Classes, Enums, Extensions

Abstract Classes and Interface Contracts

abstract class declares a type with abstract members that concrete classes must implement. A function that takes the abstract type works on any concrete implementer; that is polymorphic dispatch. Any class can be used as an interface with implements, which forces the implementer to re-declare every member.

Program

Play the program to dispatch a describe call through the abstract Shape type.

abstract_interface.dart
Replay: real traced execution (multi-file project)
abstract class Shape {
  String get name;
  int area();
}

class Square implements Shape {
  final int side;
  Square(this.side);
  @override
  String get name => 'square';
  @override
  int area() => side * side;
}

void describe(Shape s) {
  print('${s.name} area=${s.area()}');
}

void main() {
  Shape s = Square(4);
  describe(s);
}

  1. call ← Square(4)

    19void main() {20  Shape s = Square(4);21  describe(s);
    values this stepSquare(4)call

  2. Square built ← side=4

    7final int side;8Square(this.side);9@override
    values this stepside=4Square built4this.side

  3. s ← Square(side: 4) as Shape

    19void main() {20  Shape s = Square(4);21  describe(s);
    values this stepSquare(side: 4) as Shapes

  4. call ← describe(s)

    20  Shape s = Square(4);21  describe(s);22}
    values this stepdescribe(s)callSquare instances

  5. calls ← s.name, s.area()

    15void describe(Shape s) {16  print('${s.name} area=${s.area()}');17}
    values this steps.name, s.area()callsSquare instances

  6. return value ← square

    9@override10String get name => 'square';11@override
    values this stepsquarereturn value

  7. return value ← 16

    11  @override12  int area() => side * side;13}
    values this step16return value4side

  8. print('${s.name} area=${s.area()}');

    15void describe(Shape s) {16  print('${s.name} area=${s.area()}');17}
    outputsquare area=16
abstract class `abstract class Shape` declares a type that cannot be instantiated directly.
abstract member `String get name;` and `int area();` end with `;` and no body, so implementers must provide one.
polymorphic dispatch `describe(Shape s)` accepts any `Shape`; calling `s.name` and `s.area()` runs the concrete `Square` implementation.