Dart Private Fields

Summary: in this tutorial, you’ll learn how to create Dart private fields of a class by prefixing variables with an underscore (_).

Introduction to the Dart private fields

When you define a property for a class, you can access it from the outside of a class. The accessibility of the property is public.

To make the property private, you need to prefix its name with an underscore (_):

type _privateVariable;Code language: Dart (dart)

Note that a variable of a class is generally called a field. However, if it can be accessed from outside of a class, it is called a property.

The following example defines the Point class with the private fields _x and _y:

class Point {
  int _x = 0;
  int _y = 0;

  Point({int x = 0, int y = 0}) {
    this._x = x;
    this._y = y;
  }
  show() {
    print('Point(x=$_x,y=$_y)');
  }
}

void main() {
  var p1 = Point(x: 10, y: 20);
  p1.show();
}Code language: Dart (dart)

Output:

Point(x=10,y=20)Code language: Dart (dart)

How it works.

First, define _x and _y as the private properties for the Point class:

int _x = 0;
int _y = 0;Code language: Dart (dart)

Second, initialize the _x and _y in the constructor:

Point({int x = 0, int y = 0}) {
  this._x = x;
  this._y = y;
}Code language: Dart (dart)

In this example, we define _x and _y as the private field for the Point class. The constructor initializes _x and _y with the corresponding arguments.

Third, display the _x and _y in the show() method:

show() {
  print('Point(x=$_x,y=$_y)');
}Code language: Dart (dart)

Finally, create a new instance of the Point class and call the show() method:

void main() {
  var p1 = Point(x: 10, y: 20);
  p1.show();
}Code language: Dart (dart)

The following example attempts to access the _x and _y private fields in the main() function:

void main() {
  var p1 = Point(x: 10, y: 20);
  p1.show();

  p1._x = 100;
  p1._y = 200;
  p1.show();
}Code language: Dart (dart)

Output:

Point(x=10,y=20)
Point(x=100,y=200)Code language: Dart (dart)

The code works in a way that you may not expect because _x and _y fields are supposed to be private.

In Dart, privacy is at the library level rather than the class level. Adding an underscore to a variable makes it library private not class private.

For now, you can understand that a library is simply a Dart file. Because both the Point class and the main() function are on the same file, they’re in the same library. Therefore, the main() function can access the private fields of the Point class.

To prevent other functions from accessing the private fields of the Point class, you need to create a new library and place the class in it.

First, create a new file called point.dart and add the Point class to the file:

class Point {
  int _x = 0;
  int _y = 0;

  Point({int x = 0, int y = 0}) {
    this._x = x;
    this._y = y;
  }
  show() {
    print('Point(x=$_x,y=$_y)');
  }
}Code language: Dart (dart)

Second, import the point.dart library into the main.dart so that you can reference the Point class:

import 'point.dart';

void main() {
  var p1 = Point(x: 10, y: 20);
  p1.show();
}Code language: Dart (dart)

If you compile and run the program, it should work as expected.

Third, if you attempt to access the _x and _y properties from the main.dart, you’ll get an error:

import 'point.dart';

void main() {
  var p1 = Point(x: 10, y: 20);
  p1.show();

  // errors
  p1._x = 100;
  p1._y = 200;
}Code language: Dart (dart)

Initializer list

If you use private fields, you may find a problem with an unnamed constructor. For example, the following code causes an error:

class Point {
  int _x = 0;
  int _y = 0;

  Point({this._x = 0, this._y = 0});

  show() {
    print('Point(x=$_x,y=$_y)');
  }
}Code language: Dart (dart)

To fix it, you need to use something calls an initializer list. For example:

class Point {
  int _x = 0;
  int _y = 0;

  Point({int x = 0, int y = 0})
      : _x = x,
        _y = y;

  show() {
    print('Point(x=$_x,y=$_y)');
  }
}Code language: Dart (dart)

In this example, the comma-separated list that appears after the semicolon (:) is called an initializer list.

Summary

• Add underscore (_) to fields to make them private at the library level rather than the class level.
• A library is a source code file in Dart.
• Use an initializer list in the unnamed constructor to initialize private fields.