# Classes

> Summary of classes, class instances, and their members.



<?code-excerpt replace="/ *\/\/\s+ignore_for_file:[^\n]+\n//g; /(^|\n) *\/\/\s+ignore:[^\n]+\n/$1/g; /(\n[^\n]+) *\/\/\s+ignore:[^\n]+\n/$1\n/g; / *\/\/\s+ignore:[^\n]+//g; /([A-Z]\w*)\d\b/$1/g"?>

Dart is an object-oriented language with classes and mixin-based
inheritance. Every object is an instance of a class, and all classes
except `Null` descend from [`Object`][].
*Mixin-based inheritance* means that although every class
(except for the [top class][top-and-bottom], `Object?`)
has exactly one superclass, a class body can be reused in
multiple class hierarchies.
[Extension methods][] are a way to
add functionality to a class without changing the class or creating a subclass.
[Class modifiers][] allow you to control how libraries can subtype a class.


## 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:

<?code-excerpt "misc/test/language_tour/classes_test.dart (object-members)"?>
```dart
var p = Point(2, 2);

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

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

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

<?code-excerpt "misc/test/language_tour/classes_test.dart (safe-member-access)"?>
```dart
// If p is non-null, set a variable equal to its y value.
var a = p?.y;
```


## Using constructors

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

<?code-excerpt "misc/test/language_tour/classes_test.dart (object-creation)" replace="/ as .*?;/;/g"?>
```dart
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:

<?code-excerpt "misc/test/language_tour/classes_test.dart (object-creation-new)" replace="/ as .*?;/;/g"?>
```dart
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:

<?code-excerpt "misc/test/language_tour/classes_test.dart (const)"?>
```dart
var p = const ImmutablePoint(2, 2);
```

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

<?code-excerpt "misc/test/language_tour/classes_test.dart (identical)"?>
```dart
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:

<?code-excerpt "misc/test/language_tour/classes_test.dart (const-context-withconst)" replace="/pointAndLine1/pointAndLine/g"?>
```dart
// 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:

<?code-excerpt "misc/test/language_tour/classes_test.dart (const-context-noconst)" replace="/pointAndLine2/pointAndLine/g"?>
```dart
// 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**:

<?code-excerpt "misc/test/language_tour/classes_test.dart (nonconst-const-constructor)"?>
```dart
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 the `Object` property `runtimeType`,
which returns a [`Type`][] object.

<?code-excerpt "misc/test/language_tour/classes_test.dart (runtime-type)"?>
```dart
print('The type of a is ${a.runtimeType}');
```

:::warning
Use a [type test operator][] rather than `runtimeType`
to test an object's type.
In production environments, the test `object is Type` is more stable
than the test `object.runtimeType == Type`.
:::

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:

<?code-excerpt "misc/lib/language_tour/classes/point_with_main.dart (class)"?>
```dart
class Point {
  double? x; // Declare instance variable x, initially null.
  double? y; // Declare y, initially null.
  double z = 0; // Declare z, initially 0.
}
```

An uninitialized instance variable declared with a
[nullable type][] has the value `null`.
Non-nullable instance variables [must be initialized][] at declaration.

All instance variables generate an implicit *getter* method.
Non-final instance variables and
`late final` instance variables without initializers also generate
an implicit *setter* method. For details,
check out [Getters and setters][].

<?code-excerpt "misc/lib/language_tour/classes/point_with_main.dart (class-main)" replace="/(double .*?;).*/$1/g" plaster="none"?>
```dart
class Point {
  double? x; // Declare instance variable x, initially null.
  double? y; // Declare y, initially null.
}

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.
}
```

Initializing a non-`late` instance variable where it's declared
sets the value when the instance is created,
before the constructor and its initializer list execute.
As a result, the initializing expression (after the `=`)
of a non-`late` instance variable can't access `this`.

<?code-excerpt "misc/lib/language_tour/classes/point_this.dart"?>
```dart
double initialX = 1.5;

class Point {
  // OK, can access declarations that do not depend on `this`:
  double? x = initialX;

  // ERROR, can't access `this` in non-`late` initializer:
  double? y = this.x;

  // OK, can access `this` in `late` initializer:
  late double? z = this.x;

  // OK, `this.x` and `this.y` are parameter declarations, not expressions:
  Point(this.x, this.y);
}
```

Instance variables can be `final`,
in which case they must be set exactly once.
Initialize `final`, non-`late` instance variables
at declaration,
using a constructor parameter, or
using a constructor's [initializer list][]:

<?code-excerpt "misc/lib/effective_dart/usage_good.dart (field-init-at-decl)"?>
```dart
class ProfileMark {
  final String name;
  final DateTime start = DateTime.now();

  ProfileMark(this.name);
  ProfileMark.unnamed() : name = '';
}
```

If you need to assign the value of a `final` instance variable
after the constructor body starts, you can use one of the following:

* Use a [factory constructor][].
* Use `late final`, but [_be careful:_][late-final-ivar]
  a `late final` without an initializer adds a setter to the API.

## 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:

<?code-excerpt "misc/lib/language_tour/classes/impostor.dart"?>
```dart
// A person. The implicit interface contains greet().
class Person {
  // In the interface, but visible only in this library.
  final String _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 {
  String 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:

<?code-excerpt "misc/lib/language_tour/classes/misc.dart (point-interfaces)"?>
```dart
class Point implements Comparable, Location {
  ...
}
```


## 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:

<?code-excerpt "misc/lib/language_tour/classes/misc.dart (static-field)"?>
```dart
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](/effective-dart/style#identifiers)
of preferring `lowerCamelCase` for constant names.
:::

### Static methods

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

<?code-excerpt "misc/lib/language_tour/classes/point_with_distance_method.dart"?>
```dart
import 'dart:math';

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

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

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

:::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.


[`Object`]: https://api.dart.dev/dart-core/Object-class.html
[top-and-bottom]: /null-safety/understanding-null-safety#top-and-bottom
[Extension methods]: /language/extension-methods
[Class modifiers]: /language/class-modifiers
[constant constructors]: /language/constructors#constant-constructors
[`Type`]: https://api.dart.dev/dart-core/Type-class.html
[type test operator]: /language/operators#type-test-operators
[Getters and setters]: /language/methods#getters-and-setters
[initializer list]: /language/constructors#use-an-initializer-list
[factory constructor]: /language/constructors#factory-constructors
[late-final-ivar]: /effective-dart/design#avoid-public-late-final-fields-without-initializers
[nullable type]: /null-safety/understanding-null-safety#using-nullable-types
[must be initialized]: /null-safety/understanding-null-safety#uninitialized-variables

