Overriding methods and properties in Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of overridden method call

What is the output of this Swift code when calling dog.makeSound()?

Swift

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

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

let dog = Dog()
dog.makeSound()

A Some generic sound

B Bark

C Compilation error due to missing override keyword

D Runtime error

💡 Hint

Remember that the override keyword allows a subclass to provide its own implementation of a method.

The Dog class overrides the makeSound() method of Animal. When calling dog.makeSound(), the overridden method in Dog runs, printing "Bark".

❓ Predict Output

intermediate

Output of overridden computed property

What will be printed when accessing circle.description in this Swift code?

Swift

class Shape {
    var description: String {
        return "A shape"
    }
}

class Circle: Shape {
    override var description: String {
        return "A circle"
    }
}

let circle = Circle()
print(circle.description)

A A shape

B Runtime error

C Compilation error: cannot override stored property

D A circle

💡 Hint

Check how computed properties can be overridden with override var.

The Circle class overrides the computed property description from Shape. Accessing circle.description calls the overridden property, printing "A circle".

🔧 Debug

advanced

Identify the error in method overriding

Which option correctly explains the error in this Swift code?

Swift

class Vehicle {
    func startEngine() {
        print("Engine started")
    }
}

class Car: Vehicle {
    func startEngine() {
        print("Car engine started")
    }
}

A No error, code runs and prints "Car engine started"

B Runtime error due to method ambiguity

C Compilation error: missing 'override' keyword before 'startEngine()' in Car

D Compilation error: cannot override a method without 'final' keyword

💡 Hint

In Swift, overriding a method requires the override keyword.

The Car class tries to override startEngine() but misses the override keyword, causing a compilation error.

❓ Predict Output

advanced

Output of property observer in overridden property

What is the output when setting mySquare.sideLength = 10 in this Swift code?

Swift

class Square {
    var sideLength: Int = 0 {
        willSet {
            print("Square will set sideLength to \(newValue)")
        }
    }
}

class ColoredSquare: Square {
    override var sideLength: Int {
        get {
            super.sideLength
        }
        set {
            print("ColoredSquare will set sideLength to \(newValue)")
            super.sideLength = newValue
        }
    }
}

let mySquare = ColoredSquare()
mySquare.sideLength = 10

A ColoredSquare will set sideLength to 10

B Square will set sideLength to 10

C Both "Square will set sideLength to 10" and "ColoredSquare will set sideLength to 10"

D No output

💡 Hint

Overridden property observers replace the superclass observers.

The ColoredSquare overrides the property observer willSet. Only the overridden observer runs, printing "ColoredSquare will set sideLength to 10".

🧠 Conceptual

expert

Behavior of overriding a property with different types

Consider this Swift code. What error will occur and why?

Swift

class Parent {
    var value: Int {
        return 5
    }
}

class Child: Parent {
    override var value: String {
        return "Five"
    }
}

A Compilation error: Cannot override property with a different type

B Runtime error: Type mismatch when accessing value

C No error, Child's value returns "Five" as String

D Compilation error: Missing 'override' keyword

💡 Hint

Overriding properties must keep the same type in Swift.

Swift requires the overridden property to have the same type as the original. Changing from Int to String causes a compilation error.

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