Base class and subclass in Swift - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a base class named Vehicle.

Swift

class [1] {
    var speed: Int = 0
}

Drag options to blanks, or click blank then click option'

ACar

BTransport

CBike

DVehicle

Attempts:

3 left

💡 Hint

Common Mistakes

Using a subclass name like Car instead of the base class name Vehicle.

The base class is named Vehicle. This is the main class from which others can inherit.

2fill in blank

medium

Complete the code to declare a subclass named Car that inherits from Vehicle.

Swift

class Car: [1] {
    var brand: String = ""
}

Drag options to blanks, or click blank then click option'

AVehicle

BMachine

CBike

DTransport

Attempts:

3 left

💡 Hint

Common Mistakes

Using a wrong class name after the colon that is not the base class.

The subclass Car inherits from the base class Vehicle using a colon :.

3fill in blank

hard

Fix the error in the subclass initializer to call the base class initializer.

Swift

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

class Car: Vehicle {
    var brand: String
    init(brand: String, speed: Int) {
        self.brand = brand
        [1]
    }
}

Drag options to blanks, or click blank then click option'

Aself.init(speed: speed)

BVehicle.init(speed)

Csuper.init(speed: speed)

Dinit(speed)

Attempts:

3 left

💡 Hint

Common Mistakes

Trying to call the base class initializer without super.

Calling self.init inside the initializer causing recursion.

In Swift, the subclass must call the base class initializer using super.init(...) to properly initialize inherited properties.

4fill in blank

hard

Fill both blanks to override the base class method and add a new method in the subclass.

Swift

class Vehicle {
    func description() -> String {
        return "Vehicle moving at speed 0"
    }
}

class Car: Vehicle {
    override func [1]() -> String {
        return "Car moving at speed 0"
    }
    func [2]() -> String {
        return "This is a car"
    }
}

Drag options to blanks, or click blank then click option'

Adescription

Binfo

Cdetails

Dsummary

Attempts:

3 left

💡 Hint

Common Mistakes

Using a different name for the overridden method.

Forgetting the override keyword.

The method description is overridden in the subclass using override. The new method info is added to provide extra information.

5fill in blank

hard

Fill all three blanks to create a subclass that overrides a method and calls the base class method inside it.

Swift

class Vehicle {
    func move() -> String {
        return "Vehicle is moving"
    }
}

class Bike: Vehicle {
    override func [1]() -> String {
        let baseMessage = super.[2]()
        return baseMessage + ", Bike is moving fast"
    }
    func [3]() -> String {
        return "Bike details"
    }
}

Drag options to blanks, or click blank then click option'

Amove

Cdetails

Dinfo

Attempts:

3 left

💡 Hint

Common Mistakes

Not calling the base class method inside the override.

Using different method names for override.

The method move is overridden and calls the base class move using super.move(). A new method details is added.

Practice

1. What is the main purpose of a base class in Swift?

easy

A. To prevent other classes from inheriting

B. To provide a general template for other classes to build upon

C. To store data only without any functions

D. To create instances that cannot be used

Solution

1. Step 1: Understand the role of a base class

   A base class defines common properties and methods that other classes can reuse.

2. Step 2: Compare options with this role

   To provide a general template for other classes to build upon correctly states the base class is a general template. Other options describe incorrect or unrelated roles.

3. Final Answer:

   To provide a general template for other classes to build upon -> Option B

4. Quick Check:

   Base class = general template [OK]

Hint: Base class = general template for subclasses [OK]

Common Mistakes:

• Thinking base class cannot be inherited
• Confusing base class with instances
• Assuming base class only stores data

2. Which of the following is the correct way to declare a subclass named Dog inheriting from a base class Animal in Swift?

easy

A. class Dog inherits Animal { }

B. class Dog extends Animal { }

C. class Dog: Animal { }

D. class Dog < Animal { }

Solution

1. Step 1: Recall Swift subclass syntax

   In Swift, a subclass is declared using a colon (:) after the subclass name followed by the base class name.

2. Step 2: Match options to syntax

   class Dog: Animal { } uses class Dog: Animal { }, which is correct. Other options use incorrect keywords or symbols.

3. Final Answer:

   class Dog: Animal { } -> Option C

4. Quick Check:

   Subclass syntax = colon (:) [OK]

Hint: Use colon (:) to inherit in Swift class declaration [OK]

Common Mistakes:

• Using 'inherits' or 'extends' keywords (not Swift)
• Using wrong symbols like <
• Omitting the colon between subclass and base class

3. Consider this Swift code:

class Vehicle {
    func sound() -> String {
        return "Vroom"
    }
}

class Car: Vehicle {
    override func sound() -> String {
        return "Beep"
    }
}

let myCar = Car()
print(myCar.sound())

What will be printed when this code runs?

medium

A. "Beep"

B. "Vroom"

C. Compilation error

D. "Car"

Solution

1. Step 1: Understand method overriding in subclass

   The subclass Car overrides the sound() method to return "Beep" instead of the base class "Vroom".

2. Step 2: Identify which method is called

   When calling myCar.sound(), the overridden method in Car runs, returning "Beep".

3. Final Answer:

   "Beep" -> Option A

4. Quick Check:

   Overridden method runs = "Beep" [OK]

Hint: Overridden methods in subclass replace base methods [OK]

Common Mistakes:

• Expecting base class method output
• Confusing method override with overload
• Thinking print shows class name

4. Identify the error in this Swift subclass declaration:

class Animal {
    func speak() {
        print("Animal sound")
    }
}

class Cat: Animal {
    func speak() {
        print("Meow")
    }
}

medium

A. Class names must be lowercase

B. Base class cannot have methods

C. Subclass cannot have methods with same name

D. Missing override keyword before speak() in subclass

Solution

1. Step 1: Check method overriding rules in Swift

   When a subclass provides its own version of a base class method, it must use the override keyword.

2. Step 2: Identify missing override keyword

   The speak() method in Cat lacks override, causing a compile error.

3. Final Answer:

   Missing override keyword before speak() in subclass -> Option D

4. Quick Check:

   Override keyword required for method override [OK]

Hint: Use override keyword when redefining base methods [OK]

Common Mistakes:

• Forgetting override keyword
• Thinking subclass methods cannot have same name
• Assuming class names must be lowercase

5. You want to create a base class Employee with a property name and a method work() that prints "Working". Then create a subclass Manager that overrides work() to print "Managing team" and adds a new method holdMeeting() that prints "Meeting started".

Which code correctly implements this?

hard

A. class Employee {
var name: String
init(name: String) {
self.name = name
}
func work() {
print("Working")
}
}

class Manager: Employee {
override func work() {
print("Managing team")
}
func holdMeeting() {
print("Meeting started")
}
}

B. class Employee {
var name: String
func work() {
print("Working")
}
}

class Manager: Employee {
func work() {
print("Managing team")
}
func holdMeeting() {
print("Meeting started")
}
}

C. class Employee {
var name: String
init(name: String) {
self.name = name
}
func work() {
print("Working")
}
}

class Manager: Employee {
func work() {
print("Managing team")
}
func holdMeeting() {
print("Meeting started")
}
}

D. class Employee {
var name: String
init(name: String) {
self.name = name
}
func work() {
print("Working")
}
}

class Manager: Employee {
override func work() {
print("Managing team")
}
}

Solution

1. Step 1: Check base class implementation

   The base class Employee must have a name property with initializer and a work() method printing "Working". class Employee { var name: String init(name: String) { self.name = name } func work() { print("Working") } } class Manager: Employee { override func work() { print("Managing team") } func holdMeeting() { print("Meeting started") } } and C have correct base class with initializer.

2. Step 2: Check subclass overrides and new method

   The subclass Manager must override work() with override keyword and add holdMeeting(). class Employee { var name: String init(name: String) { self.name = name } func work() { print("Working") } } class Manager: Employee { override func work() { print("Managing team") } func holdMeeting() { print("Meeting started") } } correctly uses override and adds holdMeeting(). class Employee { var name: String init(name: String) { self.name = name } func work() { print("Working") } } class Manager: Employee { func work() { print("Managing team") } func holdMeeting() { print("Meeting started") } } misses override. class Employee { var name: String func work() { print("Working") } } class Manager: Employee { func work() { print("Managing team") } func holdMeeting() { print("Meeting started") } } misses initializer in base class. class Employee { var name: String init(name: String) { self.name = name } func work() { print("Working") } } class Manager: Employee { override func work() { print("Managing team") } } misses holdMeeting().

3. Final Answer:

   Option A code correctly implements all requirements -> Option A

4. Quick Check:

   Override + new method + initializer = class Employee { var name: String init(name: String) { self.name = name } func work() { print("Working") } } class Manager: Employee { override func work() { print("Managing team") } func holdMeeting() { print("Meeting started") } } [OK]

Hint: Use override keyword and add new methods in subclass [OK]

Common Mistakes:

• Forgetting override keyword in subclass
• Missing initializer in base class
• Not adding new methods in subclass