Constructors in Dart

What is a Constructor?

A constructor is a special method that runs when you create an object. It initializes the object's fields. In Dart, a constructor has the same name as the class.

Default Constructor

class Point {
  double x;
  double y;

  // Constructor
  Point(double x, double y) {
    this.x = x;
    this.y = y;
  }
}

var p = Point(3.0, 4.0);
print('$${p.x}, $${p.y}');  // 3.0, 4.0

Initializing Formal Parameters

Dart provides a shorthand to initialize fields directly in the constructor signature using this.fieldName:

class Point {
  double x;
  double y;

  // Shorthand — automatically assigns x and y
  Point(this.x, this.y);
}

// Equivalent to the verbose version above

Initializer List

Run code before the constructor body using an initializer list (after :). Useful for computing final field values or validation:

class Circle {
  final double radius;
  final double area;

  Circle(double r)
      : radius = r,
        area = 3.14159 * r * r {
    print('Circle created with radius $radius');
  }
}

var c = Circle(5.0);
print('Area: $${c.area.toStringAsFixed(2)}');

Circle created with radius 5.0
Area: 78.54

Example

class Employee {
  final String id;
  final String name;
  final String department;
  double salary;

  Employee({
    required this.id,
    required this.name,
    required this.department,
    this.salary = 50000.0,
  });

  void applyRaise(double percent) {
    salary *= (1 + percent / 100);
    print('$name raised to \$$${salary.toStringAsFixed(2)}');
  }
}

void main() {
  var emp1 = Employee(id: 'E001', name: 'Alice', department: 'Engineering');
  var emp2 = Employee(id: 'E002', name: 'Bob', department: 'Sales', salary: 45000);

  emp1.applyRaise(10);  // Alice raised to $55000.00
  emp2.applyRaise(5);   // Bob raised to $47250.00
}

🧠 Quiz

Summary

Constructors initialize object fields when an instance is created. Use the this.field shorthand for concise initialization. Initializer lists (:) run before the constructor body, useful for final fields and computed values. Named parameters with required make constructors explicit and safe.