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.
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.
Tip: You can play with most of Dart’s language features using DartPad (learn more).
A basic Dart program
The following code uses many of Dart’s most basic features:
// Define a function. printInteger(int aNumber) { print('The number is $aNumber.'); // Print to console. } // This is where the app starts executing. 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.
int
-
A type. Some of the other built-in types are
String
,List
, andbool
. 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.
Note: This site’s code follows the conventions in the Dart style guide.
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 typeint
. When you want to explicitly say that no type is expected, use the special typedynamic
. -
Dart supports generic types, like
List<int>
(a list of integers) orList<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
, andprivate
. 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 ofexpr1
orexpr2
. 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.
| abstract 2 | dynamic 2 | implements 2 | show 1 |
| as 2 | else | import 2 | static 2 |
| assert | enum | in | super |
| async 1 | export 2 | interface 2 | switch |
| await 3 | extends | is | sync 1 |
| break | external 2 | library 2 | this |
| case | factory 2 | mixin 2 | throw |
| catch | false | new | true |
| class | final | null | try |
| const | finally | on 1 | typedef 2 |
| continue | for | operator 2 | var |
| covariant 2 | Function 2 | part 2 | void |
| default | get 2 | rethrow | while |
| deferred 2 | hide 1 | return | with |
| do | if | set 2 | yield 3 |
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 newer, limited reserved words related to the asynchrony support that was added after Dart’s 1.0 release. You can’t use
await
oryield
as an identifier in any function body marked withasync
,async*
, orsync*
.
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';
Note:
This page follows the
style guide recommendation
of using var
, rather than type annotations, for local variables.
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);
Note:
Production code ignores the assert()
call.
During development, on the other hand,
assert(condition)
throws an exception if
condition is false.
For details, see Assert.
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.
Note:
Instance variables can be final
but not const
.
Final instance variables must be initialized before
the constructor body starts —
at the variable declaration, by a constructor parameter,
or in the constructor’s initializer list.
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.
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;
As of Dart 2.1, 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!');
Note:
The ==
operator tests whether two objects are equivalent. Two
strings are equivalent if they contain the same sequence of code
units.
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 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];
Lists use zero-based indexing, where 0 is the index of the first element
and list.length - 1
is the index of the last element. You can get a
list’s length and refer to list elements 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; // Uncommenting this causes an error.
Dart 2.3 introduced the spread operator (...
) and the
null-aware spread operator (...?
),
which provide a concise way to insert multiple elements into a collection.
For example, you can use the spread operator (...
) to insert
all the elements 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 2.3 also introduced 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'); // Uncommenting this causes an error.
As of Dart 2.3, 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'; // Uncommenting this causes an error.
As of Dart 2.3, 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
In Dart, runes are the UTF-32 code points of a string.
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 32-bit Unicode values 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{1f600}
.
The String
class has several properties you can use to extract rune information.
The codeUnitAt
and codeUnit
properties return 16-bit code
units. Use the runes
property to get the runes of a string.
The following example illustrates the relationship between runes, 16-bit code units, and 32-bit code points. Click Run to see runes in action.
Note: Be careful when manipulating runes using list operations. This approach can easily break down, depending on the particular language, character set, and operation. For more information, see How do I reverse a String in Dart? on Stack Overflow.
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.
A function can have two types of parameters: required and optional. The required parameters are listed first, followed by any optional parameters. Optional parameters can be named or positional.
Optional parameters
Optional parameters can be either named or positional, but not both.
Named parameters
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
.
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 $device'; } return result; }
Here’s an example of calling this function without the optional parameter:
assert(say('Bob', 'Howdy') == 'Bob says Howdy');
And here’s an example of calling this function with the third parameter:
assert(say('Bob', 'Howdy', 'smoke signal') == 'Bob says Howdy with a smoke signal');
Default parameter values
Your function can use =
to define default values for both named and positional
parameters. The default values must be compile-time constants.
If no default value is provided, the default value is null
.
Here’s an example of setting default values for named parameters:
/// Sets the [bold] and [hidden] flags ... void enableFlags({bool bold = false, bool hidden = false}) {...} // bold will be true; hidden will be false. enableFlags(bold: true);
Deprecation note:
Old code might use a colon (:
) instead of =
to set default values of named parameters.
The reason is that originally, only :
was supported for named parameters.
That support might be deprecated,
so we recommend that you
use =
to specify default values.
The next example shows how to set default values for positional parameters:
String say(String from, String msg, [String device = 'carrier pigeon', String mood]) { var result = '$from says $msg'; if (device != null) { result = '$result with a $device'; } if (mood != null) { result = '$result (in a $mood mood)'; } 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); }
Note:
The ..
syntax in the preceding code is called a cascade.
With cascades,
you can perform multiple operations on the members of a single object.
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)}: $item'); });
Click Run to execute the code.
If the function contains only one statement, you can shorten it using arrow notation. Paste the following line into DartPad and click Run to verify that it is functionally equivalent.
list.forEach( (item) => print('${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(num addBy) { return (num 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 defines the operators shown in the following table. You can override many of these operators, as described in Overridable operators.
| Description | Operator |
|---|---|
| unary postfix |
expr++ expr-- () [] . ?.
|
| unary prefix |
-expr !expr ~expr ++expr --expr |
| multiplicative |
* / % ~/
|
| additive |
+ -
|
| shift |
<< >> >>>
|
| bitwise AND | & |
| bitwise XOR | ^ |
| bitwise OR | | |
| relational and type test |
>= > <= < as is is!
|
| equality |
== != |
| logical AND | && |
| logical OR | || |
| if null | ?? |
| conditional | expr1 ? expr2 : expr3 |
| cascade | .. |
| assignment |
= *= /= += -= &= ^= etc.
|
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) ...
Warning:
For operators that work on two operands, the leftmost operand
determines which version of the operator is used. For example, if you
have a Vector object and a Point object, aVector + aPoint
uses the
Vector version of +.
Arithmetic operators
Dart supports the usual arithmetic operators, as shown in the following table.
| Operator | Meaning |
|---|---|
+ |
Add |
– |
Subtract |
-expr |
Unary minus, also known as negation (reverse the sign of the expression) |
* |
Multiply |
/ |
Divide |
~/ |
Divide, returning an integer result |
% |
Get the remainder of an integer division (modulo) |
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.
| Operator | Meaning |
|---|---|
++var |
var = var + 1 (expression value is var + 1 ) |
var++ |
var = var + 1 (expression value is var ) |
--var |
var = var – 1 (expression value is var – 1 ) |
var-- |
var = var – 1 (expression value is var ) |
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.
| Operator | Meaning |
|---|---|
== |
Equal; see discussion below |
!= |
Not equal |
> |
Greater than |
< |
Less than |
>= |
Greater than or equal to |
<= |
Less than or equal to |
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:
-
If x or y is null, return true if both are null, and false if only one is null.
-
Return the result of the method invocation
x.==(y)
. (That’s right, operators such as==
are methods that are invoked on their first operand. You can even override many operators, including==
, as you’ll see in Overridable 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.
| Operator | Meaning |
|---|---|
as |
Typecast (also used to specify library prefixes) |
is |
True if the object has the specified type |
is! |
False if the object has the specified type |
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. In
general, you should use it as a shorthand for an is
test on an object
following by an expression using that object. For example, consider the
following code:
if (emp is Person) { // Type check emp.firstName = 'Bob'; }
You can make the code shorter using the as
operator:
(emp as Person).firstName = 'Bob';
Note:
The code isn’t equivalent. If emp
is null or not a Person, the
first example (with is
) does nothing; the second (with as
) throws
an exception.
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:
| Compound assignment | Equivalent expression | |
|---|---|---|
| For an operator op: | a op= b |
a = a op b |
| Example: | a += b |
a = a + b |
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.
| Operator | Meaning |
|---|---|
!expr |
inverts the following expression (changes false to true, and vice versa) |
|| |
logical OR |
&& |
logical AND |
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.
| Operator | Meaning |
|---|---|
& |
AND |
| |
OR |
^ |
XOR |
~expr |
Unary bitwise complement (0s become 1s; 1s become 0s) |
<< |
Shift left |
>> |
Shift right |
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 =<EMAIL_ADDRESS>..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
.
Note: Strictly speaking, the “double dot” notation for cascades is not an operator. It’s just part of the Dart syntax.
Other operators
You’ve seen most of the remaining operators in other examples:
| Operator | Name | Meaning |
|---|---|---|
() |
Function application | Represents a function call |
[] |
List access | Refers to the value at the specified index in the list |
. |
Member access | Refers to a property of an expression; example: foo.bar selects property bar from expression foo
|
?. |
Conditional member access | Like . , but the leftmost operand can be null; example: foo?.bar selects property bar from expression foo unless foo is null (in which case the value of foo?.bar is null) |
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
andelse
-
for
loops -
while
anddo
-while
loops -
break
andcontinue
-
switch
andcase
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 = [0, 1, 2]; for (var x in collection) { print(x); // 0 1 2 }
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.
Note: Switch statements in Dart are intended for limited circumstances, such as in interpreters or scanners.
Each non-empty case
clause ends with a break
statement, as a rule.

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
.
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!';
 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.  }
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.
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); // Set the value of the instance variable y. p.y = 3; // Get the value of y. assert(p.y == 3); // Invoke distanceTo() on p. num 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 its y value to 4. p?.y = 4;
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 { num x; // Declare instance variable x, initially null. num y; // Declare y, initially null. num 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 { num x; num 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 { num x, y; Point(num x, num 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.
Note:
Use this
only when there is a name conflict. Otherwise, Dart style
omits the this
.
The pattern of assigning a constructor argument to an instance variable is so common, Dart has syntactic sugar to make it easy:
class Point { num 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 { num 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:
- initializer list
- superclass’s no-arg constructor
- 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(getDefaultData()); // ··· }
Warning:
Arguments to the superclass constructor do not have access to this
.
For example, arguments can call static methods but not instance methods.
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, num> json) : x = json['x'], y = json['y'] { print('In Point.fromJson(): ($x, $y)'); }
Warning:
The right-hand side of an initializer does not have access to this
.
During development, you can validate inputs by using assert
in the
initializer list.
Point.withAssert(this.x, this.y) : assert(x >= 0) { print('In Point.withAssert(): ($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 { num x, y; // The main constructor for this class. Point(this.x, this.y); // Delegates to the main constructor. Point.alongXAxis(num 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 final ImmutablePoint origin = const ImmutablePoint(0, 0); final num 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.
The following example demonstrates a factory constructor returning objects from a cache:
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)); } Logger._internal(this.name); void log(String msg) { if (!mute) print(msg); } }
Note:
Factory constructors have no access to this
.
Invoke a factory constructor just like you would any other constructor:
var logger = Logger('UI'); logger.log('Button clicked');
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 { num x, y; Point(this.x, this.y); num distanceTo(Point other) { var dx = x - other.x; var dy = y - other.y; return sqrt(dx * dx + dy * dy); } }
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 { num left, top, width, height; Rectangle(this.left, this.top, this.width, this.height); // Define two calculated properties: right and bottom. num get right => left + width; set right(num value) => left = value - width; num get bottom => top + height; set bottom(num 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.
Note: Operators such as increment (++) work in the expected way, whether or not a getter is explicitly defined. To avoid any unexpected side effects, the operator calls the getter exactly once, saving its value in a temporary variable.
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, 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.
Overridable operators
You can override the operators shown in the following table.
For example, if you define a
Vector class, you might define a +
method to add two vectors.
< |
+ |
| |
[] |
> |
/ |
^ |
[]= |
<= |
~/ |
& |
~ |
>= |
* |
<< |
== |
– |
% |
>> |
Note: You may have noticed that !=
is not an overridable operator.
The expression e1 != e2
is just syntactic sugar for !(e1 == e2)
.
Here’s an example of a class that overrides the +
and -
operators:
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. For details, see note below. // ··· } void main() { final v = Vector(2, 3); final w = Vector(2, 2); assert(v + w == Vector(4, 5)); assert(v - w == Vector(0, 1)); }
If you override ==
, you should also override Object’s hashCode
getter.
For an example of overriding ==
and hashCode
, see
Implementing map keys.
For more information on overriding, in general, see Extending a class.
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 classObject
.
For more information, see the informal noSuchMethod forwarding specification.
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 }
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'); } } }
To specify that only certain types can use the mixin — for example,
so your mixin can invoke a method that it doesn’t define —
use on
to specify the required superclass:
mixin MusicalPerformer on Musician { // ··· }
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.
Note:
This page follows the style guide
recommendation
of preferring lowerCamelCase
for constant names.
Static methods
Static methods (class methods) do not operate on an instance, and thus
do not have access to this
. For example:
import 'dart:math'; class Point { num x, y; Point(this.x, this.y); static num 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); }
Note: Consider using top-level functions, instead of static methods, for common or widely used utilities and functionality.
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.  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 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
Note:
In contrast, generics in Java use erasure, which means that generic
type parameters are removed at runtime. In Java, you can test whether
an object is a List, but you can’t test whether it’s a List<String>
.
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
).
For more information about generics, see Using Generic Methods.
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';
Note: URI stands for uniform resource identifier. URLs (uniform resource locators) are a common kind of URI.
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 usingdeferred as namespace
. TheloadLibrary()
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.
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
andawait
. - 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:
- Wait until the stream emits a value.
- Execute the body of the for loop, with the variable set to that emitted value.
- 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:
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
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.


Video di presentazione
Video di presentazione
Dal team-building al sostegno delle famiglie dei dipendenti
Scopri comeSome quick example text to build on the card title and make up the bulk of the card's content.
Some quick example text to build on the card title and make up the bulk of the card's content.
Some quick example text to build on the card title and make up the bulk of the card's content.


US Man Warns of 'Russian Mind Controlling Devices' in Americans' Ears, Turns Himself In to FBI
An FBI spokesperson explained that the man who painted this message about Russia allegedly controlling people via micro devices inserted into their ears appears to be suffering from some kind of mental illness.
A man who recently left a car with a cryptic warning painted on it next to the FBI office in Pittsburgh has finally turned himself in, according to local NBC affiliate WPXI.
The message painted on the man’s car said: "Russia plants micro radio devices in ppls ears and controls them using a sound similar to a dog whistle. Subliminal hypnosis."
While the vehicle was examined by a bomb squad, and with FBI agents and Pittsburgh police moved to investigate this incident, an FBI spokesperson explained that the man turned himself in voluntarily and appears to be suffering from "an unknown mental illness."
News of this development, perhaps predictably, seemed to amuse a number of social media users who joked about the contents of the "mind control" warning.


Arrest Made In Drive-By Shooting Murder Of Dutch Footballer
by SL InternationalAuthorities confirmed on Thursday that a suspect has been arrested in connection with a drive-by shooting that resulted in the death of a Dutch footballer in September.
Read: Ronaldo Facing Two-Year Ban?
In September, former Volendam and Burton Albion defender Kelvin Maynard was killed after being shot in his car by two unidentified suspects on a motorcycle.
Authorities have now confirmed that a person has been arrested in connection with Maynard's murder.
Click through the gallery below to see 10 footballers who were arrested:
"Kelvin Maynard died on 18 September 2019 after a shooting incident on the Langbroekdreef in Amsterdam-Zuidoost," an official statement read.
"He was fired (at) on that evening at around 8.30 p.m. Since then, the criminal investigation department has been conducting a very extensive investigation.
"Investigations are being carried out on forensic, technical and financial matters. Thanks to this investigation, a suspect was arrested on 14 November.
"The suspect was taken to a police station for questioning. More information about this suspect and the suspicion against him will follow later."
The motive for the shooting has not yet been made apparent.
Read: Policeman Charged With Murder Over Ex-EPL Star's Death
Maynard was contracted to Alphense Boys at the time of his death, and previously played for the likes of Royal Antwerp and FC Emmen during his 13-year career.


After submitting your protein sequences you will be redirected to the results page with token {{upload.id}}. To visit your results page for this submission at a later time press button to copy link address to the results page.


History of SMPV
-
2011
Mr. Shuwei Zong and colleagues started ginseng peptide research and development.
-
2012
Mr. Shuwei Zong co-founded Jilin TaiGu (Peptide Valley) Bioengineering Co. (the predecessor of SMPV).
-
2014
Jilin TaiGu built 15,000 square meters (3.7 acres) facility with two automatic production lines with proprietary technology to process natural plants such as ginseng into the form of peptides.
-
2014
Jilin TaiGu started to collaborate with Peking University Health Science Center to conduct 24 ginseng peptide related research projects.
-
2015
Jilin Peptide Bioengineering Co., Ltd. was grated the Innovative Patent "Extraction Method for Ginseng Peptide" (ZL201510622841.0).
-
2017
With strategic investment from Sinopharm Group Co., Ltd, Jilin TaiGu Bioengineering Co. was merged into Guoyao TaiGu (SinoMed Peptide Valley).


This demo shows how to use an {@link jidejs/base/ObservableList} to store a collection of data.
require(['jidejs/base/ObservableList'], function(ObservableList) { // create a list of people, we represent a person as a string to keep the example small var people = new ObservableList(); // add some people people.add('George', 'Patrick', 'David', 'John', 'Sarah'); log(people.toArray().join(', ')); // now remove John people.removeAt(3); log(people.toArray().join(', ')); // and now let's remove George people.remove('George'); log(people.toArray().join(', ')); // and insert them back people.insertAt(2, 'John', 'George'); log(people.toArray().join(', ')); // constantly having to log the changes get's boring, listen to the changes instead function changeLogger(event) { // log the new contents of the list log('The people list was changed: '+people.toArray().join(', ')); // and now list the changes var changes = event.enumerator(); while(changes.moveNext()) { var change = changes.current; if(change.isUpdate) { log('Update: '+change.oldValue+' => '+change.newValue); } else if(change.isDelete) { log('Delete: '+change.oldValue); } else if(change.isInsert) { log('Insert: '+change.newValue); } } } people.on('change', changeLogger); // and change the list again people.splice(2, 1, 'Jessica', 'Naomi'); // we can also sort the list: var sortedPeople = people.sort(function(firstPerson, secondPerson) { return firstPerson.localeCompare(secondPerson); }); // ignore changes to the original list now people.removeListener('change', changeLogger); // and listen to the sorted list instead sortedPeople.on('change', changeLogger); // sortedPeople is actually a jidejs/base/Collection so we can't modify it directly, but we can still change the original // list: people.splice(0, 2, 'Michael', 'Peter', 'Xavier'); // this utility function appends an entry to our visible log function log(msg) { root.querySelector('#xlog').appendChild(function() { var li = document.createElement('li'); li.innerHTML = msg; return li; }()); } });
Whenever you need to store a collection of objects that should be observable, you should consider to use a {@link jidejs/base/Collection} or an {@link jidejs/base/ObservableList} instead of an array.
The {@link jidejs/base/Collection} API provides detailed change events and the power to create sorted, filtered, transformed and grouped collections of the data stored within them that are automatically updated when the original collection changed.
To get comfortable with {@link jidejs/base/ObservableList}, as you really should be, try to modify the example so
that the list contains an actual Person object with name and age properties and use {@link jidejs/base/Collection~filter}
to filter the list to contain only people below 40. In order to see the result, you might need to implement a
toString
method for your Person object.


Driver-Navigator pairing involves the driver typing and the navigator talking and leading the driver. The driver gives input, but should be focused on the area of code instructed by the navigator. The driver presses the gas, but the navigator says when to turn.


|
LabBot
|
A class for controlling a LabJack U6 or U6-Pro data aquisition device. More...
#import <DRHLabjackU6.h>
Instance Methods | |
| (DRHLabjackU6 *) | - initWithSerialNum: |
| (DRHLabjackU6 *) | - initForStreamingWithSerialNum:AndSettings: |
| (void *) | - handle |
| (NSNumber *) | - readOneSampleFromAnalogueChannel: |
| (BOOL) | - configureStream |
| (BOOL) | - startStream |
| (BOOL) | - stopStream |
| (NSArray *) | - readStreamData |
| (void) | - close |
Class Methods | |
| (DRHLabjackU6 *) | + u6WithSerialNum: |
| (DRHLabjackU6 *) | + u6ForStreamingWithSerialNum:AndSettings: |
| (NSNumber *) | + readOneSampleFromAnalogueChannel:AndDeviceWithSerialNum: |
| Class Methods inherited from DRHLabJack | |
| (NSDictionary *) | + numberOfConnectedLabJackDevices |
Public Attributes | |
| NSString *const | DRHLabJackU6ConfigNumAnalogueChanKey |
| NSString *const | DRHLabJackU6ConfigScanRateKey |
| NSString *const | DRHLabJackU6ConfigSamplesPerPacketKey |
Protected Attributes | |
| NSInteger | numAnalogueChan |
| The number of sampling analogChannels. | |
| NSInteger | samplesPerPacket |
| The number of samples received on each read stream loop. At this stage this should be an integer multiple of the value set for numAnalogueChan. | |
| NSInteger | scanRate |
| The rate that scans are collected in Hz. 1 scan equals 1 sample from each configured channel. | |
A class for controlling a LabJack U6 or U6-Pro data aquisition device.
This class provides properties and methods to facilitate interacting with a LabJack U6 or U6-Pro device. This class is specific to the U6 and U6-Pro. If you have a different LabJack device you should be using a different subclass of DRHLabJack
. If your data aqusition hardware is not a LabJack device you should be using a different subclass of DRHDaqDevice
.
| - (void) close |
Closes the LabJack U6
| - (BOOL) configureStream |
Sends a low-level command to configure the LabJack for analogue streaming.
0
if successful, else -1
. | - (HANDLE) handle |
Getter method for the device handle of this initialised LabJack U6 device ID.
HANDLE
of the LabJack U6 device being controlled by the receiver. | - (DRHLabjackU6 *) initForStreamingWithSerialNum: | (int) | serialNum | |
| AndSettings: | (NSDictionary *) | settings | |
Initialises the receiver with the specified device handle.
| serialNum | The device handle of an exisiting LabJack U6 device. |
| settings | An NSDictionary of the configuration settings. Keys are defined under class constants. |
| - (DRHLabjackU6 *) initWithSerialNum: | (int) | serialNum |
Initialises the receiver with the specified device serial number.
| serialNum | The serial number of a LabJack U6 device that is connected to a USB port. |
| - (NSNumber *) readOneSampleFromAnalogueChannel: | (uint8) | channel |
Reads a single sample from a single analogue channel. Intended for calibration routines.
| channel | The number of the analogue channel to read one sample from. |
NSNumber
. | + (NSNumber *) readOneSampleFromAnalogueChannel: | (uint8) | channel | |
| AndDeviceWithSerialNum: | (int) | serialNum | |
Reads a single sample from a single analogue channel and then closes the device. Intended for calibration routines.
| channel | The number of the analogue channel to read one sample from. |
| serialNum | The serial number of the U6 device to sample from. |
NSNumber
. | - (NSArray *) readStreamData |
Reads the StreamData low-level function response in a loop. All voltages from the stream are stored in the voltages 2D array.
Nill
if error, otherwise an array of arrays where each sub array represents a scan and contains NSNumbers of the sampled data from each streaming analogue channel. Reimplemented from DRHLabJack.
| - (BOOL) startStream |
Sends a low-level command to start the configured stream.
0
if successful, else -1
. Reimplemented from DRHLabJack.
| - (BOOL) stopStream |
Sends a low-level command to stop any exisiting streams.
0
if successful, else -1
. Reimplemented from DRHLabJack.
| + (DRHLabjackU6 *) u6ForStreamingWithSerialNum: | (int) | serialNum | |
| AndSettings: | (NSDictionary *) | settings | |
Creates a new instance of DRHLabJackU6
and initialises it with the specified device handle.
| serialNum | The device handle of an exisiting LabJack U6 device. |
| settings | An NSDictionary of the configuration settings. Keys are defined under class constants. |
| + (DRHLabjackU6 *) u6WithSerialNum: | (int) | serialNum |
Creates a new instance of DRHLabJackU6
and initialises it with the the device with the specified serial number.
| serialNum | The serial number of a LabJack U6 device that is connected to a USB port. |
| - (NSString* const) DRHLabJackU6ConfigNumAnalogueChanKey |
Key for the number of analogue channels in the initialising setting dictionary.
| - (NSString* const) DRHLabJackU6ConfigSamplesPerPacketKey |
Key for the samples per packet setting in the initialising setting dictionary. At this stage this should be an integer multiple of the value set for DRHLabJackU6ConfigNumAnalogueChanKey.
| - (NSString* const) DRHLabJackU6ConfigScanRateKey |
Key for the scan rate in the initialising setting dictionary. 1 scan is 1 sample of each configured channel.


I am a Responsive Developer who thrives on the best user experience. When I'm not working, pursuing side projects or getting lost in the world of blogs and books, I can be found in the back yard with my fur-baby Curly.
I am a Responsive Developer who thrives on the best user experience. When I'm not working, pursuing side projects or getting lost in the world of blogs and books, I can be found in the back yard with my fur-baby Curly.
I was born in Jonquiere, Quebec, Canada and my parents moved me to East Hartford, Connecticut when I was just 12 days old. When I was 14 years old my parents moved back to Quebec but in Laval this time. I continued this back and forth across the border a few times and eventually landing in Kingston, Ontario, Canada.
My passion: I LOVE coming up with new recipes! I enjoy coming up with different twists to old favorites as well as tring something for the first time. I am so happy when others try my creations and find they really like them.
Fun fact: I was barely computer-literate when I started to learn coding. I'm sure I exasperated a few of my instructors with some of the questions I asked. After much confusion and hair pulling look at me now! Who would have guessed what I would be the "go-to person" for computer issues?


HLA-C*15:10
Back to allele selection.
Motif
Length distribution
Extracted MHC positions (pseudosequence)
YYAGYRENYRQTDVNKLYGIRYDDLYTWAELAYTWYX
Alleles with identical predictions
HLA-C*15:10 has identical extracted MHC positions (pseudosequence) as the following others. Predictions for these alleles are the same.
- HLA-C*15:100
- HLA-C*15:101
- HLA-C*15:106
- HLA-C*15:109
- HLA-C*15:112
- HLA-C*15:118
- HLA-C*15:119
- HLA-C*15:120
- HLA-C*15:121
- HLA-C*15:124
- HLA-C*15:131
- HLA-C*15:135
- HLA-C*15:136
- HLA-C*15:137
- HLA-C*15:141
- HLA-C*15:150
- HLA-C*15:162
- HLA-C*15:166
- HLA-C*15:28
- HLA-C*15:33
- HLA-C*15:34
- HLA-C*15:38
- HLA-C*15:49
- HLA-C*15:50
- HLA-C*15:53
- HLA-C*15:56
- HLA-C*15:64
- HLA-C*15:67
- HLA-C*15:68
- HLA-C*15:73
- HLA-C*15:86
- HLA-C*15:89
- HLA-C*15:91
- HLA-C*15:93
- HLA-C*15:94
- HLA-C*15:98