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class="site-toc__title">Contents</header><ul class="section-nav"><li class="toc-entry nav-item"><a class="nav-link" href="#why-use-generics">Why use generics?</a></li><li class="toc-entry nav-item"><a class="nav-link" href="#using-collection-literals">Using collection literals</a></li><li class="toc-entry nav-item"><a class="nav-link" href="#using-parameterized-types-with-constructors">Using parameterized types with constructors</a></li><li class="toc-entry nav-item"><a class="nav-link" href="#generic-collections-and-the-types-they-contain">Generic collections and the types they contain</a></li><li class="toc-entry nav-item"><a class="nav-link" href="#restricting-the-parameterized-type">Restricting the parameterized type</a><ul class="nav"><li class="toc-entry nav-item"><a class="nav-link" href="#f-bounds">Self-referential type parameter restrictions (F-bounds)</a></li></ul></li><li class="toc-entry nav-item"><a class="nav-link" href="#using-generic-methods">Using generic methods</a></li></ul></div><article><div class="content"><div id="site-content-title"><h1>Generics</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="#why-use-generics">Why use generics?</a></li><li class="toc-entry"><a href="#using-collection-literals">Using collection literals</a></li><li class="toc-entry"><a href="#using-parameterized-types-with-constructors">Using parameterized types with constructors</a></li><li class="toc-entry"><a href="#generic-collections-and-the-types-they-contain">Generic collections and the types they contain</a></li><li class="toc-entry"><a href="#restricting-the-parameterized-type">Restricting the parameterized type</a><ul><li class="toc-entry"><a href="#f-bounds">Self-referential type parameter restrictions (F-bounds)</a></li></ul></li><li class="toc-entry"><a href="#using-generic-methods">Using generic methods</a></li></ul></div> <?code-excerpt replace="/ *\/\/\s+ignore_for_file:[^\n]+\n//g; /(^|\n) *\/\/\s+ignore:[^\n]+\n/$1/g; /(\n[^\n]+) *\/\/\s+ignore:[^\n]+\n/$1\n/g; / *\/\/\s+ignore:[^\n]+//g; /([A-Z]\w*)\d\b/$1/g"?> <p>If you look at the API documentation for the basic array type, <a href="https://api.dart.dev/dart-core/List-class.html"><code>List</code></a>, you'll see that the type is actually <code>List<E></code>. The <...> notation marks List as a <em>generic</em> (or <em>parameterized</em>) type—a type that has formal type parameters. <a href="/effective-dart/design#do-follow-existing-mnemonic-conventions-when-naming-type-parameters">By convention</a>, most type variables have single-letter names, such as E, T, S, K, and V.</p><div class="header-wrapper"><h2 id="why-use-generics">Why use generics?</h2><a class="heading-link" href="#why-use-generics" aria-label="Link to 'Why use generics?' section">#</a></div><p>Generics are often required for type safety, but they have more benefits than just allowing your code to run:</p><ul><li>Properly specifying generic types results in better generated code.</li><li>You can use generics to reduce code duplication.</li></ul><p>If you intend for a list to contain only strings, you can declare it as <code>List<String></code> (read that as "list of string"). That way you, your fellow programmers, and your tools can detect that assigning a non-string to the list is probably a mistake. Here's an example:</p><div class="code-block-wrapper language-dart"><div class="code-block-body has-tag tag-fails-sa"><span class="code-block-tag">✗ static analysis: failure</span><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">var</span><span style="color:#222222"> names = <</span><span style="color:#0468D7">String</span><span style="color:#222222">>[];</span></span> <span class="line"><span style="color:#222222">names.</span><span style="color:#6200EE">addAll</span><span style="color:#222222">([</span><span style="color:#11796D">'Seth'</span><span style="color:#222222">, </span><span style="color:#11796D">'Kathy'</span><span style="color:#222222">, </span><span style="color:#11796D">'Lars'</span><span style="color:#222222">]);</span></span> <span class="line"><span style="color:#222222">names.</span><span style="color:#6200EE">add</span><span style="color:#222222">(</span><span style="color:#11796D">42</span><span style="color:#222222">); </span><span style="color:#6E6E70">// Error</span></span></code></pre></div></div><p>Another reason for using generics is to reduce code duplication. Generics let you share a single interface and implementation between many types, while still taking advantage of static analysis. For example, say you create an interface for caching an object:</p> <?code-excerpt "misc/lib/language_tour/generics/cache.dart (object-cache)"?> <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">abstract</span><span style="color:#D43324"> class</span><span style="color:#0468D7"> ObjectCache</span><span style="color:#222222"> {</span></span> <span class="line"><span style="color:#0468D7"> Object</span><span style="color:#6200EE"> getByKey</span><span style="color:#222222">(</span><span style="color:#0468D7">String</span><span style="color:#222222"> key);</span></span> <span class="line"><span style="color:#D43324"> void</span><span style="color:#6200EE"> setByKey</span><span style="color:#222222">(</span><span style="color:#0468D7">String</span><span style="color:#222222"> key, </span><span style="color:#0468D7">Object</span><span style="color:#222222"> value);</span></span> <span class="line"><span style="color:#222222">}</span></span></code></pre></div></div><p>You discover that you want a string-specific version of this interface, so you create another interface:</p> <?code-excerpt "misc/lib/language_tour/generics/cache.dart (string-cache)"?> <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">abstract</span><span style="color:#D43324"> class</span><span style="color:#0468D7"> StringCache</span><span style="color:#222222"> {</span></span> <span class="line"><span style="color:#0468D7"> String</span><span style="color:#6200EE"> getByKey</span><span style="color:#222222">(</span><span style="color:#0468D7">String</span><span style="color:#222222"> key);</span></span> <span class="line"><span style="color:#D43324"> void</span><span style="color:#6200EE"> setByKey</span><span style="color:#222222">(</span><span style="color:#0468D7">String</span><span style="color:#222222"> key, </span><span style="color:#0468D7">String</span><span style="color:#222222"> value);</span></span> <span class="line"><span style="color:#222222">}</span></span></code></pre></div></div><p>Later, you decide you want a number-specific version of this interface... You get the idea.</p><p>Generic types can save you the trouble of creating all these interfaces. Instead, you can create a single interface that takes a type parameter:</p> <?code-excerpt "misc/lib/language_tour/generics/cache.dart (cache)"?> <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">abstract</span><span style="color:#D43324"> class</span><span style="color:#0468D7"> Cache</span><span style="color:#222222"><</span><span style="color:#0468D7">T</span><span style="color:#222222">> {</span></span> <span class="line"><span style="color:#0468D7"> T</span><span style="color:#6200EE"> getByKey</span><span style="color:#222222">(</span><span style="color:#0468D7">String</span><span style="color:#222222"> key);</span></span> <span class="line"><span style="color:#D43324"> void</span><span style="color:#6200EE"> setByKey</span><span style="color:#222222">(</span><span style="color:#0468D7">String</span><span style="color:#222222"> key, </span><span style="color:#0468D7">T</span><span style="color:#222222"> value);</span></span> <span class="line"><span style="color:#222222">}</span></span></code></pre></div></div><p>In this code, T is the stand-in type. It's a placeholder that you can think of as a type that a developer will define later.</p><div class="header-wrapper"><h2 id="using-collection-literals">Using collection literals</h2><a class="heading-link" href="#using-collection-literals" aria-label="Link to 'Using collection literals' section">#</a></div><p>List, set, and map literals can be parameterized. Parameterized literals are just like the literals you've already seen, except that you add <code><<em>type</em>></code> (for lists and sets) or <code><<em>keyType</em>, <em>valueType</em>></code> (for maps) before the opening bracket. Here is an example of using typed literals:</p> <?code-excerpt "misc/lib/language_tour/generics/misc.dart (collection-literals)"?> <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">var</span><span style="color:#222222"> names = <</span><span style="color:#0468D7">String</span><span style="color:#222222">>[</span><span style="color:#11796D">'Seth'</span><span style="color:#222222">, </span><span style="color:#11796D">'Kathy'</span><span style="color:#222222">, </span><span style="color:#11796D">'Lars'</span><span style="color:#222222">];</span></span> <span class="line"><span style="color:#D43324">var</span><span style="color:#222222"> uniqueNames = <</span><span style="color:#0468D7">String</span><span style="color:#222222">>{</span><span style="color:#11796D">'Seth'</span><span style="color:#222222">, </span><span style="color:#11796D">'Kathy'</span><span style="color:#222222">, </span><span style="color:#11796D">'Lars'</span><span style="color:#222222">};</span></span> <span class="line"><span style="color:#D43324">var</span><span style="color:#222222"> pages = <</span><span style="color:#0468D7">String</span><span style="color:#222222">, </span><span style="color:#0468D7">String</span><span style="color:#222222">>{</span></span> <span class="line"><span style="color:#11796D"> 'index.html'</span><span style="color:#222222">: </span><span style="color:#11796D">'Homepage'</span><span style="color:#222222">,</span></span> <span class="line"><span style="color:#11796D"> 'robots.txt'</span><span style="color:#222222">: </span><span style="color:#11796D">'Hints for web robots'</span><span style="color:#222222">,</span></span> <span class="line"><span style="color:#11796D"> 'humans.txt'</span><span style="color:#222222">: </span><span style="color:#11796D">'We are people, not machines'</span><span style="color:#222222">,</span></span> <span class="line"><span style="color:#222222">};</span></span></code></pre></div></div><div class="header-wrapper"><h2 id="using-parameterized-types-with-constructors">Using parameterized types with constructors</h2><a class="heading-link" href="#using-parameterized-types-with-constructors" aria-label="Link to 'Using parameterized types with constructors' section">#</a></div><p>To specify one or more types when using a constructor, put the types in angle brackets (<code><...></code>) just after the class name. For example:</p> <?code-excerpt "misc/test/language_tour/generics_test.dart (constructor-1)"?> <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">var</span><span style="color:#222222"> nameSet = </span><span style="color:#0468D7">Set</span><span style="color:#222222"><</span><span style="color:#0468D7">String</span><span style="color:#222222">>.</span><span style="color:#6200EE">of</span><span style="color:#222222">(names);</span></span></code></pre></div></div><p>The following code creates a <code>SplayTreeMap</code> that has integer keys and values of type <code>View</code>:</p> <?code-excerpt "misc/test/language_tour/generics_test.dart (constructor-2)"?> <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">var</span><span style="color:#222222"> views = </span><span style="color:#0468D7">SplayTreeMap</span><span style="color:#222222"><</span><span style="color:#0468D7">int</span><span style="color:#222222">, </span><span style="color:#0468D7">View</span><span style="color:#222222">>();</span></span></code></pre></div></div><div class="header-wrapper"><h2 id="generic-collections-and-the-types-they-contain">Generic collections and the types they contain</h2><a class="heading-link" href="#generic-collections-and-the-types-they-contain" aria-label="Link to 'Generic collections and the types they contain' section">#</a></div><p>Dart generic types are <em>reified</em>, which means that they carry their type information around at runtime. For example, you can test the type of a collection:</p> <?code-excerpt "misc/test/language_tour/generics_test.dart (generic-collections)"?> <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">var</span><span style="color:#222222"> names = <</span><span style="color:#0468D7">String</span><span style="color:#222222">>[];</span></span> <span class="line"><span style="color:#222222">names.</span><span style="color:#6200EE">addAll</span><span style="color:#222222">([</span><span style="color:#11796D">'Seth'</span><span style="color:#222222">, </span><span style="color:#11796D">'Kathy'</span><span style="color:#222222">, </span><span style="color:#11796D">'Lars'</span><span style="color:#222222">]);</span></span> <span class="line"><span style="color:#6200EE">print</span><span style="color:#222222">(names is </span><span style="color:#0468D7">List</span><span style="color:#222222"><</span><span style="color:#0468D7">String</span><span style="color:#222222">>); </span><span style="color:#6E6E70">// true</span></span></code></pre></div></div><aside class="alert alert-info"><div class="alert-header"><span class="material-symbols" aria-hidden="true">info</span> <span>Note</span></div><div class="alert-content"><p>In contrast, generics in Java use <em>erasure</em>, which means that generic type parameters are removed at runtime. In Java, you can test whether an object is a List, but you can't test whether it's a <code>List<String></code>.</p></div></aside><div class="header-wrapper"><h2 id="restricting-the-parameterized-type">Restricting the parameterized type</h2><a class="heading-link" href="#restricting-the-parameterized-type" aria-label="Link to 'Restricting the parameterized type' section">#</a></div><p>When implementing a generic type, you might want to limit the types that can be provided as arguments, so that the argument must be a subtype of a particular type. This restriction is called a bound. You can do this using <code>extends</code>.</p><p>A common use case is ensuring that a type is non-nullable by making it a subtype of <code>Object</code> (instead of the default, <a href="/null-safety/understanding-null-safety#top-and-bottom"><code>Object?</code></a>).</p> <?code-excerpt "misc/lib/language_tour/generics/misc.dart (non-nullable)"?> <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"> Foo</span><span style="color:#222222"><</span><span style="color:#0468D7">T</span><span style="color:#D43324"> extends</span><span style="color:#0468D7"> Object</span><span style="color:#222222">> {</span></span> <span class="line"><span style="color:#6E6E70"> // Any type provided to Foo for T must be non-nullable.</span></span> <span class="line"><span style="color:#222222">}</span></span></code></pre></div></div><p>You can use <code>extends</code> with other types besides <code>Object</code>. Here's an example of extending <code>SomeBaseClass</code>, so that members of <code>SomeBaseClass</code> can be called on objects of type <code>T</code>:</p> <?code-excerpt "misc/lib/language_tour/generics/base_class.dart (generic)" replace="/extends SomeBaseClass(?=. \{)/[!$&!]/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"> Foo</span><span style="color:#222222"><</span><span style="color:#0468D7">T</span><span style="color:#D43324"> </span><mark class="highlight"><span style="color:#D43324">extends</span><span style="color:#0468D7"> SomeBaseClass</span></mark><span style="color:#222222">> {</span></span> <span class="line"><span style="color:#6E6E70"> // Implementation goes here...</span></span> <span class="line"><span style="color:#0468D7"> String</span><span style="color:#6200EE"> toString</span><span style="color:#222222">() => </span><span style="color:#11796D">"Instance of 'Foo<</span><span style="color:#11796D">$</span><span style="color:#222222">T</span><span style="color:#11796D">>'"</span><span style="color:#222222">;</span></span> <span class="line"><span style="color:#222222">}</span></span> <span class="line"></span> <span class="line"><span style="color:#D43324">class</span><span style="color:#0468D7"> Extender</span><span style="color:#D43324"> extends</span><span style="color:#0468D7"> SomeBaseClass</span><span style="color:#222222"> {</span></span> <span class="line"><span style="color:#222222"> ...</span></span> <span class="line"><span style="color:#222222">}</span></span></code></pre></div></div><p>It's OK to use <code>SomeBaseClass</code> or any of its subtypes as the generic argument:</p> <?code-excerpt "misc/test/language_tour/generics_test.dart (SomeBaseClass-ok)" replace="/Foo.\w+./[!$&!]/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">var</span><span style="color:#222222"> someBaseClassFoo = </span><mark class="highlight"><span style="color:#0468D7">Foo</span><span style="color:#222222"><</span><span style="color:#0468D7">SomeBaseClass</span><span style="color:#222222">></span></mark><span style="color:#222222">();</span></span> <span class="line"><span style="color:#D43324">var</span><span style="color:#222222"> extenderFoo = </span><mark class="highlight"><span style="color:#0468D7">Foo</span><span style="color:#222222"><</span><span style="color:#0468D7">Extender</span><span style="color:#222222">></span></mark><span style="color:#222222">();</span></span></code></pre></div></div><p>It's also OK to specify no generic argument:</p> <?code-excerpt "misc/test/language_tour/generics_test.dart (no-generic-arg-ok)" replace="/expect$(.*?).toString\($, .(.*?).\);/print($1); \/\/ $2/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">var</span><span style="color:#222222"> foo = </span><span style="color:#0468D7">Foo</span><span style="color:#222222">();</span></span> <span class="line"><span style="color:#6200EE">print</span><span style="color:#222222">(foo); </span><span style="color:#6E6E70">// Instance of 'Foo<SomeBaseClass>'</span></span></code></pre></div></div><p>Specifying any non-<code>SomeBaseClass</code> type results in an error:</p><div class="code-block-wrapper language-dart"><div class="code-block-body has-tag tag-fails-sa"><span class="code-block-tag">✗ static analysis: failure</span><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">var</span><span style="color:#222222"> foo = </span><mark class="highlight"><span style="color:#0468D7">Foo</span><span style="color:#222222"><</span><span style="color:#0468D7">Object</span><span style="color:#222222">></span></mark><span style="color:#222222">();</span></span></code></pre></div></div><div class="header-wrapper"><h3 id="f-bounds">Self-referential type parameter restrictions (F-bounds)</h3><a class="heading-link" href="#f-bounds" aria-label="Link to 'Self-referential type parameter restrictions (F-bounds)' section">#</a></div><p>When using bounds to restrict parameter types, you can refer the bound back to the type parameter itself. This creates a self-referential constraint, or F-bound. For example:</p> <?code-excerpt "misc/test/language_tour/generics_test.dart (f-bound)"?> <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">abstract</span><span style="color:#D43324"> interface</span><span style="color:#D43324"> class</span><span style="color:#0468D7"> Comparable</span><span style="color:#222222"><</span><span style="color:#0468D7">T</span><span style="color:#222222">> {</span></span> <span class="line"><span style="color:#0468D7"> int</span><span style="color:#6200EE"> compareTo</span><span style="color:#222222">(</span><span style="color:#0468D7">T</span><span style="color:#222222"> o);</span></span> <span class="line"><span style="color:#222222">}</span></span> <span class="line"></span> <span class="line"><span style="color:#0468D7">int</span><span style="color:#6200EE"> compareAndOffset</span><span style="color:#222222"><</span><span style="color:#0468D7">T</span><span style="color:#D43324"> extends</span><span style="color:#0468D7"> Comparable</span><span style="color:#222222"><</span><span style="color:#0468D7">T</span><span style="color:#222222">>>(</span><span style="color:#0468D7">T</span><span style="color:#222222"> t1, </span><span style="color:#0468D7">T</span><span style="color:#222222"> t2) =></span></span> <span class="line"><span style="color:#222222"> t1.</span><span style="color:#6200EE">compareTo</span><span style="color:#222222">(t2) + </span><span style="color:#11796D">1</span><span style="color:#222222">;</span></span> <span class="line"></span> <span class="line"><span style="color:#D43324">class</span><span style="color:#0468D7"> A</span><span style="color:#D43324"> implements</span><span style="color:#0468D7"> Comparable</span><span style="color:#222222"><</span><span style="color:#0468D7">A</span><span style="color:#222222">> {</span></span> <span class="line"><span style="color:#D43324"> @override</span></span> <span class="line"><span style="color:#0468D7"> int</span><span style="color:#6200EE"> compareTo</span><span style="color:#222222">(</span><span style="color:#0468D7">A</span><span style="color:#222222"> other) => </span><span style="color:#6E6E70">/*...implementation...*/</span><span style="color:#11796D"> 0</span><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">var</span><span style="color:#222222"> useIt = </span><span style="color:#6200EE">compareAndOffset</span><span style="color:#222222">(</span><span style="color:#0468D7">A</span><span style="color:#222222">(), </span><span style="color:#0468D7">A</span><span style="color:#222222">());</span></span></code></pre></div></div><p>The F-bound <code>T extends Comparable<T></code> means <code>T</code> must be comparable to itself. So, <code>A</code> can only be compared to other instances of the same type.</p><div class="header-wrapper"><h2 id="using-generic-methods">Using generic methods</h2><a class="heading-link" href="#using-generic-methods" aria-label="Link to 'Using generic methods' section">#</a></div><p>Methods and functions also allow type arguments:</p> <?code-excerpt "misc/test/language_tour/generics_test.dart (method)" replace="/<T.(?=\()|T/[!$&!]/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"><mark class="highlight"><span style="color:#0468D7">T</span></mark><span style="color:#6200EE"> first</span><mark class="highlight"><span style="color:#222222"><</span><span style="color:#0468D7">T</span><span style="color:#222222">></span></mark><span style="color:#222222">(</span><span style="color:#0468D7">List</span><span style="color:#222222"><</span><mark class="highlight"><span style="color:#0468D7">T</span></mark><span style="color:#222222">> ts) {</span></span> <span class="line"><span style="color:#6E6E70"> // Do some initial work or error checking, then...</span></span> <span class="line"><span style="color:#0468D7"> </span><mark class="highlight"><span style="color:#0468D7">T</span></mark><span style="color:#222222"> tmp = ts[</span><span style="color:#11796D">0</span><span style="color:#222222">];</span></span> <span class="line"><span style="color:#6E6E70"> // Do some additional checking or processing...</span></span> <span class="line"><span style="color:#D43324"> return</span><span style="color:#222222"> tmp;</span></span> <span class="line"><span style="color:#222222">}</span></span></code></pre></div></div><p>Here the generic type parameter on <code>first</code> (<code><T></code>) allows you to use the type argument <code>T</code> in several places:</p><ul><li>In the function's return type (<code>T</code>).</li><li>In the type of an argument (<code>List<T></code>).</li><li>In the type of a local variable (<code>T tmp</code>).</li></ul><nav id="prev-next"><ul><li class="previous"><a href="/language/collections">&lang;  Collections</a></li><li class="next"><a href="/language/typedefs">Typedefs  &rang;</a></li></ul></nav><p id="page-github-links"><span>Unless stated otherwise, the documentation on this site reflects Dart 3.7.1. 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