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Extend a class | Dart
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site</a></li></ul></li></ul></div></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"><nav id="subnav"><ul><li class="previous"><a href="/language/methods">⟨ Methods</a></li><li class="next"><a href="/language/mixins">Mixins ⟩</a></li></ul></nav><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> <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:#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="/guides/language/sound-problems#the-covariant-keyword"><code>covariant</code> keyword</a> in a parameter declaration. For details, see the <a href="/guides/language/spec">Dart language specification</a>.</p><aside class="alert alert-warning"><div class="alert-header"><i class="material-symbols" aria-hidden="true">warning</i> <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(?!,)/[!$&!]/g"?> <div class="code-block-wrapper language-dart"><div class="code-block-body"><span class="code-block-language" title="Language dart">dart</span><pre class="shiki dash-light" tabindex="0"><code><span class="line"><span style="color:#D43324">class</span><span style="color:#0468D7"> A</span><span style="color:#222222"> {</span></span> <span class="line"><span style="color:#6E6E70"> // Unless you override noSuchMethod, using a</span></span> <span class="line"><span style="color:#6E6E70"> // non-existent member results in a NoSuchMethodError.</span></span> <span class="line"><span style="color:#D43324"> @override</span></span> <span class="line"><span style="color:#D43324"> void</span><span style="color:#6200EE"> </span><mark class="highlight"><span style="color:#6200EE">noSuchMethod</span></mark><span style="color:#222222">(</span><span style="color:#0468D7">Invocation</span><span style="color:#222222"> invocation) {</span></span> <span class="line"><span style="color:#6200EE"> print</span><span style="color:#222222">(</span><span style="color:#11796D">'You tried to use a non-existent member: '</span></span> <span class="line"><span style="color:#11796D"> '</span><span style="color:#11796D">${</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></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="subnav"><ul><li class="previous"><a href="/language/methods">⟨ Methods</a></li><li class="next"><a href="/language/mixins">Mixins ⟩</a></li></ul></nav><p id="page-github-links"><span>Unless stated otherwise, the documentation on this site reflects Dart 3.5.4. 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