Metadata
- Dart course
- Intermediate
- Lesson 76/116
- 5 min
Learn metadata with @annotation.
Metadata with annotations
An annotation is a label attached on top of a code element. In Dart it starts with the @ sign, for example @override. Annotations give extra information to the compiler and to tools.
The best known example is @override. When a subclass method redefines a superclass method you write @override. This is common for overriding methods such as toString.
Annotations can tag classes, methods, fields and parameters. You can also write your own annotation; the next lesson covers that.
Where annotations attach
Annotations can attach to many code elements. They are written above a class, method, field or parameter. For example @Deprecated('old') marks a method as deprecated.
Multiple annotations can sit side by side: @override @Deprecated('old'). Dart reads them left to right.
Annotations are signals for tools. @override warns the compiler about a wrong override; @deprecated guides the user to a new method.
Practice Point and @override
Define a Point class with int x and int y fields and a Point(this.x, this.y) constructor. Add a String toString() method marked with @override that returns '($x, $y)'. In main create Point(3, 4) and print it with print.
Starter code
// Define the Point class: x, y, constructor and a toString method
void main() {
// create a Point and print it
}Sample solution
class Point {
int x;
int y;
Point(this.x, this.y);
@override
String toString() => '($x, $y)';
}
void main() {
print(Point(3, 4));
}Expected output
(3, 4)
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 What does an annotation do?
- Adds metadata to code Correct answer
- Deletes a variable
- Stops a loop
- Makes a class inherit
Explanation: an annotation adds metadata to code.
2 Writing @override
class Dog extends Animal {
@____
String toString() => 'Köpek';
}class Dog extends Animal {
@override
String toString() => 'Köpek';
}Explanation: @override is written lowercase.
3 Match the annotation parts
| Term | Meaning |
|---|---|
@override |
Override |
@ |
Annotation sign |
@deprecated |
Deprecated mark |
toString() |
Overridden method |