Basic Inheritance

Part of the Object Oriented Programming section of Coddy's Dart journey — lesson 36 of 110.

Inheritance allows a class to acquire the properties and methods of another class. Instead of rewriting the same code, you create a new class that builds upon an existing one. The original class is called the parent (or superclass), and the new class is the child (or subclass).

In Dart, you use the extends keyword to inherit from a parent class:

class Animal {
  String name;
  
  Animal(this.name);
  
  void eat() {
    print('$name is eating');
  }
}

class Dog extends Animal {
  Dog(String name) : super(name);
  
  void bark() {
    print('$name says woof!');
  }
}

void main() {
  var dog = Dog('Buddy');
  dog.eat();   // Inherited from Animal
  dog.bark();  // Defined in Dog
}

The Dog class automatically has access to the name property and eat() method from Animal, plus its own bark() method. This creates an "is-a" relationship - a Dog is an Animal.

The child class can add new properties and methods, but it inherits everything public from the parent. Note that private members (those with _) are not directly accessible in subclasses from different libraries, though they still exist in the inherited object.

Challenge

Easy

Let's build a vehicle system that demonstrates how inheritance allows child classes to reuse and extend parent class functionality. You'll create a parent class for general vehicles and a child class for a specific type of vehicle.

You'll organize your code into two files:

vehicle.dart: Define your classes here:
- A Vehicle class (the parent) with a String brand property, a constructor that takes the brand, and a method start() that prints [brand] engine starting...
- A Motorcycle class that extends Vehicle. It should have an additional int cc property (engine displacement), a constructor that takes both the brand and cc (passing the brand to the parent using super), and its own method wheelie() that prints [brand] is doing a wheelie!
main.dart: Import your vehicle file and demonstrate inheritance in action:
- Create a Vehicle with brand 'Toyota'
- Call its start() method
- Create a Motorcycle with brand 'Harley' and cc 1200
- Call the motorcycle's start() method (inherited from Vehicle)
- Call the motorcycle's wheelie() method (defined in Motorcycle)
- Print Motorcycle engine: [cc]cc to show the child's own property

Notice how the Motorcycle class automatically gains access to the brand property and start() method from Vehicle, while also having its own unique cc property and wheelie() method. This is the power of inheritance - building upon existing code.

Expected output:

Toyota engine starting...
Harley engine starting...
Harley is doing a wheelie!
Motorcycle engine: 1200cc

Cheat sheet

Use the extends keyword to create a child class that inherits from a parent class:

class Animal {
  String name;
  
  Animal(this.name);
  
  void eat() {
    print('$name is eating');
  }
}

class Dog extends Animal {
  Dog(String name) : super(name);
  
  void bark() {
    print('$name says woof!');
  }
}

The child class inherits all public properties and methods from the parent class. Use super in the child constructor to pass parameters to the parent constructor.

Child classes can add their own properties and methods while still accessing inherited ones:

var dog = Dog('Buddy');
dog.eat();   // Inherited from Animal
dog.bark();  // Defined in Dog

This creates an "is-a" relationship - a Dog is an Animal.

Try it yourself

import 'vehicle.dart';

void main() {
  // TODO: Create a Vehicle with brand 'Toyota'
  
  // TODO: Call the vehicle's start() method
  
  // TODO: Create a Motorcycle with brand 'Harley' and cc 1200
  
  // TODO: Call the motorcycle's start() method (inherited from Vehicle)
  
  // TODO: Call the motorcycle's wheelie() method
  
  // TODO: Print 'Motorcycle engine: [cc]cc' using the motorcycle's cc property
}