Named Constructors

Part of the Object Oriented Programming section of Coddy's Dart journey — lesson 11 of 110.

Named constructors allow you to create multiple constructors for a class, each with a descriptive name that clarifies its purpose. This is especially useful when you need different ways to initialize an object.

The syntax uses the class name followed by a dot and the constructor name:

class Point {
  int x;
  int y;

  // Regular constructor
  Point(this.x, this.y);

  // Named constructor for origin point
  Point.origin() : x = 0, y = 0;

  // Named constructor from a single value
  Point.diagonal(int value) : x = value, y = value;
}

Each named constructor provides a clear, readable way to create objects:

Point p1 = Point(3, 5);
Point p2 = Point.origin();
Point p3 = Point.diagonal(10);

print('p1: (${p1.x}, ${p1.y})'); // p1: (3, 5)
print('p2: (${p2.x}, ${p2.y})'); // p2: (0, 0)
print('p3: (${p3.x}, ${p3.y})'); // p3: (10, 10)

Named constructors make your code more expressive. Instead of guessing what Point(0, 0) means, Point.origin() immediately communicates the intent. This pattern is common in Dart libraries and helps create self-documenting code.

Challenge

Easy

Let's build a temperature converter that showcases the power of named constructors. You'll create a Temperature class that can be initialized in different ways depending on the unit you're starting with.

You'll organize your code into two files:

temperature.dart: Define a Temperature class with a double celsius instance variable. Your class should provide three ways to create a temperature:
- A regular constructor Temperature(this.celsius) that takes a Celsius value directly
- A named constructor Temperature.fromFahrenheit that accepts a Fahrenheit value and converts it to Celsius using the formula: (fahrenheit - 32) * 5 / 9
- A named constructor Temperature.freezing() that creates a temperature at the freezing point (0 degrees Celsius)
Add a display() method that prints: [celsius]C
main.dart: Import your temperature class and create three temperature objects:
- One using the regular constructor with 25.0
- One using fromFahrenheit with 98.6
- One using freezing()
Call display() on each temperature in the order listed above.

Expected output format:

25.0C
37.0C
0.0C

Cheat sheet

Named constructors allow you to create multiple constructors for a class with descriptive names that clarify their purpose.

The syntax uses the class name followed by a dot and the constructor name:

class Point {
  int x;
  int y;

  // Regular constructor
  Point(this.x, this.y);

  // Named constructor for origin point
  Point.origin() : x = 0, y = 0;

  // Named constructor from a single value
  Point.diagonal(int value) : x = value, y = value;
}

Using named constructors:

Point p1 = Point(3, 5);
Point p2 = Point.origin();
Point p3 = Point.diagonal(10);

print('p1: (${p1.x}, ${p1.y})'); // p1: (3, 5)
print('p2: (${p2.x}, ${p2.y})'); // p2: (0, 0)
print('p3: (${p3.x}, ${p3.y})'); // p3: (10, 10)

Named constructors make code more expressive and self-documenting by clearly communicating intent.

Try it yourself

import 'temperature.dart';

void main() {
  // TODO: Create a Temperature using the regular constructor with 25.0
  
  // TODO: Create a Temperature using fromFahrenheit with 98.6
  
  // TODO: Create a Temperature using freezing()
  
  // TODO: Call display() on each temperature in order
}