A tour of the Dart language

This page 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. For a briefer, less complete introduction to the language, see the language samples page.

To learn more about Dart’s core libraries, see the library tour. Whenever you want more details about a language feature, consult the Dart language specification.

A basic Dart program

The following code uses many of Dart’s most basic features:

// Define a function.
void printInteger(int aNumber) {
  print('The number is $aNumber.'); // Print to console.
}

// This is where the app starts executing.
void main() {
  var number = 42; // Declare and initialize a variable.
  printInteger(number); // Call a function.
}

Here’s what this program uses that applies to all (or almost all) Dart apps:

// This is a comment.

A single-line comment. Dart also supports multi-line and document comments. For details, see Comments.

void

A special type that indicates a value that’s never used. Functions like printInteger() and main() that don’t explicitly return a value have the void return type. For more information, see this article.

int

Another type, indicating an integer. Some additional built-in types are String, List, and bool.

42

A number literal. Number literals are a kind of compile-time constant.

print()

A handy way to display output.

'...' (or "...")

A string literal.

$variableName (or ${expression})

String interpolation: including a variable or expression’s string equivalent inside of a string literal. For more information, see Strings.

main()

The special, required, top-level function where app execution starts. For more information, see The main() function.

var

A way to declare a variable without specifying its type.

Important concepts

As you learn about the Dart language, keep these facts and concepts in mind:

• Everything you can place in a variable is an object, and every object is an instance of a class. Even numbers, functions, and null are objects. All objects inherit from the Object class.

• Although Dart is strongly typed, type annotations are optional because Dart can infer types. In the code above, number is inferred to be of type int. When you want to explicitly say that no type is expected, use the special type dynamic.

• Dart supports generic types, like List<int> (a list of integers) or List<dynamic> (a list of objects of any type).

• Dart supports top-level functions (such as main()), as well as functions tied to a class or object (static and instance methods, respectively). You can also create functions within functions (nested or local functions).

• Similarly, Dart supports top-level variables, as well as variables tied to a class or object (static and instance variables). Instance variables are sometimes known as fields or properties.

• Unlike Java, Dart doesn’t have the keywords public, protected, and private. If an identifier starts with an underscore (_), it’s private to its library. For details, see Libraries and visibility.

• Identifiers can start with a letter or underscore (_), followed by any combination of those characters plus digits.

• Dart has both expressions (which have runtime values) and statements (which don’t). For example, the conditional expression condition ? expr1 : expr2 has a value of expr1 or expr2. Compare that to an if-else statement, which has no value. A statement often contains one or more expressions, but an expression can’t directly contain a statement.

• Dart tools can report two kinds of problems: warnings and errors. Warnings are just indications that your code might not work, but they don’t prevent your program from executing. Errors can be either compile-time or run-time. A compile-time error prevents the code from executing at all; a run-time error results in an exception being raised while the code executes.

Keywords

The following table lists the words that the Dart language treats specially.

Avoid using these words as identifiers. However, if necessary, the keywords marked with superscripts can be identifiers:

• Words with the superscript 1 are contextual keywords, which have meaning only in specific places. They’re valid identifiers everywhere.

• Words with the superscript 2 are built-in identifiers. To simplify the task of porting JavaScript code to Dart, these keywords are valid identifiers in most places, but they can’t be used as class or type names, or as import prefixes.

• Words with the superscript 3 are limited reserved words related to asynchrony support. You can’t use await or yield as an identifier in any function body marked with async, async*, or sync*.

All other words in the table are reserved words, which can’t be identifiers.

Variables

Here’s an example of creating a variable and initializing it:

var name = 'Bob';

Variables store references. The variable called name contains a reference to a String object with a value of “Bob”.

The type of the name variable is inferred to be String, but you can change that type by specifying it. If an object isn’t restricted to a single type, specify the Object or dynamic type, following design guidelines.

dynamic name = 'Bob';

Another option is to explicitly declare the type that would be inferred:

String name = 'Bob';

Default value

Uninitialized variables have an initial value of null. Even variables with numeric types are initially null, because numbers—like everything else in Dart—are objects.

int lineCount;
assert(lineCount == null);

Final and const

If you never intend to change a variable, use final or const, either instead of var 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.) A final top-level or class variable is initialized the first time it’s used.

Here’s an example of creating and setting a final variable:

final name = 'Bob'; // Without a type annotation
final String nickname = 'Bobby';

You can’t change the value of a final variable:

name = 'Alice'; // Error: a final variable can only be set once.

Use const for variables that you want to be compile-time constants. If the const variable is at the class level, mark it static const. 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:

const bar = 1000000; // Unit of pressure (dynes/cm2)
const double atm = 1.01325 * bar; // Standard atmosphere

The const keyword isn’t just for declaring constant variables. You can also use it to create constant values, as well as to declare constructors that create constant values. Any variable can have a constant value.

var foo = const [];
final bar = const [];
const baz = []; // Equivalent to `const []`

You can omit const from the initializing expression of a const declaration, like for baz above. For details, see DON’T use const redundantly.

You can change the value of a non-final, non-const variable, even if it used to have a const value:

foo = [1, 2, 3]; // Was const []

You can’t change the value of a const variable:

baz = [42]; // Error: Constant variables can't be assigned a value.

You can define constants that use type checks and casts (is and as), collection if, and spread operators (... and ...?):

const Object i = 3; // Where i is a const Object with an int value...
const list = [i as int]; // Use a typecast.
const map = {if (i is int) i: "int"}; // Use is and collection if.
const set = {if (list is List<int>) ...list}; // ...and a spread.

For more information on using const to create constant values, see Lists, Maps, and Classes.

Built-in types

The Dart language has special support for the following types:

• numbers
• strings
• booleans
• lists (also known as arrays)
• sets
• maps
• runes (for expressing Unicode characters in a string)
• symbols

You can initialize an object of any of these special types using a literal. For example, 'this is a string' is a string literal, and true is a boolean literal.

Because every variable in Dart refers to an object—an instance of a class—you can usually use constructors to initialize variables. Some of the built-in types have their own constructors. For example, you can use the Map() constructor to create a map.

Numbers

Dart numbers come in two flavors:

int

Integer values no larger than 64 bits, depending on the platform. On the Dart VM, values can be from -2⁶³ to 2⁶³ - 1. Dart that’s compiled to JavaScript uses JavaScript numbers, allowing values from -2⁵³ to 2⁵³ - 1.

double

64-bit (double-precision) floating-point numbers, as specified by the IEEE 754 standard.

Both int and double are subtypes of num. The num type includes basic operators such as +, -, /, and *, and is also where you’ll find abs(), ceil(), and floor(), among other methods. (Bitwise operators, such as >>, are defined in the int class.) If num and its subtypes don’t have what you’re looking for, the dart:math library might.

Integers are numbers without a decimal point. Here are some examples of defining integer literals:

var x = 1;
var hex = 0xDEADBEEF;

If a number includes a decimal, it is a double. Here are some examples of defining double literals:

var y = 1.1;
var exponents = 1.42e5;

Integer literals are automatically converted to doubles when necessary:

double z = 1; // Equivalent to double z = 1.0.

Here’s how you turn a string into a number, or vice versa:

// String -> int
var one = int.parse('1');
assert(one == 1);

// String -> double
var onePointOne = double.parse('1.1');
assert(onePointOne == 1.1);

// int -> String
String oneAsString = 1.toString();
assert(oneAsString == '1');

// double -> String
String piAsString = 3.14159.toStringAsFixed(2);
assert(piAsString == '3.14');

The int type specifies the traditional bitwise shift (<<, >>), AND (&), and OR (|) operators. For example:

assert((3 << 1) == 6); // 0011 << 1 == 0110
assert((3 >> 1) == 1); // 0011 >> 1 == 0001
assert((3 | 4) == 7); // 0011 | 0100 == 0111

Literal numbers are compile-time constants. Many arithmetic expressions are also compile-time constants, as long as their operands are compile-time constants that evaluate to numbers.

const msPerSecond = 1000;
const secondsUntilRetry = 5;
const msUntilRetry = secondsUntilRetry * msPerSecond;

Strings

A Dart string is a sequence of UTF-16 code units. You can use either single or double quotes to create a string:

var s1 = 'Single quotes work well for string literals.';
var s2 = "Double quotes work just as well.";
var s3 = 'It\'s easy to escape the string delimiter.';
var s4 = "It's even easier to use the other delimiter.";

You can put the value of an expression inside a string by using ${expression}. If the expression is an identifier, you can skip the {}. To get the string corresponding to an object, Dart calls the object’s toString() method.

var s = 'string interpolation';

assert('Dart has $s, which is very handy.' ==
    'Dart has string interpolation, ' +
        'which is very handy.');
assert('That deserves all caps. ' +
        '${s.toUpperCase()} is very handy!' ==
    'That deserves all caps. ' +
        'STRING INTERPOLATION is very handy!');

You can concatenate strings using adjacent string literals or the + operator:

var s1 = 'String '
    'concatenation'
    " works even over line breaks.";
assert(s1 ==
    'String concatenation works even over '
        'line breaks.');

var s2 = 'The + operator ' + 'works, as well.';
assert(s2 == 'The + operator works, as well.');

Another way to create a multi-line string: use a triple quote with either single or double quotation marks:

var s1 = '''
You can create
multi-line strings like this one.
''';

var s2 = """This is also a
multi-line string.""";

You can create a “raw” string by prefixing it with r:

var s = r'In a raw string, not even \n gets special treatment.';

See Runes and grapheme clusters for details on how to express Unicode characters in a string.

Literal strings are compile-time constants, as long as any interpolated expression is a compile-time constant that evaluates to null or a numeric, string, or boolean value.

// These work in a const string.
const aConstNum = 0;
const aConstBool = true;
const aConstString = 'a constant string';

// These do NOT work in a const string.
var aNum = 0;
var aBool = true;
var aString = 'a string';
const aConstList = [1, 2, 3];

const validConstString = '$aConstNum $aConstBool $aConstString';
// const invalidConstString = '$aNum $aBool $aString $aConstList';

For more information on using strings, see Strings and regular expressions.

Booleans

To represent boolean values, Dart has a type named bool. Only two objects have type bool: the boolean literals true and false, which are both compile-time constants.

Dart’s type safety means that you can’t use code like if (nonbooleanValue) or assert (nonbooleanValue). Instead, explicitly check for values, like this:

// Check for an empty string.
var fullName = '';
assert(fullName.isEmpty);

// Check for zero.
var hitPoints = 0;
assert(hitPoints <= 0);

// Check for null.
var unicorn;
assert(unicorn == null);

// Check for NaN.
var iMeantToDoThis = 0 / 0;
assert(iMeantToDoThis.isNaN);

Lists

Perhaps the most common collection in nearly every programming language is the array, or ordered group of objects. In Dart, arrays are List objects, so most people just call them lists.

Dart list literals look like JavaScript array literals. Here’s a simple Dart list:

var list = [1, 2, 3];

You can add a comma after the last item in a Dart collection literal. This trailing comma doesn’t affect the collection, but it can help prevent copy-paste errors.

var list = [
  'Car',
  'Boat',
  'Plane',
];

Lists use zero-based indexing, where 0 is the index of the first value and list.length - 1 is the index of the last value. You can get a list’s length and refer to list values just as you would in JavaScript:

var list = [1, 2, 3];
assert(list.length == 3);
assert(list[1] == 2);

list[1] = 1;
assert(list[1] == 1);

To create a list that’s a compile-time constant, add const before the list literal:

var constantList = const [1, 2, 3];
// constantList[1] = 1; // This line will cause an error.

Dart 2.3 introduced the spread operator (...) and the null-aware spread operator (...?), which provide a concise way to insert multiple values into a collection.

For example, you can use the spread operator (...) to insert all the values of a list into another list:

var list = [1, 2, 3];
var list2 = [0, ...list];
assert(list2.length == 4);

If the expression to the right of the spread operator might be null, you can avoid exceptions by using a null-aware spread operator (...?):

var list;
var list2 = [0, ...?list];
assert(list2.length == 1);

For more details and examples of using the spread operator, see the spread operator proposal.

Dart also offers collection if and collection for, which you can use to build collections using conditionals (if) and repetition (for).

Here’s an example of using collection if to create a list with three or four items in it:

var nav = [
  'Home',
  'Furniture',
  'Plants',
  if (promoActive) 'Outlet'
];

Here’s an example of using collection for to manipulate the items of a list before adding them to another list:

var listOfInts = [1, 2, 3];
var listOfStrings = [
  '#0',
  for (var i in listOfInts) '#$i'
];
assert(listOfStrings[1] == '#1');

For more details and examples of using collection if and for, see the control flow collections proposal.

The List type has many handy methods for manipulating lists. For more information about lists, see Generics and Collections.

Sets

A set in Dart is an unordered collection of unique items. Dart support for sets is provided by set literals and the Set type.

Here is a simple Dart set, created using a set literal:

var halogens = {'fluorine', 'chlorine', 'bromine', 'iodine', 'astatine'};

To create an empty set, use {} preceded by a type argument, or assign {} to a variable of type Set:

var names = <String>{};
// Set<String> names = {}; // This works, too.
// var names = {}; // Creates a map, not a set.

Add items to an existing set using the add() or addAll() methods:

var elements = <String>{};
elements.add('fluorine');
elements.addAll(halogens);

Use .length to get the number of items in the set:

var elements = <String>{};
elements.add('fluorine');
elements.addAll(halogens);
assert(elements.length == 5);

To create a set that’s a compile-time constant, add const before the set literal:

final constantSet = const {
  'fluorine',
  'chlorine',
  'bromine',
  'iodine',
  'astatine',
};
// constantSet.add('helium'); // This line will cause an error.

Sets support spread operators (... and ...?) and collection ifs and fors, just like lists do. For more information, see the list spread operator and list collection operator discussions.

For more information about sets, see Generics and Sets.

Maps

In general, a map is an object that associates keys and values. Both keys and values can be any type of object. Each key occurs only once, but you can use the same value multiple times. Dart support for maps is provided by map literals and the Map type.

Here are a couple of simple Dart maps, created using map literals:

var gifts = {
  // Key:    Value
  'first': 'partridge',
  'second': 'turtledoves',
  'fifth': 'golden rings'
};

var nobleGases = {
  2: 'helium',
  10: 'neon',
  18: 'argon',
};

You can create the same objects using a Map constructor:

var gifts = Map();
gifts['first'] = 'partridge';
gifts['second'] = 'turtledoves';
gifts['fifth'] = 'golden rings';

var nobleGases = Map();
nobleGases[2] = 'helium';
nobleGases[10] = 'neon';
nobleGases[18] = 'argon';

Add a new key-value pair to an existing map just as you would in JavaScript:

var gifts = {'first': 'partridge'};
gifts['fourth'] = 'calling birds'; // Add a key-value pair

Retrieve a value from a map the same way you would in JavaScript:

var gifts = {'first': 'partridge'};
assert(gifts['first'] == 'partridge');

If you look for a key that isn’t in a map, you get a null in return:

var gifts = {'first': 'partridge'};
assert(gifts['fifth'] == null);

Use .length to get the number of key-value pairs in the map:

var gifts = {'first': 'partridge'};
gifts['fourth'] = 'calling birds';
assert(gifts.length == 2);

To create a map that’s a compile-time constant, add const before the map literal:

final constantMap = const {
  2: 'helium',
  10: 'neon',
  18: 'argon',
};

// constantMap[2] = 'Helium'; // This line will cause an error.

Maps support spread operators (... and ...?) and collection if and for, just like lists do. For details and examples, see the spread operator proposal and the control flow collections proposal.

For more information about maps, see Generics and Maps.

Runes and grapheme clusters

In Dart, runes expose the Unicode code points of a string. You can use the characters package to view or manipulate user-perceived characters, also known as Unicode (extended) grapheme clusters.

Unicode defines a unique numeric value for each letter, digit, and symbol used in all of the world’s writing systems. Because a Dart string is a sequence of UTF-16 code units, expressing Unicode code points within a string requires special syntax. The usual way to express a Unicode code point is \uXXXX, where XXXX is a 4-digit hexadecimal value. For example, the heart character (♥) is \u2665. To specify more or less than 4 hex digits, place the value in curly brackets. For example, the laughing emoji (😆) is \u{1f606}.

If you need to read or write individual Unicode characters, use the characters getter defined on String by the characters package. The returned Characters object is the string as a sequence of grapheme clusters. Here’s an example of using the characters API:

import 'package:characters/characters.dart';
...
var hi = 'Hi 🇩🇰';
print(hi);
print('The end of the string: ${hi.substring(hi.length - 1)}');
print('The last character: ${hi.characters.last}\n');

The output, depending on your environment, looks something like this:

$ dart bin/main.dart
Hi 🇩🇰
The end of the string: ???
The last character: 🇩🇰

For details on using the characters package to manipulate strings, see the example and API reference for the characters package.

Symbols

A Symbol object represents an operator or identifier declared in a Dart program. You might never need to use symbols, but they’re invaluable for APIs that refer to identifiers by name, because minification changes identifier names but not identifier symbols.

To get the symbol for an identifier, use a symbol literal, which is just # followed by the identifier:

#radix
#bar

Symbol literals are compile-time constants.

Functions

Dart is a true object-oriented language, so even functions are objects and have a type, Function. This means that functions can be assigned to variables or passed as arguments to other functions. You can also call an instance of a Dart class as if it were a function. For details, see Callable classes.

Here’s an example of implementing a function:

bool isNoble(int atomicNumber) {
  return _nobleGases[atomicNumber] != null;
}

Although Effective Dart recommends type annotations for public APIs, the function still works if you omit the types:

isNoble(atomicNumber) {
  return _nobleGases[atomicNumber] != null;
}

For functions that contain just one expression, you can use a shorthand syntax:

bool isNoble(int atomicNumber) => _nobleGases[atomicNumber] != null;

The => expr syntax is a shorthand for { return expr; }. The => notation is sometimes referred to as arrow syntax.

Parameters

A function can have any number of required positional parameters. These can be followed either by named parameters or by optional positional parameters (but not both).

You can use trailing commas when you pass arguments to a function or when you define function parameters.

Named parameters

Named parameters are optional unless they’re specifically marked as required.

When calling a function, you can specify named parameters using paramName: value. For example:

enableFlags(bold: true, hidden: false);

When defining a function, use {param1, param2, …} to specify named parameters:

/// Sets the [bold] and [hidden] flags ...
void enableFlags({bool bold, bool hidden}) {...}

Although named parameters are a kind of optional parameter, you can annotate them with @required to indicate that the parameter is mandatory — that users must provide a value for the parameter. For example:

const Scrollbar({Key key, @required Widget child})

If someone tries to create a Scrollbar without specifying the child argument, then the analyzer reports an issue.

To use the @required annotation, depend on the meta package and import package:meta/meta.dart.

Optional positional parameters

Wrapping a set of function parameters in [] marks them as optional positional parameters:

String say(String from, String msg, [String device]) {
  var result = '$from says $msg';
  if (device != null) {
    result = '$result with a $device';
  }
  return result;
}

Here’s an example of calling this function without the optional parameter:

assert(say('Bob', 'Howdy') == 'Bob says Howdy');

And here’s an example of calling this function with the third parameter:

assert(say('Bob', 'Howdy', 'smoke signal') ==
    'Bob says Howdy with a smoke signal');

Default parameter values

Your function can use = to define default values for both named and positional parameters. The default values must be compile-time constants. If no default value is provided, the default value is null.

Here’s an example of setting default values for named parameters:

/// Sets the [bold] and [hidden] flags ...
void enableFlags({bool bold = false, bool hidden = false}) {...}

// bold will be true; hidden will be false.
enableFlags(bold: true);

The next example shows how to set default values for positional parameters:

String say(String from, String msg,
    [String device = 'carrier pigeon']) {
  var result = '$from says $msg with a $device';
  return result;
}

assert(say('Bob', 'Howdy') ==
    'Bob says Howdy with a carrier pigeon');

You can also pass lists or maps as default values. The following example defines a function, doStuff(), that specifies a default list for the list parameter and a default map for the gifts parameter.

void doStuff(
    {List<int> list = const [1, 2, 3],
    Map<String, String> gifts = const {
      'first': 'paper',
      'second': 'cotton',
      'third': 'leather'
    }}) {
  print('list:  $list');
  print('gifts: $gifts');
}

The main() function

Every app must have a top-level main() function, which serves as the entrypoint to the app. The main() function returns void and has an optional List<String> parameter for arguments.

Here’s an example of the main() function for a web app:

void main() {
  querySelector('#sample_text_id')
    ..text = 'Click me!'
    ..onClick.listen(reverseText);
}

Here’s an example of the main() function for a command-line app that takes arguments:

// Run the app like this: dart args.dart 1 test
void main(List<String> arguments) {
  print(arguments);

  assert(arguments.length == 2);
  assert(int.parse(arguments[0]) == 1);
  assert(arguments[1] == 'test');
}

You can use the args library to define and parse command-line arguments.

Functions as first-class objects

You can pass a function as a parameter to another function. For example:

void printElement(int element) {
  print(element);
}

var list = [1, 2, 3];

// Pass printElement as a parameter.
list.forEach(printElement);

You can also assign a function to a variable, such as:

var loudify = (msg) => '!!! ${msg.toUpperCase()} !!!';
assert(loudify('hello') == '!!! HELLO !!!');

This example uses an anonymous function. More about those in the next section.

Anonymous functions

Most functions are named, such as main() or printElement(). You can also create a nameless function called an anonymous function, or sometimes a lambda or closure. You might assign an anonymous function to a variable so that, for example, you can add or remove it from a collection.

An anonymous function looks similar to a named function— zero or more parameters, separated by commas and optional type annotations, between parentheses.

The code block that follows contains the function’s body:

([[Type] param1[, …]]) {
  codeBlock;
};


The following example defines an anonymous function with an untyped parameter, item. The function, invoked for each item in the list, prints a string that includes the value at the specified index.

var list = ['apples', 'bananas', 'oranges'];
list.forEach((item) {
  print('${list.indexOf(item)}: $item');
});

Click Run to execute the code.

If the function contains only one return statement, you can shorten it using arrow notation. Paste the following line into DartPad and click Run to verify that it is functionally equivalent.

list.forEach(
    (item) => print('${list.indexOf(item)}: $item'));

Lexical scope

Dart is a lexically scoped language, which means that the scope of variables is determined statically, simply by the layout of the code. You can “follow the curly braces outwards” to see if a variable is in scope.

Here is an example of nested functions with variables at each scope level:

bool topLevel = true;

void main() {
  var insideMain = true;

  void myFunction() {
    var insideFunction = true;

    void nestedFunction() {
      var insideNestedFunction = true;

      assert(topLevel);
      assert(insideMain);
      assert(insideFunction);
      assert(insideNestedFunction);
    }
  }
}

Notice how nestedFunction() can use variables from every level, all the way up to the top level.

Lexical closures

A closure is a function object that has access to variables in its lexical scope, even when the function is used outside of its original scope.

Functions can close over variables defined in surrounding scopes. In the following example, makeAdder() captures the variable addBy. Wherever the returned function goes, it remembers addBy.

/// Returns a function that adds [addBy] to the
/// function's argument.
Function makeAdder(int addBy) {
  return (int i) => addBy + i;
}

void main() {
  // Create a function that adds 2.
  var add2 = makeAdder(2);

  // Create a function that adds 4.
  var add4 = makeAdder(4);

  assert(add2(3) == 5);
  assert(add4(3) == 7);
}

Testing functions for equality

Here’s an example of testing top-level functions, static methods, and instance methods for equality:

void foo() {} // A top-level function

class A {
  static void bar() {} // A static method
  void baz() {} // An instance method
}

void main() {
  var x;

  // Comparing top-level functions.
  x = foo;
  assert(foo == x);

  // Comparing static methods.
  x = A.bar;
  assert(A.bar == x);

  // Comparing instance methods.
  var v = A(); // Instance #1 of A
  var w = A(); // Instance #2 of A
  var y = w;
  x = w.baz;

  // These closures refer to the same instance (#2),
  // so they're equal.
  assert(y.baz == x);

  // These closures refer to different instances,
  // so they're unequal.
  assert(v.baz != w.baz);
}

Return values

All functions return a value. If no return value is specified, the statement return null; is implicitly appended to the function body.

foo() {}

assert(foo() == null);

Operators

Dart supports the operators shown in the following table. You can implement many of these operators as class members.

When you use operators, you create expressions. Here are some examples of operator expressions:

a++
a + b
a = b
a == b
c ? a : b
a is T

In the operator table, each operator has higher precedence than the operators in the rows that follow it. For example, the multiplicative operator % has higher precedence than (and thus executes before) the equality operator ==, which has higher precedence than the logical AND operator &&. That precedence means that the following two lines of code execute the same way:

// Parentheses improve readability.
if ((n % i == 0) && (d % i == 0)) ...

// Harder to read, but equivalent.
if (n % i == 0 && d % i == 0) ...

Arithmetic operators

Dart supports the usual arithmetic operators, as shown in the following table.

Example:

assert(2 + 3 == 5);
assert(2 - 3 == -1);
assert(2 * 3 == 6);
assert(5 / 2 == 2.5); // Result is a double
assert(5 ~/ 2 == 2); // Result is an int
assert(5 % 2 == 1); // Remainder

assert('5/2 = ${5 ~/ 2} r ${5 % 2}' == '5/2 = 2 r 1');

Dart also supports both prefix and postfix increment and decrement operators.

Example:

var a, b;

a = 0;
b = ++a; // Increment a before b gets its value.
assert(a == b); // 1 == 1

a = 0;
b = a++; // Increment a AFTER b gets its value.
assert(a != b); // 1 != 0

a = 0;
b = --a; // Decrement a before b gets its value.
assert(a == b); // -1 == -1

a = 0;
b = a--; // Decrement a AFTER b gets its value.
assert(a != b); // -1 != 0

Equality and relational operators

The following table lists the meanings of equality and relational operators.

To test whether two objects x and y represent the same thing, use the == operator. (In the rare case where you need to know whether two objects are the exact same object, use the identical() function instead.) Here’s how the == operator works:

1. If x or y is null, return true if both are null, and false if only one is null.

2. Return the result of the method invocation x.==(y). (That’s right, operators such as == are methods that are invoked on their first operand. For details, see Operators.)

Here’s an example of using each of the equality and relational operators:

assert(2 == 2);
assert(2 != 3);
assert(3 > 2);
assert(2 < 3);
assert(3 >= 3);
assert(2 <= 3);

Type test operators

The as, is, and is! operators are handy for checking types at runtime.

The result of obj is T is true if obj implements the interface specified by T. For example, obj is Object is always true.

Use the as operator to cast an object to a particular type if and only if you are sure that the object is of that type. Example:

(emp as Person).firstName = 'Bob';

If you aren’t sure that the object is of type T, then use is T to check the type before using the object.

if (emp is Person) {
  // Type check
  emp.firstName = 'Bob';
}

Assignment operators

As you’ve already seen, you can assign values using the = operator. To assign only if the assigned-to variable is null, use the ??= operator.

// Assign value to a
a = value;
// Assign value to b if b is null; otherwise, b stays the same
b ??= value;

Compound assignment operators such as += combine an operation with an assignment.

Here’s how compound assignment operators work:

The following example uses assignment and compound assignment operators:

var a = 2; // Assign using =
a *= 3; // Assign and multiply: a = a * 3
assert(a == 6);

Logical operators

You can invert or combine boolean expressions using the logical operators.

Here’s an example of using the logical operators:

if (!done && (col == 0 || col == 3)) {
  // ...Do something...
}

Bitwise and shift operators

You can manipulate the individual bits of numbers in Dart. Usually, you’d use these bitwise and shift operators with integers.

Here’s an example of using bitwise and shift operators:

final value = 0x22;
final bitmask = 0x0f;

assert((value & bitmask) == 0x02); // AND
assert((value & ~bitmask) == 0x20); // AND NOT
assert((value | bitmask) == 0x2f); // OR
assert((value ^ bitmask) == 0x2d); // XOR
assert((value << 4) == 0x220); // Shift left
assert((value >> 4) == 0x02); // Shift right

Conditional expressions

Dart has two operators that let you concisely evaluate expressions that might otherwise require if-else statements:

condition ? expr1 : expr2

If condition is true, evaluates expr1 (and returns its value); otherwise, evaluates and returns the value of expr2.

expr1 ?? expr2

If expr1 is non-null, returns its value; otherwise, evaluates and returns the value of expr2.

When you need to assign a value based on a boolean expression, consider using ?:.

var visibility = isPublic ? 'public' : 'private';

If the boolean expression tests for null, consider using ??.

String playerName(String name) => name ?? 'Guest';

The previous example could have been written at least two other ways, but not as succinctly:

// Slightly longer version uses ?: operator.
String playerName(String name) => name != null ? name : 'Guest';

// Very long version uses if-else statement.
String playerName(String name) {
  if (name != null) {
    return name;
  } else {
    return 'Guest';
  }
}

Cascade notation (..)

Cascades (..) allow you to make a sequence of operations on the same object. In addition to function calls, you can also access fields on that same object. This often saves you the step of creating a temporary variable and allows you to write more fluid code.

Consider the following code:

querySelector('#confirm') // Get an object.
  ..text = 'Confirm' // Use its members.
  ..classes.add('important')
  ..onClick.listen((e) => window.alert('Confirmed!'));

The first method call, querySelector(), returns a selector object. The code that follows the cascade notation operates on this selector object, ignoring any subsequent values that might be returned.

The previous example is equivalent to:

var button = querySelector('#confirm');
button.text = 'Confirm';
button.classes.add('important');
button.onClick.listen((e) => window.alert('Confirmed!'));

You can also nest your cascades. For example:

final addressBook = (AddressBookBuilder()
      ..name = 'jenny'
      ..email = 'jenny@example.com'
      ..phone = (PhoneNumberBuilder()
            ..number = '415-555-0100'
            ..label = 'home')
          .build())
    .build();

Be careful to construct your cascade on a function that returns an actual object. For example, the following code fails:

var sb = StringBuffer();
sb.write('foo')
  ..write('bar'); // Error: method 'write' isn't defined for 'void'.

The sb.write() call returns void, and you can’t construct a cascade on void.

Other operators

You’ve seen most of the remaining operators in other examples:

For more information about the ., ?., and .. operators, see Classes.

Control flow statements

You can control the flow of your Dart code using any of the following:

• if and else
• for loops
• while and do-while loops
• break and continue
• switch and case
• assert

You can also affect the control flow using try-catch and throw, as explained in Exceptions.

If and else

Dart supports if statements with optional else statements, as the next sample shows. Also see conditional expressions.

if (isRaining()) {
  you.bringRainCoat();
} else if (isSnowing()) {
  you.wearJacket();
} else {
  car.putTopDown();
}

Unlike JavaScript, conditions must use boolean values, nothing else. See Booleans for more information.

For loops

You can iterate with the standard for loop. For example:

var message = StringBuffer('Dart is fun');
for (var i = 0; i < 5; i++) {
  message.write('!');
}

Closures inside of Dart’s for loops capture the value of the index, avoiding a common pitfall found in JavaScript. For example, consider:

var callbacks = [];
for (var i = 0; i < 2; i++) {
  callbacks.add(() => print(i));
}
callbacks.forEach((c) => c());

The output is 0 and then 1, as expected. In contrast, the example would print 2 and then 2 in JavaScript.

If the object that you are iterating over is an Iterable, you can use the forEach() method. Using forEach() is a good option if you don’t need to know the current iteration counter:

candidates.forEach((candidate) => candidate.interview());

Iterable classes such as List and Set also support the for-in form of iteration:

var collection = [1, 2, 3];
for (var x in collection) {
  print(x); // 1 2 3
}

While and do-while

A while loop evaluates the condition before the loop:

while (!isDone()) {
  doSomething();
}

A do-while loop evaluates the condition after the loop:

do {
  printLine();
} while (!atEndOfPage());

Break and continue

Use break to stop looping:

while (true) {
  if (shutDownRequested()) break;
  processIncomingRequests();
}

Use continue to skip to the next loop iteration:

for (int i = 0; i < candidates.length; i++) {
  var candidate = candidates[i];
  if (candidate.yearsExperience < 5) {
    continue;
  }
  candidate.interview();
}

You might write that example differently if you’re using an Iterable such as a list or set:

candidates
    .where((c) => c.yearsExperience >= 5)
    .forEach((c) => c.interview());

Switch and case

Switch statements in Dart compare integer, string, or compile-time constants using ==. The compared objects must all be instances of the same class (and not of any of its subtypes), and the class must not override ==. Enumerated types work well in switch statements.

Each non-empty case clause ends with a break statement, as a rule. Other valid ways to end a non-empty case clause are a continue, throw, or return statement.

Use a default clause to execute code when no case clause matches:

var command = 'OPEN';
switch (command) {
  case 'CLOSED':
    executeClosed();
    break;
  case 'PENDING':
    executePending();
    break;
  case 'APPROVED':
    executeApproved();
    break;
  case 'DENIED':
    executeDenied();
    break;
  case 'OPEN':
    executeOpen();
    break;
  default:
    executeUnknown();
}

The following example omits the break statement in a case clause, thus generating an error:

var command = 'OPEN';
switch (command) {
  case 'OPEN':
    executeOpen();
    // ERROR: Missing break

  case 'CLOSED':
    executeClosed();
    break;
}

However, Dart does support empty case clauses, allowing a form of fall-through:

var command = 'CLOSED';
switch (command) {
  case 'CLOSED': // Empty case falls through.
  case 'NOW_CLOSED':
    // Runs for both CLOSED and NOW_CLOSED.
    executeNowClosed();
    break;
}

If you really want fall-through, you can use a continue statement and a label:

var command = 'CLOSED';
switch (command) {
  case 'CLOSED':
    executeClosed();
    continue nowClosed;
  // Continues executing at the nowClosed label.

  nowClosed:
  case 'NOW_CLOSED':
    // Runs for both CLOSED and NOW_CLOSED.
    executeNowClosed();
    break;
}

A case clause can have local variables, which are visible only inside the scope of that clause.

Assert

During development, use an assert statement — assert(condition, optionalMessage); — to disrupt normal execution if a boolean condition is false. You can find examples of assert statements throughout this tour. Here are some more:

// Make sure the variable has a non-null value.
assert(text != null);

// Make sure the value is less than 100.
assert(number < 100);

// Make sure this is an https URL.
assert(urlString.startsWith('https'));

To attach a message to an assertion, add a string as the second argument to assert (optionally with a trailing comma):

assert(urlString.startsWith('https'),
    'URL ($urlString) should start with "https".');

The first argument to assert can be any expression that resolves to a boolean value. If the expression’s value is true, the assertion succeeds and execution continues. If it’s false, the assertion fails and an exception (an AssertionError) is thrown.

When exactly do assertions work? That depends on the tools and framework you’re using:

• Flutter enables assertions in debug mode.
• Development-only tools such as dartdevc typically enable assertions by default.
• Some tools, such as dart and dart2js, support assertions through a command-line flag: --enable-asserts.

In production code, assertions are ignored, and the arguments to assert aren’t evaluated.

Exceptions

Your Dart code can throw and catch exceptions. Exceptions are errors 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.

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 exceptions.

Dart provides Exception and Error 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.

Throw

Here’s an example of throwing, or raising, an exception:

throw FormatException('Expected at least 1 section');

You can also throw arbitrary objects:

throw 'Out of llamas!';

Because throwing an exception is an expression, you can throw exceptions in => statements, as well as anywhere else that allows expressions:

void distanceTo(Point other) => throw UnimplementedError();

Catch

Catching, or capturing, an exception stops the exception from propagating (unless you rethrow the exception). Catching an exception gives you a chance to handle it:

try {
  breedMoreLlamas();
} on OutOfLlamasException {
  buyMoreLlamas();
}

To handle code that can throw more than one type of exception, you can specify multiple catch clauses. The first catch clause that matches the thrown object’s type handles the exception. If the catch clause does not specify a type, that clause can handle any type of thrown object:

try {
  breedMoreLlamas();
} on OutOfLlamasException {
  // A specific exception
  buyMoreLlamas();
} on Exception catch (e) {
  // Anything else that is an exception
  print('Unknown exception: $e');
} catch (e) {
  // No specified type, handles all
  print('Something really unknown: $e');
}

As the preceding code shows, you can use either on or catch or both. Use on when you need to specify the exception type. Use catch when your exception handler needs the exception object.

You can specify one or two parameters to catch(). The first is the exception that was thrown, and the second is the stack trace (a StackTrace object).

try {
  // ···
} on Exception catch (e) {
  print('Exception details:\n $e');
} catch (e, s) {
  print('Exception details:\n $e');
  print('Stack trace:\n $s');
}

To partially handle an exception, while allowing it to propagate, use the rethrow keyword.

void misbehave() {
  try {
    dynamic foo = true;
    print(foo++); // Runtime error
  } catch (e) {
    print('misbehave() partially handled ${e.runtimeType}.');
    rethrow; // Allow callers to see the exception.
  }
}

void main() {
  try {
    misbehave();
  } catch (e) {
    print('main() finished handling ${e.runtimeType}.');
  }
}

Finally

To ensure that some code runs whether or not an exception is thrown, use a finally clause. If no catch clause matches the exception, the exception is propagated after the finally clause runs:

try {
  breedMoreLlamas();
} finally {
  // Always clean up, even if an exception is thrown.
  cleanLlamaStalls();
}

The finally clause runs after any matching catch clauses:

try {
  breedMoreLlamas();
} catch (e) {
  print('Error: $e'); // Handle the exception first.
} finally {
  cleanLlamaStalls(); // Then clean up.
}

Learn more by reading the Exceptions section of the library tour.

Classes

Dart is an object-oriented language with classes and mixin-based inheritance. Every object is an instance of a class, and all classes descend from Object. Mixin-based inheritance means that although every class (except for Object) has exactly one superclass, a class body can be reused in multiple class hierarchies. Extension methods are a way to add functionality to a class without changing the class or creating a subclass.

Using class members

Objects have members consisting of functions and data (methods and instance variables, respectively). When you call a method, you invoke it on an object: the method has access to that object’s functions and data.

Use a dot (.) to refer to an instance variable or method:

var p = Point(2, 2);

// Get the value of y.
assert(p.y == 2);

// Invoke distanceTo() on p.
double distance = p.distanceTo(Point(4, 4));

Use ?. instead of . to avoid an exception when the leftmost operand is null:

// If p is non-null, set a variable equal to its y value.
var a = p?.y;

Using constructors

You can create an object using a constructor. Constructor names can be either ClassName or ClassName.identifier. For example, the following code creates Point objects using the Point() and Point.fromJson() constructors:

var p1 = Point(2, 2);
var p2 = Point.fromJson({'x': 1, 'y': 2});

The following code has the same effect, but uses the optional new keyword before the constructor name:

var p1 = new Point(2, 2);
var p2 = new Point.fromJson({'x': 1, 'y': 2});

Some classes provide constant constructors. To create a compile-time constant using a constant constructor, put the const keyword before the constructor name:

var p = const ImmutablePoint(2, 2);

Constructing two identical compile-time constants results in a single, canonical instance:

var a = const ImmutablePoint(1, 1);
var b = const ImmutablePoint(1, 1);

assert(identical(a, b)); // They are the same instance!

Within a constant context, you can omit the const before a constructor or literal. For example, look at this code, which creates a const map:

// Lots of const keywords here.
const pointAndLine = const {
  'point': const [const ImmutablePoint(0, 0)],
  'line': const [const ImmutablePoint(1, 10), const ImmutablePoint(-2, 11)],
};

You can omit all but the first use of the const keyword:

// Only one const, which establishes the constant context.
const pointAndLine = {
  'point': [ImmutablePoint(0, 0)],
  'line': [ImmutablePoint(1, 10), ImmutablePoint(-2, 11)],
};

If a constant constructor is outside of a constant context and is invoked without const, it creates a non-constant object:

var a = const ImmutablePoint(1, 1); // Creates a constant
var b = ImmutablePoint(1, 1); // Does NOT create a constant

assert(!identical(a, b)); // NOT the same instance!

Getting an object’s type

To get an object’s type at runtime, you can use Object’s runtimeType property, which returns a Type object.

print('The type of a is ${a.runtimeType}');

Up to here, you’ve seen how to use classes. The rest of this section shows how to implement classes.

Instance variables

Here’s how you declare instance variables:

class Point {
  double x; // Declare instance variable x, initially null.
  double y; // Declare y, initially null.
  double z = 0; // Declare z, initially 0.
}

All uninitialized instance variables have the value null.

All instance variables generate an implicit getter method. Non-final instance variables also generate an implicit setter method. For details, see Getters and setters.

class Point {
  double x;
  double y;
}

void main() {
  var point = Point();
  point.x = 4; // Use the setter method for x.
  assert(point.x == 4); // Use the getter method for x.
  assert(point.y == null); // Values default to null.
}

If you initialize an instance variable where it is declared (instead of in a constructor or method), the value is set when the instance is created, which is before the constructor and its initializer list execute.

Constructors

Declare a constructor by creating a function with the same name as its class (plus, optionally, an additional identifier as described in Named constructors). The most common form of constructor, the generative constructor, creates a new instance of a class:

class Point {
  double x, y;

  Point(double x, double y) {
    // There's a better way to do this, stay tuned.
    this.x = x;
    this.y = y;
  }
}

The this keyword refers to the current instance.

The pattern of assigning a constructor argument to an instance variable is so common, Dart has syntactic sugar to make it easy:

class Point {
  double x, y;

  // Syntactic sugar for setting x and y
  // before the constructor body runs.
  Point(this.x, this.y);
}

Default constructors

If you don’t declare a constructor, a default constructor is provided for you. The default constructor has no arguments and invokes the no-argument constructor in the superclass.

Constructors aren’t inherited

Subclasses don’t inherit constructors from their superclass. A subclass that declares no constructors has only the default (no argument, no name) constructor.

Named constructors

Use a named constructor to implement multiple constructors for a class or to provide extra clarity:

class Point {
  double x, y;

  Point(this.x, this.y);

  // Named constructor
  Point.origin()
      : x = 0,
        y = 0;
}

Remember that constructors are not inherited, which means that a superclass’s named constructor is not inherited by a subclass. If you want a subclass to be created with a named constructor defined in the superclass, you must implement that constructor in the subclass.

Invoking a non-default superclass constructor

By default, a constructor in a subclass calls the superclass’s unnamed, no-argument constructor. The superclass’s constructor is called at the beginning of the constructor body. If an initializer list is also being used, it executes before the superclass is called. In summary, the order of execution is as follows:

1. initializer list
2. superclass’s no-arg constructor
3. main class’s no-arg constructor

If the superclass doesn’t have an unnamed, no-argument constructor, then you must manually call one of the constructors in the superclass. Specify the superclass constructor after a colon (:), just before the constructor body (if any).

In the following example, the constructor for the Employee class calls the named constructor for its superclass, Person. Click Run to execute the code.

Because the arguments to the superclass constructor are evaluated before invoking the constructor, an argument can be an expression such as a function call:

class Employee extends Person {
  Employee() : super.fromJson(defaultData);
  // ···
}

Initializer list

Besides invoking a superclass constructor, you can also initialize instance variables before the constructor body runs. Separate initializers with commas.

// Initializer list sets instance variables before
// the constructor body runs.
Point.fromJson(Map<String, double> json)
    : x = json['x'],
      y = json['y'] {
  print('In Point.fromJson(): ($x, $y)');
}

During development, you can validate inputs by using assert in the initializer list.

Point.withAssert(this.x, this.y) : assert(x >= 0) {
  print('In Point.withAssert(): ($x, $y)');
}

Initializer lists are handy when setting up final fields. The following example initializes three final fields in an initializer list. Click Run to execute the code.

Redirecting constructors

Sometimes a constructor’s only purpose is to redirect to another constructor in the same class. A redirecting constructor’s body is empty, with the constructor call appearing after a colon (:).

class Point {
  double x, y;

  // The main constructor for this class.
  Point(this.x, this.y);

  // Delegates to the main constructor.
  Point.alongXAxis(double x) : this(x, 0);
}

Constant constructors

If your class produces objects that never change, you can make these objects compile-time constants. To do this, define a const constructor and make sure that all instance variables are final.

class ImmutablePoint {
  static const ImmutablePoint origin = ImmutablePoint(0, 0);

  final double x, y;

  const ImmutablePoint(this.x, this.y);
}

Constant constructors don’t always create constants. For details, see the section on using constructors.

Factory constructors

Use the factory keyword when implementing a constructor that doesn’t always create a new instance of its class. For example, a factory constructor might return an instance from a cache, or it might return an instance of a subtype. Another use case for factory constructors is initializing a final variable using logic that can’t be handled in the initializer list.

In the following example, the Logger factory constructor returns objects from a cache, and the Logger.fromJson factory constructor initializes a final variable from a JSON object.

class Logger {
  final String name;
  bool mute = false;

  // _cache is library-private, thanks to
  // the _ in front of its name.
  static final Map<String, Logger> _cache =
      <String, Logger>{};

  factory Logger(String name) {
    return _cache.putIfAbsent(
        name, () => Logger._internal(name));
  }

  factory Logger.fromJson(Map<String, Object> json) {
    return Logger(json['name'].toString());
  }

  Logger._internal(this.name);

  void log(String msg) {
    if (!mute) print(msg);
  }
}

Invoke a factory constructor just like you would any other constructor:

var logger = Logger('UI');
logger.log('Button clicked');

var logMap = {'name': 'UI'};
var loggerJson = Logger.fromJson(logMap);

Methods

Methods are functions that provide behavior for an object.

Instance methods

Instance methods on objects can access instance variables and this. The distanceTo() method in the following sample is an example of an instance method:

import 'dart:math';

class Point {
  double x, y;

  Point(this.x, this.y);

  double distanceTo(Point other) {
    var dx = x - other.x;
    var dy = y - other.y;
    return sqrt(dx * dx + dy * dy);
  }
}

Operators

Operators are instance methods with special names. Dart allows you to define operators with the following names:

An operator declaration is identified using the built-in identifier operator. The following example defines vector addition (+) and subtraction (-):

class Vector {
  final int x, y;

  Vector(this.x, this.y);

  Vector operator +(Vector v) => Vector(x + v.x, y + v.y);
  Vector operator -(Vector v) => Vector(x - v.x, y - v.y);

  // Operator == and hashCode not shown.
  // ···
}

void main() {
  final v = Vector(2, 3);
  final w = Vector(2, 2);

  assert(v + w == Vector(4, 5));
  assert(v - w == Vector(0, 1));
}

Getters and setters

Getters and setters are special methods that provide read and write access to an object’s properties. Recall that each instance variable has an implicit getter, plus a setter if appropriate. You can create additional properties by implementing getters and setters, using the get and set keywords:

class Rectangle {
  double left, top, width, height;

  Rectangle(this.left, this.top, this.width, this.height);

  // Define two calculated properties: right and bottom.
  double get right => left + width;
  set right(double value) => left = value - width;
  double get bottom => top + height;
  set bottom(double value) => top = value - height;
}

void main() {
  var rect = Rectangle(3, 4, 20, 15);
  assert(rect.left == 3);
  rect.right = 12;
  assert(rect.left == -8);
}

With getters and setters, you can start with instance variables, later wrapping them with methods, all without changing client code.

Abstract methods

Instance, getter, and setter methods can be abstract, defining an interface but leaving its implementation up to other classes. Abstract methods can only exist in abstract classes.

To make a method abstract, use a semicolon (;) instead of a method body:

abstract class Doer {
  // Define instance variables and methods...

  void doSomething(); // Define an abstract method.
}

class EffectiveDoer extends Doer {
  void doSomething() {
    // Provide an implementation, so the method is not abstract here...
  }
}

Abstract classes

Use the abstract modifier to define an abstract class—a class that can’t be instantiated. Abstract classes are useful for defining interfaces, often with some implementation. If you want your abstract class to appear to be instantiable, define a factory constructor.

Abstract classes often have abstract methods. Here’s an example of declaring an abstract class that has an abstract method:

// This class is declared abstract and thus
// can't be instantiated.
abstract class AbstractContainer {
  // Define constructors, fields, methods...

  void updateChildren(); // Abstract method.
}

Implicit interfaces

Every class implicitly defines an interface containing all the instance members of the class and of any interfaces it implements. If you want to create a class A that supports class B’s API without inheriting B’s implementation, class A should implement the B interface.

A class implements one or more interfaces by declaring them in an implements clause and then providing the APIs required by the interfaces. For example:

// A person. The implicit interface contains greet().
class Person {
  // In the interface, but visible only in this library.
  final _name;

  // Not in the interface, since this is a constructor.
  Person(this._name);

  // In the interface.
  String greet(String who) => 'Hello, $who. I am $_name.';
}

// An implementation of the Person interface.
class Impostor implements Person {
  get _name => '';

  String greet(String who) => 'Hi $who. Do you know who I am?';
}

String greetBob(Person person) => person.greet('Bob');

void main() {
  print(greetBob(Person('Kathy')));
  print(greetBob(Impostor()));
}

Here’s an example of specifying that a class implements multiple interfaces:

class Point implements Comparable, Location {...}

Extending a class

Use extends to create a subclass, and super to refer to the superclass:

class Television {
  void turnOn() {
    _illuminateDisplay();
    _activateIrSensor();
  }
  // ···
}

class SmartTelevision extends Television {
  void turnOn() {
    super.turnOn();
    _bootNetworkInterface();
    _initializeMemory();
    _upgradeApps();
  }
  // ···
}

Overriding members

Subclasses can override instance methods (including operators), getters, and setters. You can use the @override annotation to indicate that you are intentionally overriding a member:

class SmartTelevision extends Television {
  @override
  void turnOn() {...}
  // ···
}

To narrow the type of a method parameter or instance variable in code that is type safe, you can use the covariant keyword.

noSuchMethod()

To detect or react whenever code attempts to use a non-existent method or instance variable, you can override noSuchMethod():

class A {
  // Unless you override noSuchMethod, using a
  // non-existent member results in a NoSuchMethodError.
  @override
  void noSuchMethod(Invocation invocation) {
    print('You tried to use a non-existent member: ' +
        '${invocation.memberName}');
  }
}

You can’t invoke an unimplemented method unless one of the following is true:

• The receiver has the static type dynamic.

• The receiver has a static type that defines the unimplemented method (abstract is OK), and the dynamic type of the receiver has an implemention of noSuchMethod() that’s different from the one in class Object.

For more information, see the informal noSuchMethod forwarding specification.

Extension methods

Extension methods are a way to add functionality to existing libraries. You might use extension methods without even knowing it. For example, when you use code completion in an IDE, it suggests extension methods alongside regular methods.

Here’s an example of using an extension method on String named parseInt() that’s defined in string_apis.dart:

import 'string_apis.dart';
...
print('42'.padLeft(5)); // Use a String method.
print('42'.parseInt()); // Use an extension method.

For details of using and implementing extension methods, see the extension methods page.

Enumerated types

Enumerated types, often called enumerations or enums, are a special kind of class used to represent a fixed number of constant values.

Using enums

Declare an enumerated type using the enum keyword:

enum Color { red, green, blue }

You can use trailing commas when declaring an enumerated type.

Each value in an enum has an index getter, which returns the zero-based position of the value in the enum declaration. For example, the first value has index 0, and the second value has index 1.

assert(Color.red.index == 0);
assert(Color.green.index == 1);
assert(Color.blue.index == 2);

To get a list of all of the values in the enum, use the enum’s values constant.

List<Color> colors = Color.values;
assert(colors[2] == Color.blue);

You can use enums in switch statements, and you’ll get a warning if you don’t handle all of the enum’s values:

var aColor = Color.blue;

switch (aColor) {
  case Color.red:
    print('Red as roses!');
    break;
  case Color.green:
    print('Green as grass!');
    break;
  default: // Without this, you see a WARNING.
    print(aColor); // 'Color.blue'
}

Enumerated types have the following limits:

• You can’t subclass, mix in, or implement an enum.
• You can’t explicitly instantiate an enum.

For more information, see the Dart language specification.

Adding features to a class: mixins

Mixins are a way of reusing a class’s code in multiple class hierarchies.

To use a mixin, use the with keyword followed by one or more mixin names. The following example shows two classes that use mixins:

class Musician extends Performer with Musical {
  // ···
}

class Maestro extends Person
    with Musical, Aggressive, Demented {
  Maestro(String maestroName) {
    name = maestroName;
    canConduct = true;
  }
}

To implement a mixin, create a class that extends Object and declares no constructors. Unless you want your mixin to be usable as a regular class, use the mixin keyword instead of class. For example:

mixin Musical {
  bool canPlayPiano = false;
  bool canCompose = false;
  bool canConduct = false;

  void entertainMe() {
    if (canPlayPiano) {
      print('Playing piano');
    } else if (canConduct) {
      print('Waving hands');
    } else {
      print('Humming to self');
    }
  }
}

Sometimes you might want to restrict the types that can use a mixin. For example, the mixin might depend on being able to invoke a method that the mixin doesn’t define. As the following example shows, you can restrict a mixin’s use by using the on keyword to specify the required superclass:

class Musician {
  // ...
}
mixin MusicalPerformer on Musician {
  // ...
}
class SingerDancer extends Musician with MusicalPerformer {
  // ...
}

In the preceding code, only classes that extend or implement the Musician class can use the mixin MusicalPerformer. Because SingerDancer extends Musician, SingerDancer can mix in MusicalPerformer.

Class variables and methods

Use the static keyword to implement class-wide variables and methods.

Static variables

Static variables (class variables) are useful for class-wide state and constants:

class Queue {
  static const initialCapacity = 16;
  // ···
}

void main() {
  assert(Queue.initialCapacity == 16);
}

Static variables aren’t initialized until they’re used.

Static methods

Static methods (class methods) don’t operate on an instance, and thus don’t have access to this. They do, however, have access to static variables. As the following example shows, you invoke static methods directly on a class:

import 'dart:math';

class Point {
  double x, y;
  Point(this.x, this.y);

  static double distanceBetween(Point a, Point b) {
    var dx = a.x - b.x;
    var dy = a.y - b.y;
    return sqrt(dx * dx + dy * dy);
  }
}

void main() {
  var a = Point(2, 2);
  var b = Point(4, 4);
  var distance = Point.distanceBetween(a, b);
  assert(2.8 < distance && distance < 2.9);
  print(distance);
}

You can use static methods as compile-time constants. For example, you can pass a static method as a parameter to a constant constructor.

Generics

If you look at the API documentation for the basic array type, List, you’ll see that the type is actually List<E>. The <…> notation marks List as a generic (or parameterized) type—a type that has formal type parameters. By convention, most type variables have single-letter names, such as E, T, S, K, and V.

Why use generics?

Generics are often required for type safety, but they have more benefits than just allowing your code to run:

• Properly specifying generic types results in better generated code.
• You can use generics to reduce code duplication.

If you intend for a list to contain only strings, you can declare it as List<String> (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:

var names = List<String>();
names.addAll(['Seth', 'Kathy', 'Lars']);
names.add(42); // Error

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:

abstract class ObjectCache {
  Object getByKey(String key);
  void setByKey(String key, Object value);
}

You discover that you want a string-specific version of this interface, so you create another interface:

abstract class StringCache {
  String getByKey(String key);
  void setByKey(String key, String value);
}

Later, you decide you want a number-specific version of this interface… You get the idea.

Generic types can save you the trouble of creating all these interfaces. Instead, you can create a single interface that takes a type parameter:

abstract class Cache<T> {
  T getByKey(String key);
  void setByKey(String key, T value);
}

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.

Using collection literals

List, set, and map literals can be parameterized. Parameterized literals are just like the literals you’ve already seen, except that you add <type> (for lists and sets) or <keyType, valueType> (for maps) before the opening bracket. Here is an example of using typed literals:

var names = <String>['Seth', 'Kathy', 'Lars'];
var uniqueNames = <String>{'Seth', 'Kathy', 'Lars'};
var pages = <String, String>{
  'index.html': 'Homepage',
  'robots.txt': 'Hints for web robots',
  'humans.txt': 'We are people, not machines'
};

Using parameterized types with constructors

To specify one or more types when using a constructor, put the types in angle brackets (<...>) just after the class name. For example:

var nameSet = Set<String>.from(names);

The following code creates a map that has integer keys and values of type View:

var views = Map<int, View>();

Generic collections and the types they contain

Dart generic types are reified, which means that they carry their type information around at runtime. For example, you can test the type of a collection:

var names = List<String>();
names.addAll(['Seth', 'Kathy', 'Lars']);
print(names is List<String>); // true

Restricting the parameterized type

When implementing a generic type, you might want to limit the types of its parameters. You can do this using extends.

class Foo<T extends SomeBaseClass> {
  // Implementation goes here...
  String toString() => "Instance of 'Foo<$T>'";
}

class Extender extends SomeBaseClass {...}

It’s OK to use SomeBaseClass or any of its subclasses as generic argument:

var someBaseClassFoo = Foo<SomeBaseClass>();
var extenderFoo = Foo<Extender>();

It’s also OK to specify no generic argument:

var foo = Foo();
print(foo); // Instance of 'Foo<SomeBaseClass>'

Specifying any non-SomeBaseClass type results in an error:

var foo = Foo<Object>();

Using generic methods

Initially, Dart’s generic support was limited to classes. A newer syntax, called generic methods, allows type arguments on methods and functions:

T first<T>(List<T> ts) {
  // Do some initial work or error checking, then...
  T tmp = ts[0];
  // Do some additional checking or processing...
  return tmp;
}

Here the generic type parameter on first (<T>) allows you to use the type argument T in several places:

• In the function’s return type (T).
• In the type of an argument (List<T>).
• In the type of a local variable (T tmp).

Libraries and visibility

The import and library directives can help you create a modular and shareable code base. Libraries not only provide APIs, but are a unit of privacy: identifiers that start with an underscore (_) are visible only inside the library. Every Dart app is a library, even if it doesn’t use a library directive.

Libraries can be distributed using packages.

Using libraries

Use import to specify how a namespace from one library is used in the scope of another library.

For example, Dart web apps generally use the dart:html library, which they can import like this:

import 'dart:html';

The only required argument to import is a URI specifying the library. For built-in libraries, the URI has the special dart: scheme. For other libraries, you can use a file system path or the package: scheme. The package: scheme specifies libraries provided by a package manager such as the pub tool. For example:

import 'package:test/test.dart';

Specifying a library prefix

If you import two libraries that have conflicting identifiers, then you can specify a prefix for one or both libraries. For example, if library1 and library2 both have an Element class, then you might have code like this:

import 'package:lib1/lib1.dart';
import 'package:lib2/lib2.dart' as lib2;

// Uses Element from lib1.
Element element1 = Element();

// Uses Element from lib2.
lib2.Element element2 = lib2.Element();

Importing only part of a library

If you want to use only part of a library, you can selectively import the library. For example:

// Import only foo.
import 'package:lib1/lib1.dart' show foo;

// Import all names EXCEPT foo.
import 'package:lib2/lib2.dart' hide foo;

Lazily loading a library

Deferred loading (also called lazy loading) allows a web app to load a library on demand, if and when the library is needed. Here are some cases when you might use deferred loading:

• To reduce a web app’s initial startup time.
• To perform A/B testing—trying out alternative implementations of an algorithm, for example.
• To load rarely used functionality, such as optional screens and dialogs.

To lazily load a library, you must first import it using deferred as.

import 'package:greetings/hello.dart' deferred as hello;

When you need the library, invoke loadLibrary() using the library’s identifier.

Future greet() async {
  await hello.loadLibrary();
  hello.printGreeting();
}

In the preceding code, the await keyword pauses execution until the library is loaded. For more information about async and await, see asynchrony support.

You can invoke loadLibrary() multiple times on a library without problems. The library is loaded only once.

Keep in mind the following when you use deferred loading:

• A deferred library’s constants aren’t constants in the importing file. Remember, these constants don’t exist until the deferred library is loaded.
• You can’t use types from a deferred library in the importing file. Instead, consider moving interface types to a library imported by both the deferred library and the importing file.
• Dart implicitly inserts loadLibrary() into the namespace that you define using deferred as namespace. The loadLibrary() function returns a Future.

Implementing libraries

See Create Library Packages for advice on how to implement a library package, including:

• How to organize library source code.
• How to use the export directive.
• When to use the part directive.
• When to use the library directive.
• How to use conditional imports and exports to implement a library that supports multiple platforms.

Asynchrony support

Dart libraries are full of functions that return Future or Stream objects. These functions are asynchronous: they return after setting up a possibly time-consuming operation (such as I/O), without waiting for that operation to complete.

The async and await keywords support asynchronous programming, letting you write asynchronous code that looks similar to synchronous code.

Handling Futures

When you need the result of a completed Future, you have two options:

• Use async and await.
• Use the Future API, as described in the library tour.

Code that uses async and await is asynchronous, but it looks a lot like synchronous code. For example, here’s some code that uses await to wait for the result of an asynchronous function:

await lookUpVersion();

To use await, code must be in an async function—a function marked as async:

Future checkVersion() async {
  var version = await lookUpVersion();
  // Do something with version
}

Use try, catch, and finally to handle errors and cleanup in code that uses await:

try {
  version = await lookUpVersion();
} catch (e) {
  // React to inability to look up the version
}

You can use await multiple times in an async function. For example, the following code waits three times for the results of functions:

var entrypoint = await findEntrypoint();
var exitCode = await runExecutable(entrypoint, args);
await flushThenExit(exitCode);

In await expression, the value of expression is usually a Future; if it isn’t, then the value is automatically wrapped in a Future. This Future object indicates a promise to return an object. The value of await expression is that returned object. The await expression makes execution pause until that object is available.

If you get a compile-time error when using await, make sure await is in an async function. For example, to use await in your app’s main() function, the body of main() must be marked as async:

Future main() async {
  checkVersion();
  print('In main: version is ${await lookUpVersion()}');
}

Declaring async functions

An async function is a function whose body is marked with the async modifier.

Adding the async keyword to a function makes it return a Future. For example, consider this synchronous function, which returns a String:

String lookUpVersion() => '1.0.0';

If you change it to be an async function—for example, because a future implementation will be time consuming—the returned value is a Future:

Future<String> lookUpVersion() async => '1.0.0';

Note that the function’s body doesn’t need to use the Future API. Dart creates the Future object if necessary. If your function doesn’t return a useful value, make its return type Future<void>.

For an interactive introduction to using futures, async, and await, see the asynchronous programming codelab.

Handling Streams

When you need to get values from a Stream, you have two options:

• Use async and an asynchronous for loop (await for).
• Use the Stream API, as described in the library tour.

An asynchronous for loop has the following form:

await for (varOrType identifier in expression) {
  // Executes each time the stream emits a value.
}

The value of expression must have type Stream. Execution proceeds as follows:

1. Wait until the stream emits a value.
2. Execute the body of the for loop, with the variable set to that emitted value.
3. Repeat 1 and 2 until the stream is closed.

To stop listening to the stream, you can use a break or return statement, which breaks out of the for loop and unsubscribes from the stream.

If you get a compile-time error when implementing an asynchronous for loop, make sure the await for is in an async function. For example, to use an asynchronous for loop in your app’s main() function, the body of main() must be marked as async:

Future main() async {
  // ...
  await for (var request in requestServer) {
    handleRequest(request);
  }
  // ...
}

For more information about asynchronous programming, in general, see the dart:async section of the library tour.

Generators

When you need to lazily produce a sequence of values, consider using a generator function. Dart has built-in support for two kinds of generator functions:

• Synchronous generator: Returns an Iterable object.
• Asynchronous generator: Returns a Stream object.

To implement a synchronous generator function, mark the function body as sync*, and use yield statements to deliver values:

Iterable<int> naturalsTo(int n) sync* {
  int k = 0;
  while (k < n) yield k++;
}

To implement an asynchronous generator function, mark the function body as async*, and use yield statements to deliver values:

Stream<int> asynchronousNaturalsTo(int n) async* {
  int k = 0;
  while (k < n) yield k++;
}

If your generator is recursive, you can improve its performance by using yield*:

Iterable<int> naturalsDownFrom(int n) sync* {
  if (n > 0) {
    yield n;
    yield* naturalsDownFrom(n - 1);
  }
}

Callable classes

To allow an instance of your Dart class to be called like a function, implement the call() method.

In the following example, the WannabeFunction class defines a call() function that takes three strings and concatenates them, separating each with a space, and appending an exclamation. Click Run to execute the code.

Isolates

Most computers, even on mobile platforms, have multi-core CPUs. To take advantage of all those cores, developers traditionally use shared-memory threads running concurrently. However, shared-state concurrency is error prone and can lead to complicated code.

Instead of threads, all Dart code runs inside of isolates. Each isolate has its own memory heap, ensuring that no isolate’s state is accessible from any other isolate.

For more information, see the following:

• Dart asynchronous programming: Isolates and event loops
• dart:isolate API reference, including Isolate.spawn() and TransferableTypedData
• Background parsing cookbook on the Flutter site
• Isolate sample app

Typedefs

In Dart, functions are objects, just like strings and numbers are objects. A typedef, or function-type alias, gives a function type a name that you can use when declaring fields and return types. A typedef retains type information when a function type is assigned to a variable.

Consider the following code, which doesn’t use a typedef:

class SortedCollection {
  Function compare;

  SortedCollection(int f(Object a, Object b)) {
    compare = f;
  }
}

// Initial, broken implementation.
int sort(Object a, Object b) => 0;

void main() {
  SortedCollection coll = SortedCollection(sort);

  // All we know is that compare is a function,
  // but what type of function?
  assert(coll.compare is Function);
}

Type information is lost when assigning f to compare. The type of f is (Object, Object) → int (where → means returns), yet the type of compare is Function. If we change the code to use explicit names and retain type information, both developers and tools can use that information.

typedef Compare = int Function(Object a, Object b);

class SortedCollection {
  Compare compare;

  SortedCollection(this.compare);
}

// Initial, broken implementation.
int sort(Object a, Object b) => 0;

void main() {
  SortedCollection coll = SortedCollection(sort);
  assert(coll.compare is Function);
  assert(coll.compare is Compare);
}

Because typedefs are simply aliases, they offer a way to check the type of any function. For example:

typedef Compare<T> = int Function(T a, T b);

int sort(int a, int b) => a - b;

void main() {
  assert(sort is Compare<int>); // True!
}

Metadata

Use metadata to give additional information about your code. A metadata annotation begins with the character @, followed by either a reference to a compile-time constant (such as deprecated) or a call to a constant constructor.

Two annotations are available to all Dart code: @deprecated and @override. For examples of using @override, see Extending a class. Here’s an example of using the @deprecated annotation:

class Television {
  /// _Deprecated: Use [turnOn] instead._
  @deprecated
  void activate() {
    turnOn();
  }

  /// Turns the TV's power on.
  void turnOn() {...}
}

You can define your own metadata annotations. Here’s an example of defining a @todo annotation that takes two arguments:

library todo;

class Todo {
  final String who;
  final String what;

  const Todo(this.who, this.what);
}

And here’s an example of using that @todo annotation:

import 'todo.dart';

@Todo('seth', 'make this do something')
void doSomething() {
  print('do something');
}

Metadata can appear before a library, class, typedef, type parameter, constructor, factory, function, field, parameter, or variable declaration and before an import or export directive. You can retrieve metadata at runtime using reflection.

Comments

Dart supports single-line comments, multi-line comments, and documentation comments.

Single-line comments

A single-line comment begins with //. Everything between // and the end of line is ignored by the Dart compiler.

void main() {
  // TODO: refactor into an AbstractLlamaGreetingFactory?
  print('Welcome to my Llama farm!');
}

Multi-line comments

A multi-line comment begins with /* and ends with */. Everything between /* and */ is ignored by the Dart compiler (unless the comment is a documentation comment; see the next section). Multi-line comments can nest.

void main() {
  /*
   * This is a lot of work. Consider raising chickens.

  Llama larry = Llama();
  larry.feed();
  larry.exercise();
  larry.clean();
   */
}

Documentation comments

Documentation comments are multi-line or single-line comments that begin with /// or /**. Using /// on consecutive lines has the same effect as a multi-line doc comment.

Inside a documentation comment, the Dart compiler ignores all text unless it is enclosed in brackets. Using brackets, you can refer to classes, methods, fields, top-level variables, functions, and parameters. The names in brackets are resolved in the lexical scope of the documented program element.

Here is an example of documentation comments with references to other classes and arguments:

/// A domesticated South American camelid (Lama glama).
///
/// Andean cultures have used llamas as meat and pack
/// animals since pre-Hispanic times.
class Llama {
  String name;

  /// Feeds your llama [Food].
  ///
  /// The typical llama eats one bale of hay per week.
  void feed(Food food) {
    // ...
  }

  /// Exercises your llama with an [activity] for
  /// [timeLimit] minutes.
  void exercise(Activity activity, int timeLimit) {
    // ...
  }
}

In the generated documentation, [Food] becomes a link to the API docs for the Food class.

To parse Dart code and generate HTML documentation, you can use the SDK’s documentation generation tool. For an example of generated documentation, see the Dart API documentation. For advice on how to structure your comments, see Guidelines for Dart Doc Comments.

Summary

This page 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 Dart language specification and Effective Dart.

To learn more about Dart’s core libraries, see A Tour of the Dart Libraries.