Base class and subclass in Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of method overriding in subclass

What is the output of this Swift code?

Swift

class Animal {
    func sound() -> String {
        return "Some sound"
    }
}

class Dog: Animal {
    override func sound() -> String {
        return "Bark"
    }
}

let pet: Animal = Dog()
print(pet.sound())

A Compilation error due to missing override

B Some sound

C Bark

D Runtime error

💡 Hint

Remember that subclass methods can replace base class methods when marked with override.

The Dog class overrides the sound() method of Animal. Even though the variable is typed as Animal, the actual object is a Dog, so sound() from Dog is called, printing "Bark".

❓ Predict Output

intermediate

Value of property after subclass initialization

What will be printed by this Swift code?

Swift

class Vehicle {
    var wheels: Int
    init(wheels: Int) {
        self.wheels = wheels
    }
}

class Bicycle: Vehicle {
    init() {
        super.init(wheels: 2)
    }
}

let bike = Bicycle()
print(bike.wheels)

A 2

B 0

C Compilation error due to missing initializer

D Runtime error

💡 Hint

Check how the subclass calls the base class initializer.

The Bicycle class calls super.init(wheels: 2) to set the wheels property to 2. So printing bike.wheels outputs 2.

🔧 Debug

advanced

Identify the error in subclass initializer

What error does this Swift code produce?

Swift

class Person {
    var name: String
    init(name: String) {
        self.name = name
    }
}

class Employee: Person {
    var id: Int
    init(id: Int) {
        self.id = id
    }
}

A Error: Missing call to super.init in subclass initializer

B Error: Property 'name' not initialized before use

C No error, code compiles fine

D Error: Cannot override stored property

💡 Hint

Subclass initializers must call the base class initializer to initialize inherited properties.

The Employee initializer sets id but does not call super.init(name:). This causes a compile-time error because the inherited property name is not initialized.

❓ Predict Output

advanced

Output when calling base class method from subclass

What is the output of this Swift code?

Swift

class Printer {
    func printMessage() {
        print("Base message")
    }
}

class CustomPrinter: Printer {
    override func printMessage() {
        super.printMessage()
        print("Custom message")
    }
}

let p = CustomPrinter()
p.printMessage()

A Compilation error due to super call

B Custom message

C Base message

D

Base message
Custom message

💡 Hint

Look at how super.printMessage() is used inside the subclass method.

The CustomPrinter overrides printMessage() but calls super.printMessage() first, printing "Base message" then "Custom message" on separate lines.

🧠 Conceptual

expert

Number of stored properties in subclass instance

Given these Swift classes, how many stored properties does an instance of Manager have?

class Employee {
    var name: String
    var salary: Double
    init(name: String, salary: Double) {
        self.name = name
        self.salary = salary
    }
}

class Manager: Employee {
    var department: String
    init(name: String, salary: Double, department: String) {
        self.department = department
        super.init(name: name, salary: salary)
    }
}

A 4

B 3

C 1

D 2

💡 Hint

Count all stored properties declared in base and subclass.

An instance of Manager has name and salary from Employee, plus department from Manager, totaling 3 stored properties.