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iOS Object-Oriented Swift 2.0 Class Inheritance Creating a Subclass

Posted by Ary de Oliveira
Ary de Oliveira
28,298 Points

Swift 2.0

In the editor, I have provided a class named Vehicle. 
Your task is to create a subclass of Vehicle, named Car, that adds an additional stored property numberOfSeats of type Int with a default value of 4. 
Once you've implemented the Car class, create an instance and assign it to a constant named someCar.

I tRY hARD BUT NO...

classes.swift 
class Vehicle {
    var numberOfDoors: Int
    var numberOfWheels: Int

    init(withDoors doors: Int, andWheels wheels: Int) {
        self.numberOfDoors = doors
        self.numberOfWheels = wheels
    }
}

class VehicleCar {
let numberOfSeats: Int 
let: someCar
}

// Enter your code below

3 Answers

Steve Hunter
Steve Hunter
57,712 Points

Hi Ary,

Here's how that would look:

class Car: Vehicle {
  var numberOfSeats: Int 

  override init(withDoors doors: Int, andWheels wheels: Int){
      self.numberOfSeats = 4
      super.init(withDoors: doors, andWheels: wheels)
  }
}

let someCar = Car(withDoors: 4, andWheels: 4)

We start by declaring that Car is a sub-class of Vehicle. We add its unique property, the seats integer, then create the init method. This sets the unique property first (default value of 4 required), then calls the init method of the super class, Vehicle.

Make sense?

Steve.

Gregory Adams
Gregory Adams
7,416 Points

Hi Steve,

I had nearly the same code as you, but the super.init was above the self.numberOfSeats attribution. I received the following 'error: property 'self.numberOfSeats' not initialized at super.init call'

Do you know why the order in the overriding init call matters?

Steve Hunter
Steve Hunter
57,712 Points
Steve Hunter
Steve Hunter
57,712 Points

Dominic,

Yep, this works; my presumed errors were "correct", so here's my attempt at solving your requirement:

class Car: Vehicle {
    var numberOfSeats: Int

    init(withDoors doors: Int, andWheels wheels: Int, andSeats seats: Int){
        self.numberOfSeats = seats
        super.init(withDoors: doors, andWheels: wheels)
    }
}

let someCar = Car(withDoors: 4, andWheels: 4, andSeats: 4)

Steve.

Dominic Bloor
Dominic Bloor
7,857 Points
Dominic Bloor
Dominic Bloor
7,857 Points

Thanks so much for your help Steve!!!!!! :-)

Steve Hunter
Steve Hunter
57,712 Points

Hey, no problem! :+1:

Steve Hunter
Steve Hunter
57,712 Points
Steve Hunter
Steve Hunter
57,712 Points

Hi Gregory,

I think the call to super needs to be last, either explicitly or otherwise, as on completing that chain (there could be numerous super class chains) the instance must be complete. Execution shouldn't continue so you do your 'local' setup work, then pass the construction up the chain of super.

That's my understanding of it anyway!

Steve.

Dominic Bloor
Dominic Bloor
7,857 Points
Dominic Bloor
Dominic Bloor
7,857 Points

Hi Steve

If we wanted the NumberOfSeats to come up as a parameter in a new instance, rather than being given a default of 4...

e.g. let someCar = Car(withDoors: X, andWheels: X, andSeats: X)

How would we go about writing the syntax for the initializer?

Apologies if it is really obvious! Just can't get my head round it!!!

Steve Hunter
Steve Hunter
57,712 Points

Hi Dominic,

I've not touched Swift in ages ... let me do some complete guesswork while I get my unused Xcode working!

I think you should pass the value you want for numberOfSeats in with the initializer. Something like this - although I'm pretty sure this doesn't need the override keyword as it isn't an override? I'll test it all in a bit when the updates have destroyed my bandwidth! ... 

  override init(numberOfSeats: Int, withDoors doors: Int, andWheels wheels: Int){
      self.numberOfSeats = numberOfSeats
      super.init(withDoors: doors, andWheels: wheels)
  }

There's probably lots wrong with that (override and parameter names/labels for starters) ... I shall post again when I've got back into Swift mode and got Xcode running.

Steve.