Dart Constructors
Special methods for creating and initializing objects
🔧 What are Dart Constructors?
Constructors are special methods that create and initialize objects when they are instantiated. They set up initial values and prepare objects for use in your program.
// Simple constructor example
class Car {
String brand;
int year;
Car(this.brand, this.year); // Constructor
}
Car myCar = Car('Toyota', 2023);
Types of Constructors
Default Constructor
Simple constructor with no parameters
class Person {
String name = 'Unknown';
Person(); // Default constructor
}Parameterized
Constructor that accepts parameters
class Student {
String name;
int age;
Student(this.name, this.age);
}Named Constructor
Constructor with a specific name
class Point {
double x, y;
Point.origin() {
x = 0;
y = 0;
}
}Factory Constructor
Constructor that can return instances
class Logger {
static Logger _instance;
factory Logger() {
return _instance ??= Logger._();
}
}🔹 Default Constructor
The simplest type of constructor:
class Animal {
String name;
String species;
// Default constructor
Animal() {
name = 'Unknown';
species = 'Unknown';
print('Animal created with default values');
}
void displayInfo() {
print('Name: $name, Species: $species');
}
}
void main() {
Animal animal1 = Animal();
animal1.displayInfo();
// Modify properties after creation
animal1.name = 'Fluffy';
animal1.species = 'Cat';
animal1.displayInfo();
}Output:
Animal created with default values
Name: Unknown, Species: Unknown
Name: Fluffy, Species: Cat
🔹 Parameterized Constructor
Constructor that accepts parameters to initialize objects:
class Rectangle {
double width;
double height;
// Parameterized constructor
Rectangle(this.width, this.height) {
print('Rectangle created: ${width}x$height');
}
// Alternative way to write constructor
// Rectangle(double w, double h) {
// width = w;
// height = h;
// }
double getArea() {
return width * height;
}
double getPerimeter() {
return 2 * (width + height);
}
void displayInfo() {
print('Width: $width, Height: $height');
print('Area: ${getArea()}');
print('Perimeter: ${getPerimeter()}');
}
}
void main() {
Rectangle rect1 = Rectangle(5.0, 3.0);
Rectangle rect2 = Rectangle(10.0, 7.5);
print('Rectangle 1:');
rect1.displayInfo();
print('\nRectangle 2:');
rect2.displayInfo();
}Output:
Rectangle created: 5.0x3.0
Rectangle created: 10.0x7.5
Rectangle 1:
Width: 5.0, Height: 3.0
Area: 15.0
Perimeter: 16.0
Rectangle 2:
Width: 10.0, Height: 7.5
Area: 75.0
Perimeter: 35.0
🔹 Named Constructors
Create multiple constructors with different names:
class Employee {
String name;
String department;
double salary;
// Default constructor
Employee(this.name, this.department, this.salary);
// Named constructor for interns
Employee.intern(this.name, this.department) {
salary = 1000.0;
print('$name joined as intern in $department');
}
// Named constructor for managers
Employee.manager(this.name, this.department) {
salary = 8000.0;
print('$name joined as manager in $department');
}
// Named constructor for senior employees
Employee.senior(this.name, this.department, double experience) {
salary = 5000.0 + (experience * 500);
print('$name joined as senior with $experience years experience');
}
void displayInfo() {
print('Name: $name');
print('Department: $department');
print('Salary: \$${salary.toStringAsFixed(2)}');
print('---');
}
}
void main() {
Employee emp1 = Employee('John', 'IT', 4500.0);
Employee emp2 = Employee.intern('Alice', 'Marketing');
Employee emp3 = Employee.manager('Bob', 'Sales');
Employee emp4 = Employee.senior('Carol', 'Engineering', 5.0);
emp1.displayInfo();
emp2.displayInfo();
emp3.displayInfo();
emp4.displayInfo();
}Output:
Alice joined as intern in Marketing
Bob joined as manager in Sales
Carol joined as senior with 5.0 years experience
Name: John
Department: IT
Salary: $4500.00
---
Name: Alice
Department: Marketing
Salary: $1000.00
---
🔹 Constructor with Optional Parameters
Use optional parameters to make constructors more flexible:
class Product {
String name;
double price;
String category;
bool inStock;
// Constructor with optional parameters
Product(this.name, this.price, {this.category = 'General', this.inStock = true}) {
print('Product "$name" created in $category category');
}
void displayProduct() {
print('Product: $name');
print('Price: \$${price.toStringAsFixed(2)}');
print('Category: $category');
print('In Stock: ${inStock ? 'Yes' : 'No'}');
print('---');
}
}
void main() {
// Using constructor with different parameter combinations
Product product1 = Product('Laptop', 999.99);
Product product2 = Product('Phone', 599.99, category: 'Electronics');
Product product3 = Product('Book', 29.99, category: 'Education', inStock: false);
product1.displayProduct();
product2.displayProduct();
product3.displayProduct();
}Output:
Product "Laptop" created in General category
Product "Phone" created in Electronics category
Product "Book" created in Education category
Product: Laptop
Price: $999.99
Category: General
In Stock: Yes
---