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href="https://pub.dev" target="_blank" rel="noopener">Package 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="#null-safety">Null safety</a></li><li class="toc-entry nav-item"><a class="nav-link" href="#default-value">Default value</a></li><li class="toc-entry nav-item"><a class="nav-link" href="#late-variables">Late variables</a></li><li class="toc-entry nav-item"><a class="nav-link" href="#final-and-const">Final and const</a></li></ul></div><article><div class="content"><nav id="subnav"><ul><li class="previous"><a href="/language">&lang;  Basics</a></li><li class="next"><a href="/language/operators">Operators  &rang;</a></li></ul></nav><div id="site-content-title"><h1>Variables</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="#null-safety">Null safety</a></li><li class="toc-entry"><a href="#default-value">Default value</a></li><li class="toc-entry"><a href="#late-variables">Late variables</a></li><li class="toc-entry"><a href="#final-and-const">Final and const</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>Here's an example of creating a variable and initializing it:</p> <?code-excerpt "misc/lib/language_tour/variables.dart (var-decl)"?> <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"> name = </span><span style="color:#11796D">'Bob'</span><span style="color:#222222">;</span></span></code></pre></div></div><p>Variables store references. The variable called <code>name</code> contains a reference to a <code>String</code> object with a value of "Bob".</p><p>The type of the <code>name</code> variable is inferred to be <code>String</code>, but you can change that type by specifying it. If an object isn't restricted to a single type, specify the <code>Object</code> type (or <code>dynamic</code> if necessary).</p> <?code-excerpt "misc/lib/language_tour/variables.dart (type-decl)"?> <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:#0468D7">Object</span><span style="color:#222222"> name = </span><span style="color:#11796D">'Bob'</span><span style="color:#222222">;</span></span></code></pre></div></div><p>Another option is to explicitly declare the type that would be inferred:</p> <?code-excerpt "misc/lib/language_tour/variables.dart (static-types)"?> <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:#0468D7">String</span><span style="color:#222222"> name = </span><span style="color:#11796D">'Bob'</span><span style="color:#222222">;</span></span></code></pre></div></div><aside class="alert alert-info"><div class="alert-header"><i class="material-symbols" aria-hidden="true">info</i> <span>Note</span></div><div class="alert-content"><p>This page follows the <a href="/effective-dart/design#types">style guide recommendation</a> of using <code>var</code>, rather than type annotations, for local variables.</p></div></aside><div class="header-wrapper"><h2 id="null-safety">Null safety</h2><a class="heading-link" href="#null-safety" aria-label="Link to 'Null safety' section">#</a></div><p>The Dart language enforces sound null safety.</p><p>Null safety prevents an error that results from unintentional access of variables set to <code>null</code>. The error is called a null dereference error. A null dereference error occurs when you access a property or call a method on an expression that evaluates to <code>null</code>. An exception to this rule is when <code>null</code> supports the property or method, like <code>toString()</code> or <code>hashCode</code>. With null safety, the Dart compiler detects these potential errors at compile time.</p><p>For example, say you want to find the absolute value of an <code>int</code> variable <code>i</code>. If <code>i</code> is <code>null</code>, calling <code>i.abs()</code> causes a null dereference error. In other languages, trying this could lead to a runtime error, but Dart's compiler prohibits these actions. Therefore, Dart apps can't cause runtime errors.</p><p>Null safety introduces three key changes:</p><ol><li><p>When you specify a type for a variable, parameter, or another relevant component, you can control whether the type allows <code>null</code>. To enable nullability, you add a <code>?</code> to the end of the type declaration.</p><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:#0468D7">String</span><span style="color:#222222">? name </span><span style="color:#6E6E70">// Nullable type. Can be `null` or string.</span></span> <span class="line"></span> <span class="line"><span style="color:#0468D7">String</span><span style="color:#222222"> name </span><span style="color:#6E6E70">// Non-nullable type. Cannot be `null` but can be string.</span></span></code></pre></div></div></li><li><p>You must initialize variables before using them. Nullable variables default to <code>null</code>, so they are initialized by default. Dart doesn't set initial values to non-nullable types. It forces you to set an initial value. Dart doesn't allow you to observe an uninitialized variable. This prevents you from accessing properties or calling methods where the receiver's type can be <code>null</code> but <code>null</code> doesn't support the method or property used.</p></li><li><p>You can't access properties or call methods on an expression with a nullable type. The same exception applies where it's a property or method that <code>null</code> supports like <code>hashCode</code> or <code>toString()</code>.</p></li></ol><p>Sound null safety changes potential <strong>runtime errors</strong> into <strong>edit-time</strong> analysis errors. Null safety flags a non-null variable when it has been either:</p><ul><li>Not initialized with a non-null value.</li><li>Assigned a <code>null</code> value.</li></ul><p>This check allows you to fix these errors <em>before</em> deploying your app.</p><div class="header-wrapper"><h2 id="default-value">Default value</h2><a class="heading-link" href="#default-value" aria-label="Link to 'Default value' section">#</a></div><p>Uninitialized variables that have a nullable type have an initial value of <code>null</code>. Even variables with numeric types are initially null, because numbers—like everything else in Dart—are objects.</p> <?code-excerpt "misc/test/language_tour/variables_test.dart (var-null-init)"?> <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:#0468D7">int</span><span style="color:#222222">? lineCount;</span></span> <span class="line"><span style="color:#D43324">assert</span><span style="color:#222222">(lineCount == </span><span style="color:#11796D">null</span><span style="color:#222222">);</span></span></code></pre></div></div><aside class="alert alert-info"><div class="alert-header"><i class="material-symbols" aria-hidden="true">info</i> <span>Note</span></div><div class="alert-content"><p>Production code ignores the <code>assert()</code> call. During development, on the other hand, <code>assert(<em>condition</em>)</code> throws an exception if <em>condition</em> is false. For details, check out <a href="/language/error-handling#assert">Assert</a>.</p></div></aside><p>With null safety, you must initialize the values of non-nullable variables before you use them:</p> <?code-excerpt "misc/lib/language_tour/variables.dart (var-ns-init)"?> <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:#0468D7">int</span><span style="color:#222222"> lineCount = </span><span style="color:#11796D">0</span><span style="color:#222222">;</span></span></code></pre></div></div><p>You don't have to initialize a local variable where it's declared, but you do need to assign it a value before it's used. For example, the following code is valid because Dart can detect that <code>lineCount</code> is non-null by the time it's passed to <code>print()</code>:</p> <?code-excerpt "misc/lib/language_tour/variables.dart (var-ns-flow)"?> <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:#0468D7">int</span><span style="color:#222222"> lineCount;</span></span> <span class="line"></span> <span class="line"><span style="color:#D43324">if</span><span style="color:#222222"> (weLikeToCount) {</span></span> <span class="line"><span style="color:#222222"> lineCount = </span><span style="color:#6200EE">countLines</span><span style="color:#222222">();</span></span> <span class="line"><span style="color:#222222">} </span><span style="color:#D43324">else</span><span style="color:#222222"> {</span></span> <span class="line"><span style="color:#222222"> lineCount = </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:#6200EE">print</span><span style="color:#222222">(lineCount);</span></span></code></pre></div></div><p>Top-level and class variables are lazily initialized; the initialization code runs the first time the variable is used.</p><div class="header-wrapper"><h2 id="late-variables">Late variables</h2><a class="heading-link" href="#late-variables" aria-label="Link to 'Late variables' section">#</a></div><p>The <code>late</code> modifier has two use cases:</p><ul><li>Declaring a non-nullable variable that's initialized after its declaration.</li><li>Lazily initializing a variable.</li></ul><p>Often Dart's control flow analysis can detect when a non-nullable variable is set to a non-null value before it's used, but sometimes analysis fails. Two common cases are top-level variables and instance variables: Dart often can't determine whether they're set, so it doesn't try.</p><p>If you're sure that a variable is set before it's used, but Dart disagrees, you can fix the error by marking the variable as <code>late</code>:</p> <?code-excerpt "misc/lib/language_tour/variables.dart (var-late-top-level)" replace="/late/[!$&!]/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:#D43324">late</span></mark><span style="color:#0468D7"> String</span><span style="color:#222222"> description;</span></span> <span class="line"></span> <span class="line"><span style="color:#D43324">void</span><span style="color:#6200EE"> main</span><span style="color:#222222">() {</span></span> <span class="line"><span style="color:#222222"> description = </span><span style="color:#11796D">'Feijoada!'</span><span style="color:#222222">;</span></span> <span class="line"><span style="color:#6200EE"> print</span><span style="color:#222222">(description);</span></span> <span class="line"><span style="color:#222222">}</span></span></code></pre></div></div><aside class="alert alert-warning"><div class="alert-header"><i class="material-symbols" aria-hidden="true">warning</i> <span>Notice</span></div><div class="alert-content"><p>If you fail to initialize a <code>late</code> variable, a runtime error occurs when the variable is used.</p></div></aside><p>When you mark a variable as <code>late</code> but initialize it at its declaration, then the initializer runs the first time the variable is used. This lazy initialization is handy in a couple of cases:</p><ul><li>The variable might not be needed, and initializing it is costly.</li><li>You're initializing an instance variable, and its initializer needs access to <code>this</code>.</li></ul><p>In the following example, if the <code>temperature</code> variable is never used, then the expensive <code>readThermometer()</code> function is never called:</p> <?code-excerpt "misc/lib/language_tour/variables.dart (var-late-lazy)" replace="/late/[!$&!]/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:#6E6E70">// This is the program's only call to readThermometer().</span></span> <span class="line"><mark class="highlight"><span style="color:#D43324">late</span></mark><span style="color:#0468D7"> String</span><span style="color:#222222"> temperature = </span><span style="color:#6200EE">readThermometer</span><span style="color:#222222">(); </span><span style="color:#6E6E70">// Lazily initialized.</span></span></code></pre></div></div><div class="header-wrapper"><h2 id="final-and-const">Final and const</h2><a class="heading-link" href="#final-and-const" aria-label="Link to 'Final and const' section">#</a></div><p>If you never intend to change a variable, use <code>final</code> or <code>const</code>, either instead of <code>var</code> or in addition to a type. A final variable can be set only once; a const variable is a compile-time constant. (Const variables are implicitly final.)</p><aside class="alert alert-info"><div class="alert-header"><i class="material-symbols" aria-hidden="true">info</i> <span>Note</span></div><div class="alert-content"><p><a href="/language/classes#instance-variables">Instance variables</a> can be <code>final</code> but not <code>const</code>.</p></div></aside><p>Here's an example of creating and setting a <code>final</code> variable:</p> <?code-excerpt "misc/lib/language_tour/variables.dart (final)"?> <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">final</span><span style="color:#222222"> name = </span><span style="color:#11796D">'Bob'</span><span style="color:#222222">; </span><span style="color:#6E6E70">// Without a type annotation</span></span> <span class="line"><span style="color:#D43324">final</span><span style="color:#0468D7"> String</span><span style="color:#222222"> nickname = </span><span style="color:#11796D">'Bobby'</span><span style="color:#222222">;</span></span></code></pre></div></div><p>You can't change the value of a <code>final</code> variable:</p> <?code-excerpt "misc/lib/language_tour/variables.dart (cant-assign-to-final)"?> <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:#222222">name = </span><span style="color:#11796D">'Alice'</span><span style="color:#222222">; </span><span style="color:#6E6E70">// Error: a final variable can only be set once.</span></span></code></pre></div></div><p>Use <code>const</code> for variables that you want to be <strong>compile-time constants</strong>. If the const variable is at the class level, mark it <code>static const</code>. Where you declare the variable, set the value to a compile-time constant such as a number or string literal, a const variable, or the result of an arithmetic operation on constant numbers:</p> <?code-excerpt "misc/lib/language_tour/variables.dart (const)"?> <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">const</span><span style="color:#222222"> bar = </span><span style="color:#11796D">1000000</span><span style="color:#222222">; </span><span style="color:#6E6E70">// Unit of pressure (dynes/cm2)</span></span> <span class="line"><span style="color:#D43324">const</span><span style="color:#0468D7"> double</span><span style="color:#222222"> atm = </span><span style="color:#11796D">1.01325</span><span style="color:#222222"> * bar; </span><span style="color:#6E6E70">// Standard atmosphere</span></span></code></pre></div></div><p>The <code>const</code> keyword isn't just for declaring constant variables. You can also use it to create constant <em>values</em>, as well as to declare constructors that <em>create</em> constant values. Any variable can have a constant value.</p> <?code-excerpt "misc/lib/language_tour/variables.dart (const-vs-final)"?> <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:#D43324">const</span><span style="color:#222222"> [];</span></span> <span class="line"><span style="color:#D43324">final</span><span style="color:#222222"> bar = </span><span style="color:#D43324">const</span><span style="color:#222222"> [];</span></span> <span class="line"><span style="color:#D43324">const</span><span style="color:#222222"> baz = []; </span><span style="color:#6E6E70">// Equivalent to `const []`</span></span></code></pre></div></div><p>You can omit <code>const</code> from the initializing expression of a <code>const</code> declaration, like for <code>baz</code> above. For details, see <a href="/effective-dart/usage#dont-use-const-redundantly">DON'T use const redundantly</a>.</p><p>You can change the value of a non-final, non-const variable, even if it used to have a <code>const</code> value:</p> <?code-excerpt "misc/lib/language_tour/variables.dart (reassign-to-non-final)"?> <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:#222222">foo = [</span><span style="color:#11796D">1</span><span style="color:#222222">, </span><span style="color:#11796D">2</span><span style="color:#222222">, </span><span style="color:#11796D">3</span><span style="color:#222222">]; </span><span style="color:#6E6E70">// Was const []</span></span></code></pre></div></div><p>You can't change the value of a <code>const</code> variable:</p> <?code-excerpt "misc/lib/language_tour/variables.dart (cant-assign-to-const)"?> <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:#222222">baz = [</span><span style="color:#11796D">42</span><span style="color:#222222">]; </span><span style="color:#6E6E70">// Error: Constant variables can't be assigned a value.</span></span></code></pre></div></div><p>You can define constants that use <a href="/language/operators#type-test-operators">type checks and casts</a> (<code>is</code> and <code>as</code>), <a href="/language/collections#control-flow-operators">collection <code>if</code></a>, and <a href="/language/collections#spread-operators">spread operators</a> (<code>...</code> and <code>...?</code>):</p> <?code-excerpt "misc/lib/language_tour/variables.dart (const-dart-25)"?> <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">const</span><span style="color:#0468D7"> Object</span><span style="color:#222222"> i = </span><span style="color:#11796D">3</span><span style="color:#222222">; </span><span style="color:#6E6E70">// Where i is a const Object with an int value...</span></span> <span class="line"><span style="color:#D43324">const</span><span style="color:#222222"> list = [i </span><span style="color:#D43324">as</span><span style="color:#0468D7"> int</span><span style="color:#222222">]; </span><span style="color:#6E6E70">// Use a typecast.</span></span> <span class="line"><span style="color:#D43324">const</span><span style="color:#222222"> map = {</span><span style="color:#D43324">if</span><span style="color:#222222"> (i is </span><span style="color:#0468D7">int</span><span style="color:#222222">) i: </span><span style="color:#11796D">'int'</span><span style="color:#222222">}; </span><span style="color:#6E6E70">// Use is and collection if.</span></span> <span class="line"><span style="color:#D43324">const</span><span style="color:#D43324"> set</span><span style="color:#222222"> = {</span><span style="color:#D43324">if</span><span style="color:#222222"> (list is </span><span style="color:#0468D7">List</span><span style="color:#222222"><</span><span style="color:#0468D7">int</span><span style="color:#222222">>) ...list}; </span><span style="color:#6E6E70">// ...and a spread.</span></span></code></pre></div></div><aside class="alert alert-info"><div class="alert-header"><i class="material-symbols" aria-hidden="true">info</i> <span>Note</span></div><div class="alert-content"><p>Although a <code>final</code> object cannot be modified, its fields can be changed. In comparison, a <code>const</code> object and its fields cannot be changed: they're <em>immutable</em>.</p></div></aside><p>For more information on using <code>const</code> to create constant values, see <a href="/language/collections#lists">Lists</a>, <a href="/language/collections#maps">Maps</a>, and <a href="/language/classes">Classes</a>.</p><nav id="subnav"><ul><li class="previous"><a href="/language">&lang;  Basics</a></li><li class="next"><a href="/language/operators">Operators  &rang;</a></li></ul></nav><p id="page-github-links"><span>Unless stated otherwise, the documentation on this site reflects Dart 3.5.4. Page last updated on 2024-02-07.</span> <a href="https://github.com/dart-lang/site-www/tree/main/src/content/language/variables.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/variables/&page-source=https://github.com/dart-lang/site-www/tree/main/src/content/language/variables.md" title="Report an issue with this page" target="_blank" rel="noopener">report an issue</a>.</p></div></article></main><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="Medium blog"><svg><use href="/assets/img/social/medium.svg#medium"></use></svg> </a><a href="https://github.com/dart-lang" target="_blank" rel="noopener" title="GitHub"><svg><use href="/assets/img/social/github.svg#github"></use></svg> </a><a href="https://twitter.com/dart_lang" target="_blank" rel="noopener" title="X (Twitter)"><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></body></html>