private

Learn private members with _.

private fields

In Dart a field is made private by prefixing its name with an underscore: _balance. This gives library level privacy; the field is reachable inside the same library (file) but not from other libraries.

Example: class Account { double _balance; Account(this._balance); }. Here _balance is private; from outside the file of the Account class, a._balance cannot be reached. Dart has no private keyword; privacy starts with the underscore prefix. This approach blocks direct mutation from outside; access happens through methods.

Library level privacy

Dart privacy is tied to the library concept. A library is usually a single file; so underscored fields are reachable from the same file but not from other files.

Example: if _balance is defined in account.dart, writing a._balance in main.dart gives a compile error. A method in the same account.dart can reach the field freely. So Dart privacy works at the file boundary, not the class boundary. Combining many files into one library is possible with the part directive; then all files share the same library.

Exercise Add a private field to Account

Define a class named Account. Add a private String _owner field. Write an Account(this._owner) constructor. Add a void show() { print(_owner); } method. In main, create an instance with var a = Account('Alex'); and call a.show().

Show example solution

// Define the Account class
class Account {
  String _owner;
  Account(this._owner);
  void show() {
    print(_owner);
  }
}

void main() {
  var a = Account('Alex');
  a.show();
}

Test yourself

Which is a private field in Dart?
- private balance
- _balance
- balance#
- balance.private
Show answer

_balance — The underscore prefix is Dart's private marker.

Name the private field

class Account {
  double ____;
  Account(this._balance);
}

Show answer

class Account {
  double _balance;
  Account(this._balance);
}

Match the private parts

_

Private prefix
_balance

Private field
Account

Class name

Next lesson: Getters and setters

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