button" aria-expanded="false" aria-controls="-sidenav-2"><span>Core libraries</span> <span class="material-symbols expander" aria-hidden="true">expand_more</span></button><ul class="nav collapse" id="-sidenav-2"><li class="nav-item"><a class="nav-link" href="/libraries"><div><span>Overview</span></div></a></li><li class="nav-item"><a class="nav-link" href="/libraries/dart-core"><div><span>dart:core</span></div></a></li><li class="nav-item"><a class="nav-link" href="/libraries/dart-async"><div><span>dart:async</span></div></a></li><li class="nav-item"><a class="nav-link" href="/libraries/dart-math"><div><span>dart:math</span></div></a></li><li class="nav-item"><a class="nav-link" href="/libraries/dart-convert"><div><span>dart:convert</span></div></a></li><li class="nav-item"><a class="nav-link" href="/libraries/dart-io"><div><span>dart:io</span></div></a></li><li class="nav-item"><a class="nav-link" href="/interop/js-interop"><div><span>dart:js_interop</span></div></a></li><div class="sidenav-divider"></div><li class="nav-item"><a class="nav-link" href="/libraries/collections/iterables"><div><span>Iterable collections</span></div></a></li><li class="nav-item"><button class="nav-link collapsible collapsed" data-toggle="collapse" data-target="#-sidenav-2-10" role="button" aria-expanded="false" aria-controls="-sidenav-2-10"><span>Asynchronous programming</span> <span class="material-symbols expander" aria-hidden="true">expand_more</span></button><ul class="nav collapse" id="-sidenav-2-10"><li class="nav-item"><a class="nav-link" href="/libraries/async/async-await"><div><span>Tutorial</span></div></a></li><li class="nav-item"><a class="nav-link" href="/libraries/async/futures-error-handling"><div><span>Futures and error handling</span></div></a></li><li class="nav-item"><a class="nav-link" href="/libraries/async/using-streams"><div><span>Using streams</span></div></a></li><li class="nav-item"><a class="nav-link" href="/libraries/async/creating-streams"><div><span>Creating streams</span></div></a></li></ul></li></ul></li><li class="nav-item"><button class="nav-link collapsed collapsible" data-toggle="collapse" data-target="#-sidenav-3" role="button" aria-expanded="false" aria-controls="-sidenav-3"><span>Effective Dart</span> <span class="material-symbols expander" aria-hidden="true">expand_more</span></button><ul class="nav collapse" id="-sidenav-3"><li class="nav-item"><a class="nav-link" href="/effective-dart"><div><span>Overview</span></div></a></li><li class="nav-item"><a class="nav-link" href="/effective-dart/style"><div><span>Style</span></div></a></li><li class="nav-item"><a class="nav-link" href="/effective-dart/documentation"><div><span>Documentation</span></div></a></li><li class="nav-item"><a class="nav-link" href="/effective-dart/usage"><div><span>Usage</span></div></a></li><li class="nav-item"><a class="nav-link" href="/effective-dart/design"><div><span>Design</span></div></a></li></ul></li><li class="nav-item"><button class="nav-link collapsed collapsible" data-toggle="collapse" data-target="#-sidenav-4" role="button" aria-expanded="false" aria-controls="-sidenav-4"><span>Packages</span> <span class="material-symbols expander" aria-hidden="true">expand_more</span></button><ul class="nav collapse" id="-sidenav-4"><li class="nav-item"><a class="nav-link" href="/tools/pub/packages"><div><span>How to use packages</span></div></a></li><li 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class="nav-item"><a class="nav-link" href="/tools/pub/dependencies"><div><span>Dependencies</span></div></a></li><li class="nav-item"><a class="nav-link" href="/tools/pub/glossary"><div><span>Glossary</span></div></a></li><li class="nav-item"><a class="nav-link" href="/tools/pub/package-layout"><div><span>Package layout conventions</span></div></a></li><li class="nav-item"><a class="nav-link" href="/tools/pub/environment-variables"><div><span>Pub environment variables</span></div></a></li><li class="nav-item"><a class="nav-link" href="/tools/pub/pubspec"><div><span>Pubspec file</span></div></a></li><li class="nav-item"><a class="nav-link" href="/tools/pub/troubleshoot"><div><span>Troubleshooting pub</span></div></a></li><li class="nav-item"><a class="nav-link" href="/tools/pub/verified-publishers"><div><span>Verified publishers</span></div></a></li><li class="nav-item"><a class="nav-link" href="/tools/pub/security-advisories"><div><span>Security advisories</span></div></a></li><li 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representation</span></div></a></li><li class="nav-item"><a class="nav-link" href="/resources/google-apis"><div><span>Google APIs</span></div></a></li><li class="nav-item"><a class="nav-link" href="/multiplatform-apps"><div><span>Multi-platform apps</span></div></a></li><li class="nav-item"><button class="nav-link collapsible collapsed" data-toggle="collapse" data-target="#-sidenav-5-5" role="button" aria-expanded="false" aria-controls="-sidenav-5-5"><span>Command-line & server apps</span> <span class="material-symbols expander" aria-hidden="true">expand_more</span></button><ul class="nav collapse" id="-sidenav-5-5"><li class="nav-item"><a class="nav-link" href="/server"><div><span>Overview</span></div></a></li><li class="nav-item"><a class="nav-link" href="/tutorials/server/get-started"><div><span>Get started</span></div></a></li><li class="nav-item"><a class="nav-link" href="/tutorials/server/cmdline"><div><span>Write command-line apps</span></div></a></li><li class="nav-item"><a class="nav-link" href="/tutorials/server/fetch-data"><div><span>Fetch data from the internet</span></div></a></li><li class="nav-item"><a class="nav-link" href="/tutorials/server/httpserver"><div><span>Write HTTP servers</span></div></a></li><li class="nav-item"><a class="nav-link" href="/server/libraries"><div><span>Libraries & packages</span></div></a></li><li class="nav-item"><a class="nav-link" href="/server/google-cloud"><div><span>Google Cloud</span></div></a></li></ul></li><li class="nav-item"><button class="nav-link collapsible collapsed" data-toggle="collapse" data-target="#-sidenav-5-6" role="button" aria-expanded="false" aria-controls="-sidenav-5-6"><span>Web apps</span> <span class="material-symbols expander" aria-hidden="true">expand_more</span></button><ul class="nav collapse" id="-sidenav-5-6"><li class="nav-item"><a class="nav-link" href="/web"><div><span>Overview</span></div></a></li><li class="nav-item"><a class="nav-link" href="/web/get-started"><div><span>Get 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data-toggle="collapse" data-target="#-sidenav-10" role="button" aria-expanded="true" aria-controls="-sidenav-10"><span>Related sites</span> <span class="material-symbols expander" aria-hidden="true">expand_more</span></button><ul class="nav collapse show" id="-sidenav-10"><li class="nav-item"><a class="nav-link" href="https://api.dart.dev" target="_blank" rel="noopener"><div><span>API reference</span><span class="material-symbols" aria-hidden="true">open_in_new</span></div></a></li><li class="nav-item"><a class="nav-link" href="https://medium.com/dartlang" target="_blank" rel="noopener"><div><span>Blog</span><span class="material-symbols" aria-hidden="true">open_in_new</span></div></a></li><li class="nav-item"><a class="nav-link" href="https://dartpad.dev" target="_blank" rel="noopener"><div><span>DartPad (online editor)</span><span class="material-symbols" aria-hidden="true">open_in_new</span></div></a></li><li class="nav-item"><a class="nav-link" href="https://flutter.dev" target="_blank" rel="noopener"><div><span>Flutter</span><span class="material-symbols" aria-hidden="true">open_in_new</span></div></a></li><li class="nav-item"><a class="nav-link" href="https://pub.dev" target="_blank" rel="noopener"><div><span>Package site</span><span class="material-symbols" aria-hidden="true">open_in_new</span></div></a></li></ul></li></ul></div><main id="page-content"><div id="site-toc--side" class="site-toc"><header class="site-toc__title">Contents</header><ul class="section-nav"><li class="toc-entry nav-item"><a class="nav-link" href="#overriding-members">Overriding members</a></li><li class="toc-entry nav-item"><a class="nav-link" href="#nosuchmethod">noSuchMethod()</a></li></ul></div><article><div class="content"><div id="site-content-title"><h1>Extend a class</h1></div><div id="site-toc--inline" class="site-toc"><header class="site-toc__title">Contents</header><ul class="section-nav"><li class="toc-entry"><a href="#overriding-members">Overriding members</a></li><li class="toc-entry"><a href="#nosuchmethod">noSuchMethod()</a></li></ul></div><p>Use <code>extends</code> to create a subclass, and <code>super</code> to refer to the superclass:</p> <?code-excerpt "misc/lib/language_tour/classes/extends.dart (smart-tv)" replace="/extends|super/[!$&!]/g"?> <div class="code-block-wrapper language-dart"><div class="code-block-body"><span class="code-block-language" title="Language dart">dart</span><pre class="shiki dash-light" tabindex="0"><code><span class="line"><span style="color:#D43324">class</span><span style="color:#0468D7"> Television</span><span style="color:#222222"> {</span></span> <span class="line"><span style="color:#D43324"> void</span><span style="color:#6200EE"> turnOn</span><span style="color:#222222">() {</span></span> <span class="line"><span style="color:#6200EE"> _illuminateDisplay</span><span style="color:#222222">();</span></span> <span class="line"><span style="color:#6200EE"> _activateIrSensor</span><span style="color:#222222">();</span></span> <span class="line"><span style="color:#222222"> }</span></span> <span class="line"><span style="color:#6E6E70"> // 路路路</span></span> <span class="line"><span style="color:#222222">}</span></span> <span class="line"></span> <span class="line"><span style="color:#D43324">class</span><span style="color:#0468D7"> SmartTelevision</span><span style="color:#D43324"> </span><mark class="highlight"><span style="color:#D43324">extends</span></mark><span style="color:#0468D7"> Television</span><span style="color:#222222"> {</span></span> <span class="line"><span style="color:#D43324"> void</span><span style="color:#6200EE"> turnOn</span><span style="color:#222222">() {</span></span> <span class="line"><span style="color:#D43324"> </span><mark class="highlight"><span style="color:#D43324">super</span></mark><span style="color:#222222">.</span><span style="color:#6200EE">turnOn</span><span style="color:#222222">();</span></span> <span class="line"><span style="color:#6200EE"> _bootNetworkInterface</span><span style="color:#222222">();</span></span> <span class="line"><span style="color:#6200EE"> _initializeMemory</span><span style="color:#222222">();</span></span> <span class="line"><span style="color:#6200EE"> _upgradeApps</span><span style="color:#222222">();</span></span> <span class="line"><span style="color:#222222"> }</span></span> <span class="line"><span style="color:#6E6E70"> // 路路路</span></span> <span class="line"><span style="color:#222222">}</span></span></code></pre></div></div><p>For another usage of <code>extends</code>, see the discussion of <a href="/language/generics#restricting-the-parameterized-type">parameterized types</a> on the Generics page.</p><div class="header-wrapper"><h2 id="overriding-members">Overriding members</h2><a class="heading-link" href="#overriding-members" aria-label="Link to 'Overriding members' section">#</a></div><p>Subclasses can override instance methods (including <a href="/language/methods#operators">operators</a>), getters, and setters. You can use the <code>@override</code> annotation to indicate that you are intentionally overriding a member:</p> <?code-excerpt "misc/lib/language_tour/metadata/television.dart (override)" replace="/@override/[!$&!]/g"?> <div class="code-block-wrapper language-dart"><div class="code-block-body"><span class="code-block-language" title="Language dart">dart</span><pre class="shiki dash-light" tabindex="0"><code><span class="line"><span style="color:#D43324">class</span><span style="color:#0468D7"> Television</span><span style="color:#222222"> {</span></span> <span class="line"><span style="color:#6E6E70"> // 路路路</span></span> <span class="line"><span style="color:#D43324"> set</span><span style="color:#6200EE"> contrast</span><span style="color:#222222">(</span><span style="color:#0468D7">int</span><span style="color:#222222"> value) {</span></span> <span class="line"><span style="color:#222222"> ...</span></span> <span class="line"><span style="color:#222222"> }</span></span> <span class="line"><span style="color:#222222">}</span></span> <span class="line"></span> <span class="line"><span style="color:#D43324">class</span><span style="color:#0468D7"> SmartTelevision</span><span style="color:#D43324"> extends</span><span style="color:#0468D7"> Television</span><span style="color:#222222"> {</span></span> <span class="line"><span style="color:#D43324"> </span><mark class="highlight"><span style="color:#D43324">@override</span></mark></span> <span class="line"><span style="color:#D43324"> set</span><span style="color:#6200EE"> contrast</span><span style="color:#222222">(</span><span style="color:#0468D7">num</span><span style="color:#222222"> value) {</span></span> <span class="line"><span style="color:#222222"> ...</span></span> <span class="line"><span style="color:#222222"> }</span></span> <span class="line"><span style="color:#6E6E70"> // 路路路</span></span> <span class="line"><span style="color:#222222">}</span></span></code></pre></div></div><p>An overriding method declaration must match the method (or methods) that it overrides in several ways:</p><ul><li>The return type must be the same type as (or a subtype of) the overridden method's return type.</li><li>Parameter types must be the same type as (or a supertype of) the overridden method's parameter types. In the preceding example, the <code>contrast</code> setter of <code>SmartTelevision</code> changes the parameter type from <code>int</code> to a supertype, <code>num</code>.</li><li>If the overridden method accepts <em>n</em> positional parameters, then the overriding method must also accept <em>n</em> positional parameters.</li><li>A <a href="/language/generics#using-generic-methods">generic method</a> can't override a non-generic one, and a non-generic method can't override a generic one.</li></ul><p>Sometimes you might want to narrow the type of a method parameter or an instance variable. This violates the normal rules, and it's similar to a downcast in that it can cause a type error at runtime. Still, narrowing the type is possible if the code can guarantee that a type error won't occur. In this case, you can use the <a href="/language/type-system#covariant-keyword"><code>covariant</code> keyword</a> in a parameter declaration. For details, see the <a href="/resources/language/spec">Dart language specification</a>.</p><aside class="alert alert-warning"><div class="alert-header"><span class="material-symbols" aria-hidden="true">warning</span> <span>Warning</span></div><div class="alert-content"><p>If you override <code>==</code>, you should also override Object's <code>hashCode</code> getter. For an example of overriding <code>==</code> and <code>hashCode</code>, check out <a href="/libraries/dart-core#implementing-map-keys">Implementing map keys</a>.</p></div></aside><div class="header-wrapper"><h2 id="nosuchmethod">noSuchMethod()</h2><a class="heading-link" href="#nosuchmethod" aria-label="Link to 'noSuchMethod()' section">#</a></div><p>To detect or react whenever code attempts to use a non-existent method or instance variable, you can override <code>noSuchMethod()</code>:</p> <?code-excerpt "misc/lib/language_tour/classes/no_such_method.dart (no-such-method-impl)" replace="/noSuchMethod(?!,)/[!$&!]/g"?> <div class="code-block-wrapper language-dart"><div class="code-block-body"><span class="code-block-language" title="Language dart">dart</span><pre class="shiki dash-light" tabindex="0"><code><span class="line"><span style="color:#D43324">class</span><span style="color:#0468D7"> A</span><span style="color:#222222"> {</span></span> <span class="line"><span style="color:#6E6E70"> // Unless you override noSuchMethod, using a</span></span> <span class="line"><span style="color:#6E6E70"> // non-existent member results in a NoSuchMethodError.</span></span> <span class="line"><span style="color:#D43324"> @override</span></span> <span class="line"><span style="color:#D43324"> void</span><span style="color:#6200EE"> </span><mark class="highlight"><span style="color:#6200EE">noSuchMethod</span></mark><span style="color:#222222">(</span><span style="color:#0468D7">Invocation</span><span style="color:#222222"> invocation) {</span></span> <span class="line"><span style="color:#6200EE"> print</span><span style="color:#222222">(</span></span> <span class="line"><span style="color:#11796D"> 'You tried to use a non-existent member: '</span></span> <span class="line"><span style="color:#11796D"> '</span><span style="color:#11796D">${</span><span style="color:#222222">invocation</span><span style="color:#11796D">.</span><span style="color:#222222">memberName</span><span style="color:#11796D">}</span><span style="color:#11796D">'</span><span style="color:#222222">,</span></span> <span class="line"><span style="color:#222222"> );</span></span> <span class="line"><span style="color:#222222"> }</span></span> <span class="line"><span style="color:#222222">}</span></span></code></pre></div></div><p>You <strong>can't invoke</strong> an unimplemented method unless <strong>one</strong> of the following is true:</p><ul><li><p>The receiver has the static type <code>dynamic</code>.</p></li><li><p>The receiver has a static type that defines the unimplemented method (abstract is OK), and the dynamic type of the receiver has an implementation of <code>noSuchMethod()</code> that's different from the one in class <code>Object</code>.</p></li></ul><p>For more information, see the informal <a href="https://github.com/dart-lang/language/blob/main/archive/feature-specifications/nosuchmethod-forwarding.md">noSuchMethod forwarding specification.</a></p><nav id="prev-next"><ul><li class="previous"><a href="/language/methods">&lang;  Methods</a></li><li class="next"><a href="/language/mixins">Mixins  &rang;</a></li></ul></nav><p id="page-github-links"><span>Unless stated otherwise, the documentation on this site reflects Dart 3.7.0. Page last updated on 2025-02-12.</span> <a href="https://github.com/dart-lang/site-www/tree/main/src/content/language/extend.md" target="_blank" rel="noopener">View source</a> <span>or </span><a href="https://github.com/dart-lang/site-www/issues/new?template=1_page_issue.yml&page-url=https://dart.dev/language/extend/&page-source=https://github.com/dart-lang/site-www/tree/main/src/content/language/extend.md" title="Report an issue with this page" target="_blank" rel="noopener">report an issue</a>.</p></div></article></main></div><footer id="page-footer"><div class="footer-section footer-main"><a href="/" class="brand" title="Dart"><img src="/assets/img/logo/logo-white-text.svg" alt="Dart" width="164"></a><div class="footer-social-links"><a href="https://medium.com/dartlang" target="_blank" rel="noopener" title="Dart's Medium publication"><svg><use href="/assets/img/social/medium.svg#medium"></use></svg> </a><a href="https://github.com/dart-lang" target="_blank" rel="noopener" title="Dart's GitHub organization"><svg><use href="/assets/img/social/github.svg#github"></use></svg> </a><a href="https://bsky.app/profile/dart.dev" target="_blank" rel="noopener" title="Dart's Bluesky profile"><svg><use href="/assets/img/social/bluesky.svg#bluesky"></use></svg> </a><a href="https://twitter.com/dart_lang" target="_blank" rel="noopener" title="Dart's X (Twitter) profile"><svg><use href="/assets/img/social/x.svg#x"></use></svg></a></div></div><div class="footer-section footer-tray"><div class="footer-licenses">Except as otherwise noted, this site is licensed under a <a href="https://creativecommons.org/licenses/by/4.0/">Creative Commons Attribution 4.0 International License</a>, and code samples are licensed under the <a href="https://opensource.org/licenses/BSD-3-Clause">3-Clause BSD License</a>.</div><div class="footer-utility-links"><ul><li><a href="/terms" title="Terms of use">Terms</a></li><li><a href="https://policies.google.com/privacy" target="_blank" rel="noopener" title="Privacy policy">Privacy</a></li><li><a href="/security" title="Security philosophy and practices">Security</a></li></ul></div></div></footer></div></body></html>