How to create private variables in Dart?

I want to create a private variable but I cannot.

Here is my code:

void main() {
  var b = new B();
  b.testB();    
}

class A {
  int _private = 0;

  testA() {
    print('int value: $_private');
    _private = 5;
  }
}

class B extends A {
  String _private;

  testB() {
    _private = 'Hello';
    print('String value: $_private');
    testA();
    print('String value: $_private');
  }
}

When I run this code, I get the following result:

String value: Hello
int value: Hello
Breaking on exception: type 'int' is not a subtype of type 'String' of 'value'.

Also I not get any error or warnings when editing this source code.

How can I create a private variable in Dart?

Solution 1:

From Dart documentation:

Unlike Java, Dart doesn’t have the keywords public, protected, and private. If an identifier starts with an underscore _, it’s private to its library.

Libraries not only provide APIs, but are a unit of privacy: identifiers that start with an underscore _ are visible only inside the library.

A few words about libraries:

Every Dart app is a library, even if it doesn’t use a library directive. The import and library directives can help you create a modular and shareable code base.

You may have heard of the part directive, which allows you to split a library into multiple Dart files.

Solution 2:

Privacy in Dart exists at the library, rather than the class level.

If you were to put class A into a separate library file (eg, other.dart), such as:

library other;

class A {
  int _private = 0;

  testA() {
    print('int value: $_private');  // 0
    _private = 5;
    print('int value: $_private'); // 5
  }
}

and then import it into your main app, such as:

import 'other.dart';

void main() {
  var b = new B();
  b.testB();    
}


class B extends A {
  String _private;

  testB() {
    _private = 'Hello';
    print('String value: $_private'); // Hello
    testA();
    print('String value: $_private'); // Hello
  }
}

You get the expected output:

String value: Hello
int value: 0
int value: 5
String value: Hello

Solution 3:

In dart '_' is used before the variable name to declare it as private. Unlike other programming languages, here private doesn't mean it is available only to the class it is in, private means it is accessible in the file it is in and not accessible to other files.