Overriding methods and properties in Swift - Mini Project: Build & Apply

Overriding methods and properties

📖 Scenario: You are creating a simple app to manage different types of vehicles. Each vehicle has a basic way to describe itself, but some vehicles have special descriptions.

🎯 Goal: Build a Swift program that defines a base class Vehicle with a method and a property, then create a subclass Car that overrides the method and property to provide a custom description.

📋 What You'll Learn

Create a class called Vehicle with a method description() that returns a String.

Add a property type in Vehicle that returns a String.

Create a subclass called Car that inherits from Vehicle.

Override the description() method in Car to return a custom string.

Override the type property in Car to return a different string.

Print the results of calling description() and accessing type on an instance of Car.

💡 Why This Matters

🌍 Real World

Overriding methods and properties is common when you want to create specialized versions of general objects, like different types of vehicles in an app.

💼 Career

Understanding overriding is essential for iOS developers to customize behavior in subclasses and build flexible, reusable code.

1

Create the base class Vehicle

Create a class called Vehicle with a method description() that returns the string "This is a vehicle" and a computed property type that returns the string "Generic Vehicle".

Swift

# Create class Vehicle with method description() and property type
# Your code here

Hint

Use class Vehicle {} to define the class. Inside, add func description() -> String and a computed property var type: String.

2

Create subclass Car and override property

Create a subclass called Car that inherits from Vehicle. Override the computed property type to return the string "Car".

Swift

class Vehicle {
    func description() -> String {
        return "This is a vehicle"
    }
    var type: String {
        return "Generic Vehicle"
    }
}

# Create subclass Car that overrides property type
# Your code here

Hint

Use class Car: Vehicle {} to create the subclass. Use override var type: String to change the property.

3

Override the description() method in Car

In the Car class, override the method description() to return the string "This is a car".

Swift

class Vehicle {
    func description() -> String {
        return "This is a vehicle"
    }
    var type: String {
        return "Generic Vehicle"
    }
}

class Car: Vehicle {
    override var type: String {
        return "Car"
    }
    
    # Override description() method here
    # Your code here
}

Hint

Inside Car, write override func description() -> String and return the new string.

4

Create Car instance and print results

Create an instance of Car called myCar. Print the result of calling myCar.description() and then print myCar.type.

Swift

class Vehicle {
    func description() -> String {
        return "This is a vehicle"
    }
    var type: String {
        return "Generic Vehicle"
    }
}

class Car: Vehicle {
    override var type: String {
        return "Car"
    }
    
    override func description() -> String {
        return "This is a car"
    }
}

# Create instance myCar and print description() and type
# Your code here

Hint

Create let myCar = Car(). Use print(myCar.description()) and print(myCar.type) to show the results.

Practice

1.

What keyword must you use in Swift to change a method from a parent class in a subclass?

easy

A. override

B. super

C. extend

D. inherit

Solution

1. Step 1: Understand method overriding in Swift

   Swift requires a special keyword to indicate a method is being changed in a subclass.

2. Step 2: Identify the correct keyword

   The keyword to override a method or property is override.

3. Final Answer:

   override -> Option A

4. Quick Check:

   Keyword to change parent method = override [OK]

Hint: Use 'override' before methods to change them in subclasses [OK]

Common Mistakes:

• Using 'super' instead of 'override'
• Forgetting the 'override' keyword
• Using 'extend' which is not a Swift keyword

2.

Which of the following is the correct syntax to override a method named greet() in Swift?

class Parent {
    func greet() {
        print("Hello")
    }
}

class Child: Parent {
    ??? func greet() {
        print("Hi")
    }
}

easy

A. override

B. super

C. func override

D. override func

Solution

1. Step 1: Recall Swift override syntax

   In Swift, the keyword override comes before the func keyword.

2. Step 2: Match the correct syntax

   The correct syntax is override func greet().

3. Final Answer:

   override func -> Option D

4. Quick Check:

   Keyword order is 'override func' [OK]

Hint: Write 'override' before 'func' to override methods [OK]

Common Mistakes:

• Writing 'func override' instead of 'override func'
• Using 'super' keyword instead of 'override'
• Omitting 'override' keyword entirely

3.

What will be the output of the following Swift code?

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

class Dog: Animal {
    override func sound() {
        print("Bark")
    }
}

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

medium

A. Some sound

B. Bark

C. Compile error

D. Runtime error

Solution

1. Step 1: Understand method overriding and polymorphism

   Even though the variable type is Animal, the actual object is Dog, so overridden method runs.

2. Step 2: Identify which method is called

   The Dog's overridden sound() method prints "Bark".

3. Final Answer:

   Bark -> Option B

4. Quick Check:

   Overridden method runs on subclass instance [OK]

Hint: Overridden methods run on subclass instances even if typed as parent [OK]

Common Mistakes:

• Thinking parent method runs because variable type is Animal
• Expecting compile or runtime error
• Confusing method overriding with overloading

4.

Identify the error in this Swift code that tries to override a property:

class Vehicle {
    var speed: Int = 0
}

class Car: Vehicle {
    var speed: Int = 100
}

medium

A. Missing 'override' keyword before 'var speed' in subclass

B. Cannot override properties in Swift

C. Property types must differ when overriding

D. Superclass property must be 'let' to override

Solution

1. Step 1: Check property overriding rules

   To override a property in Swift, you must use the override keyword.

2. Step 2: Identify missing keyword

   The subclass property speed lacks the override keyword, causing an error.

3. Final Answer:

   Missing 'override' keyword before 'var speed' in subclass -> Option A

4. Quick Check:

   Override properties need 'override' keyword [OK]

Hint: Always add 'override' before properties you change in subclass [OK]

Common Mistakes:

• Forgetting 'override' keyword on properties
• Thinking properties cannot be overridden
• Changing property type when overriding

5.

Given this Swift code, what will be printed?

class Person {
    var description: String {
        return "Person"
    }
}

class Student: Person {
    override var description: String {
        return super.description + " - Student"
    }
}

let s = Student()
print(s.description)

hard

A. Person

B. Student

C. Person - Student

D. Compile error due to super usage

Solution

1. Step 1: Understand property overriding with super

   The subclass overrides description and calls super.description to get parent value.

2. Step 2: Combine parent and subclass strings

   The returned string is "Person" + " - Student" = "Person - Student".

3. Final Answer:

   Person - Student -> Option C

4. Quick Check:

   super.property adds parent value [OK]

Hint: Use 'super' to include parent property value when overriding [OK]

Common Mistakes:

• Expecting only 'Student' without parent text
• Thinking 'super' causes compile error
• Not using 'override' keyword on property