How to declare an interface in Dart?

In Dart, there is no explicit keyword interface. Interfaces are implicitly declared by defining a class. Any class in Dart can serve as an interface.

A class implementing an interface must declare the keyword implements before the class name and list the names of the interfaces it implements.

// Interface is implicitly defined by this class.
class MyInterface {
  void doSomething() {}
  int getData() {
    return 0;
  }
}

// Class implementing the interface.
class Implementer implements MyInterface {
  @override
  void doSomething() {
    // Implementation of the interface method
    print('Doing something...');
  }

  @override
  int getData() {
    // Implementation of the interface method
    return 42;
  }
}

void main() {
  MyInterface obj = Implementer();
  obj.doSomething();
  print(obj.getData());
}

If a class implements multiple interfaces, they can be listed separated by commas:

class AnotherInterface {
  String getName() {
    return '';
  }
}

class MultiImplementer implements MyInterface, AnotherInterface {
  @override
  void doSomething() {
    print('Doing something from MyInterface...');
  }

  @override
  int getData() {
    return 100;
  }

  @override
  String getName() {
    return 'Multirealizer';
  }
}

Using an abstract class also allows defining an interface, but with the possibility of containing non-abstract methods and instance variables. However, for a pure contract definition, a regular class or even an abstract class with all abstract methods is often used.