Dart super

Summary: in this tutorial, you’ll learn about the Dart super keyword to reference the parent class from a child class.

Introduction to Dart super keyword

Unlike other OOP languages such as C#, in Dart, a subclass doesn’t inherit constructors from its parent class.

For example, the following adds a constructor to the BankAccount class defined in the inheritance tutorial:

class BankAccount {
  double _balance = 0;

  BankAccount({double balance = 0}) : _balance = balance;

  double get balance => _balance;

  deposit(double amount) {
    _balance += amount;
  }

  bool withdraw(double amount) {
    if (amount <= _balance) {
      _balance -= amount;
      return true;
    }
    return false;
  }
}Code language: Dart (dart)

And the SavingAccount class, which inherits from the BankAcount class, doesn’t have the unnamed constructor of the BankAccount class:

class SavingAccount extends BankAccount {
  double _interestRate = 0;

  double get interestRate => _interestRate;

  set interestRate(double value) {
    if (value > 0) {
      _interestRate = value;
    }
  }Code language: Dart (dart)

To add an unnamed constructor to the SavingAccount class, you need to explicitly define it in the class.

Typically, the constructor of the child class has more parameters than the constructor of the parent class. Also, you can use the super keyword to call the constructor of the parent class to initialize the properties from the parent class.

For example, the following defines a constructor in the SavingAccount and uses the super keyword to initialize the properties of the BankAccount class:

class SavingAccount extends BankAccount {
  double _interestRate = 0;

  SavingAccount({double balance = 0, double interestRate = 0})
      : _interestRate = interestRate,
        super(balance: balance);

  double get interestRate => _interestRate;

  set interestRate(double value) {
    if (value > 0) {
      _interestRate = value;
    }
  }

  addInterest() {
    double interest = _balance * _interestRate;
    this._balance += interest;
  }
}Code language: Dart (dart)

In the constructor of the SavingAccount class, we initialize the _interestRate field and use the super to initialize the balance property. Because the constructor uses an initializer list, the super must appear last in the list:

SavingAccount({double balance = 0, double interestRate = 0})
      : _interestRate = interestRate,
        super(balance: balance);Code language: Dart (dart)

The following creates a new instance of the SavingAccount class and calls its constructor to initialize the balance and interestRate properties:

void main() {
  var account = SavingAccount(balance: 1000, interestRate: 0.05);
  account.addInterest();

  print(account.balance);
}Code language: Dart (dart)

Put it all together:

class BankAccount {
  double _balance = 0;

  BankAccount({double balance = 0}) : _balance = balance;

  double get balance => _balance;

  deposit(double amount) {
    _balance += amount;
  }

  bool withdraw(double amount) {
    if (amount <= _balance) {
      _balance -= amount;
      return true;
    }
    return false;
  }
}

class SavingAccount extends BankAccount {
  double _interestRate = 0;

  SavingAccount({double balance = 0, double interestRate = 0})
      : _interestRate = interestRate,
        super(balance: balance);

  double get interestRate => _interestRate;

  set interestRate(double value) {
    if (value > 0) {
      _interestRate = value;
    }
  }

  addInterest() {
    double interest = _balance * _interestRate;
    this._balance += interest;
  }
}

void main() {
  var account = SavingAccount(balance: 1000, interestRate: 0.05);
  account.addInterest();

  print(account.balance);
}Code language: Dart (dart)

Summary

• A child class doesn’t automatically inherit constructors from the parent class.
• Use the super keyword to reference the parent class.