Constructor
- Dart course
- Beginner
- Lesson 78/116
- 5 min
Learn to define the class constructor.
Constructor
The constructor is a special method called when an instance is created. It shares the class name. Example: class User { String name; User(this.name); }.
Here User(this.name) is the short form; it means 'assign the parameter to the same-named field'. The long form: User(String name) { this.name = name; }.
The constructor prepares the object: it sets field values, allocates resources. Calling User('Ayşe') returns a User instance with name set to 'Ayşe'. This lesson focuses on the this.name shorthand; the this keyword is covered in a separate lesson.
Short and long form
The short form is very common in Dart constructors: User(this.name). It means 'assign the parameter to the same-named field'. The field name is filled automatically.
The long form is more explicit: User(String name) { this.name = name; }. Here you assign the parameter name to the field with this.name. this means 'this instance'.
Both do the same thing. The short form is the idiomatic Dart style; it is preferred in practice. The long form helps when you need extra logic.
Practice User constructor
Define a User class with a String name field and a User(this.name) constructor. Inside main create an instance with User('Ayşe') and print it with print(u.name).
Starter code
void main() {
// define the User class and constructor
// create an instance and print
}Sample solution
class User {
String name;
User(this.name);
}
void main() {
var u = User('Ayşe');
print(u.name);
}Expected output
Ayşe
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Match the constructor parts
| Term | Meaning |
|---|---|
User |
Constructor name |
this.name |
Shorthand assign |
User('Ayşe') |
Instance creation |
name |
Property name |
2 What does User(this.name) do?
- Throws an error
- Ignores the name parameter
- Deletes the class
- Assigns the name parameter to the name field Correct answer
Explanation: this.name is the shorthand assignment.
3 Constructor short form
class User {
String name;
User(____);
}class User {
String name;
User(this.name);
}Explanation: this.name assigns the parameter to the field.