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 <div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Chapter 2. A Tour of the Dart Language</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="ch01.html">Prev</a> </td><th width="60%" align="center"> </th><td width="20%" align="right"> <a accesskey="n" href="ch03.html">Next</a></td></tr></table><hr></div><div class="chapter"><div class="titlepage"><div><div><h1 class="title"><a name="ch02"></a>Chapter 2. A Tour of the Dart Language</h1></div></div></div><div class="toc"><p><b>Table of Contents</b></p><dl class="toc"><dt><span class="sect1"><a href="ch02.html#main-print">A Basic Dart Program</a></span></dt><dt><span class="sect1"><a href="ch02.html#ch02-concepts">Important Concepts</a></span></dt><dd><dl><dt><span class="sect2"><a href="ch02.html#ch02-keywords">Keywords</a></span></dt><dt><span class="sect2"><a href="ch02.html#ch02-runtime-modes">Runtime Modes</a></span></dt></dl></dd><dt><span class="sect1"><a href="ch02.html#variables">Variables</a></span></dt><dd><dl><dt><span class="sect2"><a href="ch02.html#variables-default-value">Default Value</a></span></dt><dt><span class="sect2"><a href="ch02.html#variables-optional-types">Optional Types</a></span></dt><dt><span class="sect2"><a href="ch02.html#ch02-final-const">Final and Const</a></span></dt></dl></dd><dt><span class="sect1"><a href="ch02.html#built-in-types">Built-in Types</a></span></dt><dd><dl><dt><span class="sect2"><a href="ch02.html#numbers">Numbers</a></span></dt><dt><span class="sect2"><a href="ch02.html#strings">Strings</a></span></dt><dt><span class="sect2"><a href="ch02.html#booleans">Booleans</a></span></dt><dt><span class="sect2"><a href="ch02.html#lists">Lists</a></span></dt><dt><span class="sect2"><a href="ch02.html#maps">Maps</a></span></dt><dt><span class="sect2"><a href="ch02.html#ch02-symbols">Symbols</a></span></dt></dl></dd><dt><span class="sect1"><a href="ch02.html#functions">Functions</a></span></dt><dd><dl><dt><span class="sect2"><a href="ch02.html#functions-optional-parameters">Optional Parameters</a></span></dt><dt><span class="sect2"><a href="ch02.html#ch02-main">The main() Function</a></span></dt><dt><span class="sect2"><a href="ch02.html#ch02-functions-first-class">Functions as First-Class Objects</a></span></dt><dt><span class="sect2"><a href="ch02.html#ch02-lexical-scope">Lexical Scope</a></span></dt><dt><span class="sect2"><a href="ch02.html#ch02-functions-lexical-closures">Lexical Closures</a></span></dt><dt><span class="sect2"><a href="ch02.html#ch02-function-equality">Testing Functions for Equality</a></span></dt><dt><span class="sect2"><a href="ch02.html#ch02-functions-return-values">Return Values</a></span></dt></dl></dd><dt><span class="sect1"><a href="ch02.html#operators">Operators</a></span></dt><dd><dl><dt><span class="sect2"><a href="ch02.html#op-arithmetic">Arithmetic Operators</a></span></dt><dt><span class="sect2"><a href="ch02.html#op-equality">Equality and Relational Operators</a></span></dt><dt><span class="sect2"><a href="ch02.html#ch02-op-type-test">Type Test Operators</a></span></dt><dt><span class="sect2"><a href="ch02.html#op-assign">Assignment Operators</a></span></dt><dt><span class="sect2"><a href="ch02.html#op-logical">Logical Operators</a></span></dt><dt><span class="sect2"><a href="ch02.html#op-bit">Bitwise and Shift Operators</a></span></dt><dt><span class="sect2"><a href="ch02.html#op-other">Other Operators</a></span></dt></dl></dd><dt><span class="sect1"><a href="ch02.html#control-flow-statements">Control Flow Statements</a></span></dt><dd><dl><dt><span class="sect2"><a href="ch02.html#if-else">If and Else</a></span></dt><dt><span class="sect2"><a href="ch02.html#for-loops">For Loops</a></span></dt><dt><span class="sect2"><a href="ch02.html#while">While and Do-While</a></span></dt><dt><span class="sect2"><a href="ch02.html#break">Break and Continue</a></span></dt><dt><span class="sect2"><a href="ch02.html#switch">Switch and Case</a></span></dt><dt><span class="sect2"><a href="ch02.html#assert">Assert</a></span></dt></dl></dd><dt><span class="sect1"><a href="ch02.html#exceptions">Exceptions</a></span></dt><dd><dl><dt><span class="sect2"><a href="ch02.html#ch02-exceptions-throw">Throw</a></span></dt><dt><span class="sect2"><a href="ch02.html#ch02-exceptions-catch">Catch</a></span></dt><dt><span class="sect2"><a href="ch02.html#ch02-exceptions-finally">Finally</a></span></dt></dl></dd><dt><span class="sect1"><a href="ch02.html#classes">Classes</a></span></dt><dd><dl><dt><span class="sect2"><a href="ch02.html#ch02-instance-variables">Instance Variables</a></span></dt><dt><span class="sect2"><a href="ch02.html#ch02-constructors">Constructors</a></span></dt><dt><span class="sect2"><a href="ch02.html#ch02-methods">Methods</a></span></dt><dt><span class="sect2"><a href="ch02.html#ch02-class-abstract">Abstract Classes</a></span></dt><dt><span class="sect2"><a href="ch02.html#ch02-implicit-interfaces">Implicit Interfaces</a></span></dt><dt><span class="sect2"><a href="ch02.html#ch02-class-extend">Extending a Class</a></span></dt><dt><span class="sect2"><a href="ch02.html#ch02-mixins">Adding Features to a Class: Mixins</a></span></dt><dt><span class="sect2"><a href="ch02.html#ch02-class-vars-and-methods">Class Variables and Methods</a></span></dt></dl></dd><dt><span class="sect1"><a href="ch02.html#generics">Generics</a></span></dt><dd><dl><dt><span class="sect2"><a href="ch02.html#ch02-generics-why">Why Use Generics?</a></span></dt><dt><span class="sect2"><a href="ch02.html#generics-literals">Using Collection Literals</a></span></dt><dt><span class="sect2"><a href="ch02.html#generics-constructors">Using Parameterized Types with Constructors</a></span></dt><dt><span class="sect2"><a href="ch02.html#generics-collections">Generic Collections and the Types they Contain</a></span></dt></dl></dd><dt><span class="sect1"><a href="ch02.html#libraries">Libraries and Visibility</a></span></dt><dd><dl><dt><span class="sect2"><a href="ch02.html#libraries-import">Using Libraries</a></span></dt><dt><span class="sect2"><a href="ch02.html#libraries-implementing">Implementing Libraries</a></span></dt></dl></dd><dt><span class="sect1"><a href="ch02.html#isolates">Isolates</a></span></dt><dt><span class="sect1"><a href="ch02.html#typedefs">Typedefs</a></span></dt><dt><span class="sect1"><a href="ch02.html#ch02-metadata">Metadata</a></span></dt><dt><span class="sect1"><a href="ch02.html#comments">Comments</a></span></dt><dd><dl><dt><span class="sect2"><a href="ch02.html#ch02-comments-single-line">Single-Line Comments</a></span></dt><dt><span class="sect2"><a href="ch02.html#ch02-comments-multi-line">Multi-Line Comments</a></span></dt><dt><span class="sect2"><a href="ch02.html#comments-doc">Documentation Comments</a></span></dt></dl></dd><dt><span class="sect1"><a href="ch02.html#ch02-summary">Summary</a></span></dt></dl></div><p>This chapter shows you how to use each major Dart feature, from
   variables and operators to classes and libraries, with the assumption that
   you already know how to program in another language.</p><div class="note" style="margin-left: 0.5in; margin-right: 0.5in;"><h3 class="title">Note</h3><p>To play with each feature, create a command-line application project
@@ skipping 77 matching lines @@
         mode. In checked mode, <code class="literal">assert(<em class="replaceable"><code>condition</code></em>)</code>
         throws an exception unless <em class="replaceable"><code>condition</code></em> is
         true. For details, see <a class="xref" href="ch02.html#assert" title="Assert">the section called “Assert”</a>.</p></div></div><div class="sect2"><div class="titlepage"><div><div><h3 class="title"><a name="variables-optional-types"></a>Optional Types</h3></div></div></div><p>You have the option of adding static types to your variable
       declarations:</p><pre class="programlisting"><em><span class="remark">lang-dart
 </span></em>String name = 'Bob';</pre><p>Adding types is a way to clearly express your intent. Tools such
       as compilers and editors can use these types to help you, by providing
       code completion and early warnings for bugs and code <span class="keep-together">completion</span>.</p><div class="note" style="margin-left: 0.5in; margin-right: 0.5in;"><h3 class="title">Note</h3><p>This chapter follows the <a class="ulink" href="http://www.dartlang.org/articles/style-guide/#type-annotations" target="_top">style
         guide recommendation</a> of using <code class="literal">var</code>, rather than type annotations, for local
         variables.</p></div></div><div class="sect2"><div class="titlepage"><div><div><h3 class="title"><a name="ch02-final-const"></a>Final and Const</h3></div></div></div><p>If you never intend to change a variable, use <code class="literal">final</code> or <code class="literal">const</code>, either instead of <code class="literal">var</code> or in addition to a type. A final variable
       can be set only once; a const variable is a compile-time
-      constant.</p><p>A local, top-level, or class variable that’s declared as <code class="literal">final</code> is initialized the first time it’s
+      constant.</p><p>A top-level or class variable that’s declared as <code class="literal">final</code> is initialized the first time it’s
       used:</p><pre class="programlisting"><em><span class="remark">lang-dart
 </span></em>final name = 'Bob';   // Or: final String name = 'Bob';
 // name = 'Alice';    // Uncommenting this results in an error</pre><div class="note" style="margin-left: 0.5in; margin-right: 0.5in;"><h3 class="title">Note</h3><p>Lazy initialization of final variables helps apps start up
         faster.</p></div><p>Use <code class="literal">const</code> for variables that
       you want to be compile-time constants. If the const variable is at the
       class level, mark it <code class="literal">static const</code>. (Instance
       variables can’t be const.) Where you declare the variable, set the value
       to a compile-time constant such as a literal, a const variable, or the
       result of an arithmetic operation on constant numbers:</p><pre class="programlisting"><em><span class="remark">lang-dart
 </span></em>const bar = 1000000;       // Unit of pressure (in dynes/cm2)
 const atm = 1.01325 * bar; // Standard atmosphere</pre></div></div><div class="sect1"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="built-in-types"></a>Built-in Types</h2></div></div></div><p>The Dart language has special support for the following
     types:</p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p>numbers</p></li><li class="listitem"><p>strings</p></li><li class="listitem"><p>booleans</p></li><li class="listitem"><p>lists (also known as <span class="emphasis"><em>arrays</em></span>)</p></li><li class="listitem"><p>maps</p></li><li class="listitem"><p>symbols</p></li></ul></div><p>You can initialize an object of any of these special types using a
     literal. For example, <code class="literal">'this is a
     string'</code> is a string literal, and <code class="literal">true</code> is a boolean literal.</p><p>Because every variable in Dart refers to an object—an instance of a
     <span class="emphasis"><em>class</em></span>—you can usually use
     <span class="emphasis"><em>constructors</em></span> to initialize variables. Some of the
     built-in types have their own constructors. For example, you can use the
     <code class="literal">Map()</code> constructor to create a map, using code such as
     <code class="literal">new Map()</code>.</p><div class="sect2"><div class="titlepage"><div><div><h3 class="title"><a name="numbers"></a>Numbers</h3></div></div></div><p>Dart numbers come in two flavors:</p><div class="variablelist"><dl class="variablelist"><dt><span class="term"><a class="ulink" href="http://api.dartlang.org/dart_core/int.html" target="_top">
           <code class="literal">int</code> </a></span></dt><dd><p>Integer values, which generally should be in the range
-            -2<sup>53</sup> to 2<sup>53</sup>
-            </p></dd><dt><span class="term"><a class="ulink" href="http://api.dartlang.org/dart_core/double.html" target="_top">
+            -2<sup>53</sup> to
+            2<sup>53</sup></p></dd><dt><span class="term"><a class="ulink" href="http://api.dartlang.org/dart_core/double.html" target="_top">
           <code class="literal">double</code> </a></span></dt><dd><p>64-bit (double-precision) floating-point numbers, as
             specified by the IEEE 754 standard</p></dd></dl></div><p>Both <code class="literal">int</code> and <code class="literal">double</code> are subtypes of <a class="ulink" href="http://api.dartlang.org/dart_core/num.html" target="_top"><code class="literal">num</code>.</a> The num type includes basic
       operators such as +, -, /, and *, as well as bitwise operators such as
       &gt;&gt;. The num type is also where you’ll find
       <code class="literal">abs()</code>,<code class="literal"> ceil()</code>, and
       <code class="literal">floor()</code>, among other methods. If num and its subtypes
-      don’t have what you’re looking for, the <a class="ulink" href="http://api.dartlang.org/dart_math.html" target="_top">Math</a> library might.
-      </p><div class="warning" style="margin-left: 0.5in; margin-right: 0.5in;"><h3 class="title">Warning</h3><p>Integers outside of the -2<sup>53</sup> to
+      don’t have what you’re looking for, the <a class="ulink" href="http://api.dartlang.org/dart_math.html" target="_top">Math</a> library
+      might.</p><div class="warning" style="margin-left: 0.5in; margin-right: 0.5in;"><h3 class="title">Warning</h3><p>Integers outside of the -2<sup>53</sup> to
         2<sup>53</sup> range currently behave differently in
         JavaScript produced from Dart code than they do when the same Dart
         code runs in the Dart VM. The reason is that Dart is specified to have
         arbitrary-precision integers, but JavaScript isn't. See <a class="ulink" href="http://dartbug.com/1533" target="_top">issue 1533</a> for details.</p></div><p>Integers are numbers without a decimal point. Here are some
       examples of defining integer literals:</p><pre class="programlisting"><em><span class="remark">lang-dart
 </span></em>var x = 1;
 var hex = 0xDEADBEEF;
 var bigInt = 346534658346524376592384765923749587398457294759347029438709349347;</pre><p>If a number includes a decimal, it is a double. Here are some
       examples of defining double literals:</p><pre class="programlisting"><em><span class="remark">lang-dart
 </span></em>var y = 1.1;
@@ skipping 606 matching lines @@
     indicating that something unexpected happened. If the exception isn’t
     caught, the isolate that raised the exception is suspended, and typically
     the isolate and its program are terminated.</p><p>In contrast to Java, all of Dart’s exceptions are unchecked
     exceptions. Methods do not declare which exceptions they might throw, and
     you are not required to catch any <span class="keep-together">exceptions</span>.</p><p>Dart provides <a class="ulink" href="http://api.dartlang.org/dart_core/Exception.html" target="_top">Exception</a>
     and <a class="ulink" href="http://api.dartlang.org/docs/continuous/dart_core/Error.html" target="_top">Error</a>
     types, as well as numerous predefined subtypes. You can, of course, define
     your own exceptions. However, Dart programs can throw any non-null
     object—not just Exception and Error objects—as an exception.</p><div class="sect2"><div class="titlepage"><div><div><h3 class="title"><a name="ch02-exceptions-throw"></a>Throw</h3></div></div></div><p>Here’s an example of throwing, or <span class="emphasis"><em>raising</em></span>, an
       exception:</p><pre class="programlisting"><em><span class="remark">lang-dart
-</span></em>throw new ExpectException('Value must be greater than zero');</pre><p>You can also throw arbitrary objects:</p><pre class="programlisting"><em><span class="remark">lang-dart
+</span></em>throw new FormatException('expected at least one section');</pre><p>You can also throw arbitrary objects:</p><pre class="programlisting"><em><span class="remark">lang-dart
 </span></em>throw 'Out of llamas!';</pre><p>Because throwing an exception is an expression, you can throw
       exceptions in =&gt; statements, as well as anywhere else that allows
       expressions:</p><pre class="programlisting"><em><span class="remark">lang-dart
 </span></em>distanceTo(Point other) =&gt; throw new UnimplementedError();</pre></div><div class="sect2"><div class="titlepage"><div><div><h3 class="title"><a name="ch02-exceptions-catch"></a>Catch</h3></div></div></div><p>Catching, or capturing, an exception stops the exception from
       propagating. Catching an exception gives you a chance to handle
       it:</p><pre class="programlisting"><em><span class="remark">lang-dart
 </span></em>try {
   breedMoreLlamas();
 } on OutOfLlamasException {
   buyMoreLlamas();
@@ skipping 815 matching lines @@
     // ...
   }
 
   /// Exercises your llama with an [activity] for
   /// [timeLimit] minutes.
   void exercise(Activity activity, int timeLimit) {
     // ...
   }
 }</pre><p>In the generated documentation, <code class="literal">[Food]</code> becomes a link to the API docs for the
       Food class.</p><p>To parse Dart code and generate HTML documentation, you can use
-      the SDK’s
-      <a class="ulink" href="http://www.dartlang.org/tools/docgen/" target="_top">documentation generation tool.</a> For an example of generated
-      documentation, see the <a class="ulink" href="http://api.dartlang.org" target="_top">Dart API
-      documentation.</a> For advice on how to structure your comments, see
-      <a class="ulink" href="http://www.dartlang.org/articles/doc-comment-guidelines/" target="_top">Guidelines
+      the SDK’s <a class="ulink" href="http://www.dartlang.org/tools/docgen/" target="_top">documentation generation
+      tool.</a> For an example of generated documentation, see the <a class="ulink" href="http://api.dartlang.org" target="_top">Dart API documentation.</a> For advice
+      on how to structure your comments, see <a class="ulink" href="http://www.dartlang.org/articles/doc-comment-guidelines/" target="_top">Guidelines
       for Dart Doc Comments.</a></p></div></div><div class="sect1"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="ch02-summary"></a>Summary</h2></div></div></div><p>This chapter summarized the commonly used features in the Dart
-    language. More features are being implemented, but we expect
-    that they won’t break existing code. For more information, see the <a class="ulink" href="http://www.dartlang.org/docs/spec/" target="_top">Dart Language
+    language. More features are being implemented, but we expect that they
+    won’t break existing code. For more information, see the <a class="ulink" href="http://www.dartlang.org/docs/spec/" target="_top">Dart Language
     Specification</a> and <a class="ulink" href="http://www.dartlang.org/articles/" target="_top">articles</a> such as <a class="ulink" href="http://www.dartlang.org/articles/idiomatic-dart/" target="_top">Idiomatic Dart.</a></p></div><div class="footnotes"><br><hr style="width:100; text-align:left;margin-left: 0"><div id="ftn.ch02-footnote-1" class="footnote"><p><a href="#ch02-footnote-1" class="para"><sup class="para">[1] </sup></a>URI stands for <em class="firstterm">uniform resource
           identifier</em>. URLs (<em class="firstterm">uniform resource
           locators</em>) are a common kind of URI.</p></div></div></div><div class="navfooter"><hr><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="ch01.html">Prev</a> </td><td width="20%" align="center"> </td><td width="40%" align="right"> <a accesskey="n" href="ch03.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Chapter 1. Quick Start </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> Chapter 3. A Tour of the Dart Libraries</td></tr></table></div>