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 {
final double x;
final double y;
// Sets the x and y instance variables
// before the constructor body runs.
Point(this.x, this.y);
double distanceTo(Point other) {
var dx = x - other.x;
var dy = y - other.y;
return sqrt(dx * dx + dy * dy);
}
}
Operators
#Most operators are instance methods with special names. Dart allows you to define operators with the following names:
< | > | <= | >= | == | ~ |
- | + | / | ~/ | * | % |
| | ˆ | & | << | >>> | >> |
[]= | [] |
To declare an operator, use the built-in identifier operator
then the operator you are defining. The following example defines vector addition (+
), subtraction (-
), and equality (==
):
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);
@override
bool operator ==(Object other) =>
other is Vector && x == other.x && y == other.y;
@override
int get hashCode => Object.hash(x, y);
}
void main() {
final v = Vector(2, 3);
final w = Vector(2, 2);
assert(v + w == Vector(4, 5));
assert(v - w == Vector(0, 1));
}
Getters and setters
#Getters and setters are special methods that provide read and write access to an object's properties. Recall that each instance variable has an implicit getter, plus a setter if appropriate. You can create additional properties by implementing getters and setters, using the get
and set
keywords:
class Rectangle {
double left, top, width, height;
Rectangle(this.left, this.top, this.width, this.height);
// Define two calculated properties: right and bottom.
double get right => left + width;
set right(double value) => left = value - width;
double get bottom => top + height;
set bottom(double value) => top = value - height;
}
void main() {
var rect = Rectangle(3, 4, 20, 15);
assert(rect.left == 3);
rect.right = 12;
assert(rect.left == -8);
}
With getters and setters, you can start with instance variables, later wrapping them with methods, all without changing client code.
Abstract methods
#Instance, getter, and setter methods can be abstract, defining an interface but leaving its implementation up to other classes. Abstract methods can only exist in abstract classes or mixins.
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...
}
}
Unless stated otherwise, the documentation on this site reflects Dart 3.5.0. Page last updated on 2024-04-17. View source or report an issue.