in subclass, how we deal with the properties we didn't modified? Do we also need to initialize it?

Hey Jun!

In any Class, you need to initialize the properties you define. If you don't do it at the var/let declaration line, you get them through the init method. If you have a subclass with a new property initialized, you still have to initialize the pending properties from the superclass, but that's why you inherit the init method too, so you can use it to provide the value for them.

Here's a little example:

class Vehicle {
    let wheels: Int

    init(wheels: Int) {
        self.wheels = wheels
    }
}

class Car: Vehicle {
    let seats: Int = 4
}

let myCar = Car(wheels: 4)
myCar.seats  //this will return 4
myCar.wheels //this will return 4

myCar inherits wheels property and the Vehicle init method although it already has a seats property from his own. This time, in the subclass I don't need to modify that wheels property from the superclass since I'm already doing with my init method.

Hope this example is useful for you. Happy coding!

I like the answer above. The short answer is we don't need to initialize the superclass's variables in the subclass, because they are being inherited as is when initializing the subclass.

Hmmm I thought it was said that we had to define the init() on a class no matter what?

I agree with Stephen, you don't need to define the init() in a subclass unless you want the store properties that your subclass is inheriting from the superclass to be behave differently...for instance

class Person
{
    let firstName: String
    let lastName: String

    init(firstName: String, lastName: String)
    {
        self.firstName = firstName
        self.lastName = lastName
    }

    func fullName() -> String
    {
        return "\(firstName) \(lastName)"
    }
}


class Mr: Person
{
    override func fullName() -> String
    {
        return "Mr." + " " + lastName
    }
}

Above is the superclass called Person, and a subclass called Mr. As you see, the superclass has an init method but the subclass does not. The subclass doesn't have an init method because I don't want to change anything about how the stored properties behave--I want firstName and lastName to function like how they function in the superclass. However, I do want change how the fullName method behaves....

let somePerson = Person(firstName: "Jorge", lastName: "Solana")
somePerson.fullName()

let someMister = Mr(firstName: "Jorge", lastName: "Solana")
someMister.fullName()

As you will see in Xcode, if you call the fullName method from both classes, you will notice that they both display the full name of someone differently. The subclass fullName method performs differently I had it override the superclass's fullName method and return something else