OOP model
- Dart course
- Intermediate
- Lesson 113/116
- 6 min
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).
Practice 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).
Starter code
// Define the Trackable interface
class Task {
final String title;
bool done;
Task(this.title, {this.done = false});
}
void main() {
// create a Task instance and print isDone
}Sample 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);
}Expected output
true
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Trackable interface
abstract class Trackable { bool get ____; }abstract class Trackable { bool get isDone; }Explanation: the abstract getter isDone is declared.
2 How does Task implement Trackable?
- Task(Trackable)
- Task extends Trackable
- Task with Trackable
- Task implements Trackable Correct answer
Explanation: implements is Dart's keyword for applying an interface.
3 OOP model parts
| Term | Meaning |
|---|---|
abstract class |
Interface |
implements |
Implementation |
@override |
Override |
get isDone |
Getter |