An example of abstract fields in Dart.

abstract_fields.dart

// An interface that defines two mutable fields.
abstract class Ball {
  abstract int x;
  abstract int y;
}

// Implements the fields by initializing in constructor.
class BallImplConstructor implements Ball {
  @override
  int x;
  
  @override
  int y;
  
  BallImplConstructor(this.x, this.y);
}

// Implements the field by intitializing with values.
class BallImplDefault implements Ball {
  @override
  var x = 0;
  
  @override
  var y = 0;
}

// Implements the field by promising it will be initialized.
//
// This is pretty clearly the worst option unless there are some lifecycle
// requirements we've not aware of, like initializing these values in a
// another function.
class BallImplLate implements Ball {
  @override
  late int x;
  
  @override
  late int y;
}