Initializer list
- Dart course
- Intermediate
- Lesson 3/116
- 5 min
Learn field initialization with the initializer list.
Initializer list
An initializer list assigns values to fields before the constructor body runs. It starts with a colon and holds comma separated assignments: : width = size, height = size.
Example: Rectangle.square(double size) : width = size, height = size;. Here width and height are set before the body runs. Since final fields cannot be changed inside the body, the initializer list is the clean way to assign them. The list runs before the body, so doing calculations here is safer.
final fields and the initializer list
Fields marked final may be assigned only once; the assignment must happen before the constructor completes. The initializer list is the cleanest way to meet this requirement.
Example: class Rectangle { final double width; final double height; Rectangle(this.width, this.height); Rectangle.square(double size) : width = size, height = size; }. The square named constructor gives both final fields their values inside the initializer list. Even with an empty body, the statement ends with a semicolon. This shape lets fields produce immutable objects.
Practice Write the Rectangle.square constructor
Define a class named Rectangle. Add final double width and final double height fields. Write the default constructor Rectangle(this.width, this.height). Add the named constructor Rectangle.square(double size) : width = size, height = size;. In main, create an instance with var r = Rectangle.square(5); and call print(r.width).
Starter code
// Define the Rectangle class
// Add final width and height
// Add the default constructor and Rectangle.square
void main() {
// Create a Rectangle.square(5) instance and print r.width
}Sample solution
// Define the Rectangle class
class Rectangle {
final double width;
final double height;
Rectangle(this.width, this.height);
Rectangle.square(double size) : width = size, height = size;
}
void main() {
var r = Rectangle.square(5);
print(r.width);
}Expected output
5.0
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Match the initializer list parts
| Term | Meaning |
|---|---|
: |
List start |
width = size |
Assignment |
final |
Immutable field keyword |
2 Add the colon
class Rectangle {
final double width;
final double height;
Rectangle.square(double size)____ width = size, height = size;
}class Rectangle {
final double width;
final double height;
Rectangle.square(double size): width = size, height = size;
}Explanation: The colon separates the initializer list from the constructor body.
3 Which is valid initializer list syntax?
- Rectangle.square(double size) { width = size; height = size; }
- Rectangle.square(double size) => width = size, height = size;
- Rectangle.square(double size) : width = size, height = size; Correct answer
- Rectangle.square(double size) where width = size, height = size;
Explanation: The colon separates the initializer list from the body; the comma divides assignments.