OOP model

Set up the class hierarchy and interfaces.

OOP model

We add an interface to the Task class. In Dart an abstract class defines an interface; you implement it with implements.

abstract class Trackable { bool get isDone; } declares an abstract getter. You implement it with Task implements Trackable.

In the implementation you add @override bool get isDone => done;. This shows the OOP hierarchy and polymorphism.

Applying with @override

The Task class now says implements Trackable; it applies the abstract getter with @override bool get isDone => done;.

The @override annotation tells the compiler that a member of a parent class or interface is being overridden. This turns a typo into a compile error.

So every Task is now a Trackable; code expecting a Trackable can accept a Task (polymorphism).

Exercise Trackable interface and Task

Define abstract class Trackable { bool get isDone; }. Make Task implements Trackable and add @override bool get isDone => done;. In main create var t = Task('Test write', done: true); and print print(t.isDone).

Show example solution

abstract class Trackable {
  bool get isDone;
}

class Task implements Trackable {
  final String title;
  bool done;
  Task(this.title, {this.done = false});

  @override
  bool get isDone => done;
}

void main() {
  var t = Task('Test write', done: true);
  print(t.isDone);
}

Test yourself

Trackable interface

abstract class Trackable { bool get ____; }

Show answer

abstract class Trackable { bool get isDone; }

How does Task implement Trackable?
- Task(Trackable)
- Task extends Trackable
- Task with Trackable
- Task implements Trackable
Show answer

Task implements Trackable — implements is Dart's keyword for applying an interface.
OOP model parts

abstract class

Interface
implements

Implementation
@override

Override
get isDone

Getter

Next lesson: Collections

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