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description bug_report

Methods

Contents

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:

dart
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

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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 (==):

dart
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

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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:

dart
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

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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:

dart
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...
  }
}