Calling super for parent behavior in Swift - Mini Project: Build & Apply

Calling super for parent behavior

📖 Scenario: Imagine you are creating a simple app with different types of vehicles. Each vehicle can start its engine, but some vehicles have extra steps when starting. You will learn how to use super to call the parent class behavior and add extra steps in child classes.

🎯 Goal: You will build a small Swift program with a parent class Vehicle and a child class Car. You will override a method in the child class and call the parent method using super to keep the original behavior and add new behavior.

📋 What You'll Learn

Create a class called Vehicle with a method startEngine() that prints "Engine started".

Create a subclass called Car that inherits from Vehicle.

Override the startEngine() method in Car and call the parent method using super.startEngine().

Add an extra print statement in Car's startEngine() method that prints "Car is ready to go!".

Create an instance of Car and call its startEngine() method to see both messages.

💡 Why This Matters

🌍 Real World

In real apps, many classes inherit from others and override methods to add or change behavior. Using <code>super</code> helps keep the original behavior while extending it.

💼 Career

Understanding inheritance and calling parent methods is essential for Swift developers working on iOS apps, especially when working with UIKit or SwiftUI where subclassing is common.

1

Create the Vehicle class

Create a class called Vehicle with a method startEngine() that prints exactly "Engine started".

Swift

// Create the Vehicle class with startEngine() method
// Your code here

Hint

Use class Vehicle {} and inside it define func startEngine() that prints the message.

2

Create the Car subclass

Create a subclass called Car that inherits from Vehicle.

Swift

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

// Create the Car subclass inheriting from Vehicle
// Your code here

Hint

Use class Car: Vehicle {} to create the subclass.

3

Override startEngine() and call super

In the Car class, override the startEngine() method. Inside it, call the parent method using super.startEngine() and then print exactly "Car is ready to go!".

Swift

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

class Car: Vehicle {
    // Override startEngine() here
    // Your code here
}

Hint

Use override func startEngine() and inside call super.startEngine() before printing the extra message.

4

Create Car instance and call startEngine()

Create an instance of Car called myCar and call its startEngine() method to print both messages.

Swift

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

class Car: Vehicle {
    override func startEngine() {
        super.startEngine()
        print("Car is ready to go!")
    }
}

// Create myCar instance and call startEngine()
// Your code here

Hint

Create let myCar = Car() and then call myCar.startEngine().

Practice

1. What does calling super.methodName() inside an overridden method in Swift do?

easy

A. It runs the parent class's version of the method.

B. It creates a new method unrelated to the parent.

C. It deletes the parent class's method.

D. It calls a method from a sibling class.

Solution

1. Step 1: Understand method overriding

   When a subclass overrides a method, it replaces the parent's method behavior.

2. Step 2: Role of super

   Calling super.methodName() runs the original method from the parent class, preserving its behavior.

3. Final Answer:

   It runs the parent class's version of the method. -> Option A

4. Quick Check:

   Calling super runs parent method [OK]

Hint: Remember: super calls parent method inside override [OK]

Common Mistakes:

• Thinking super creates a new method
• Believing super deletes parent methods
• Confusing super with sibling class calls

2. Which of the following is the correct syntax to call a parent class's method named display() inside an overridden method in Swift?

easy

A. super.display()

B. parent.display()

C. self.super.display()

D. super::display()

Solution

1. Step 1: Identify Swift syntax for super calls

   Swift uses the keyword super followed by dot and method name to call parent methods.

2. Step 2: Check each option

   Only super.display() matches Swift syntax. Others are invalid or from other languages.

3. Final Answer:

   super.display() -> Option A

4. Quick Check:

   super.methodName() is correct syntax [OK]

Hint: Use super.methodName() to call parent method [OK]

Common Mistakes:

• Using parent instead of super
• Adding self before super
• Using :: like in C++

3. Consider this Swift code:

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

class Child: Parent {
    override func greet() {
        print("Child starts")
        super.greet()
        print("Child ends")
    }
}

let c = Child()
c.greet()

What is the output when c.greet() is called?

medium

A. Child starts\nChild ends\nHello from Parent

B. Child starts\nHello from Parent\nChild ends

C. Hello from Parent

D. Hello from Parent\nChild starts\nChild ends

Solution

1. Step 1: Trace the overridden method

   The greet() method in Child prints "Child starts", then calls super.greet() which prints "Hello from Parent", then prints "Child ends".

2. Step 2: Combine outputs in order

   Output lines are: "Child starts", "Hello from Parent", "Child ends" in that order.

3. Final Answer:

   Child starts Hello from Parent Child ends -> Option B

4. Quick Check:

   super call runs parent method in middle [OK]

Hint: Follow print order: before, super call, after [OK]

Common Mistakes:

• Ignoring order of prints
• Assuming super call is last
• Mixing output lines

4. This Swift code tries to override a method but causes an error:

class Parent {
    func show() {
        print("Parent show")
    }
}

class Child: Parent {
    override func show() {
        print("Child show")
        show()  // Line X
    }
}

What is the problem and how to fix it?

medium

A. Missing super keyword before override.

B. Method name mismatch; rename show() in Child.

C. Cannot override non-virtual method; remove override keyword.

D. Line X causes infinite recursion; fix by calling super.show() instead.

Solution

1. Step 1: Identify recursion cause

   Inside Child.show(), calling show() calls itself again, causing infinite recursion.

2. Step 2: Correct call to parent method

   Use super.show() to call the parent class's method and avoid recursion.

3. Final Answer:

   Line X causes infinite recursion; fix by calling super.show() instead. -> Option D

4. Quick Check:

   Call super.method() to avoid recursion [OK]

Hint: Use super.method() inside override to avoid recursion [OK]

Common Mistakes:

• Calling method without super inside override
• Removing override keyword incorrectly
• Changing method name unnecessarily

5. You have a class hierarchy where Grandparent has a method log(), Parent overrides it and calls super.log(), and Child overrides it too. You want Child.log() to add its own message, then call Parent.log(), which calls Grandparent.log(). How should you write Child.log() to achieve this?

hard

A. override func log() { print("Child log") }

B. override func log() { super.log() ; print("Child log") }

C. override func log() { print("Child log") ; super.log() }

D. override func log() { Parent.log() ; print("Child log") }

Solution

1. Step 1: Understand call chain

   Calling super.log() in Child calls Parent.log(), which calls Grandparent.log().

2. Step 2: Add Child's message before calling super

   To add Child's message first, print it, then call super.log().

3. Final Answer:

   override func log() { print("Child log") ; super.log() } -> Option C

4. Quick Check:

   Print then super.log() for correct order [OK]

Hint: Print first, then call super to keep parent behavior [OK]

Common Mistakes:

• Calling super before print changes order
• Not calling super loses parent logs
• Trying to call Parent.log() directly (not allowed)