Inner classes and nested classes in Kotlin - Mini Project: Build & Apply

Inner classes and nested classes

📖 Scenario: You are creating a simple Kotlin program to understand how inner classes and nested classes work. Imagine you have a Car that has an Engine. The Engine can be represented as an inner class because it needs to access the Car details. You also want to have a CarInfo nested class that just holds some static information about cars.

🎯 Goal: Build a Kotlin program that defines a Car class with an Engine inner class and a CarInfo nested class. Then create objects and print details showing how inner and nested classes work.

📋 What You'll Learn

Create a class called Car with a property brand of type String set to "Toyota"

Inside Car, create an inner class called Engine with a function start() that returns a string "Engine of [brand] started"

Inside Car, create a nested class called CarInfo with a function info() that returns the string "Cars are vehicles"

Create an instance of Car, then create an instance of Engine from that car, and call start()

Call the info() function from the CarInfo nested class

Print the results of both function calls

💡 Why This Matters

🌍 Real World

Inner and nested classes help organize code that logically belongs together, like parts of a car. This keeps code clean and easier to maintain.

💼 Career

Understanding inner and nested classes is important for Kotlin developers working on Android apps or backend services, where encapsulation and code organization matter.

1

Create the Car class with brand property

Create a class called Car with a property brand of type String set to "Toyota".

Kotlin

class Car {
    // Your code here
}

Hint

Use val brand: String = "Toyota" inside the Car class.

2

Add inner Engine class with start() function

Inside the Car class, create an inner class called Engine with a function start() that returns the string "Engine of [brand] started" using the brand property.

Kotlin

class Car {
    val brand: String = "Toyota"

    // Your code here
}

Hint

Remember to use the inner keyword so Engine can access brand.

3

Add nested CarInfo class with info() function

Inside the Car class, create a nested class called CarInfo with a function info() that returns the string "Cars are vehicles".

Kotlin

class Car {
    val brand: String = "Toyota"

    inner class Engine {
        fun start(): String {
            return "Engine of $brand started"
        }
    }

    // Your code here
}

Hint

Use a normal nested class (no inner) for CarInfo.

4

Create instances and print results

Create an instance of Car called myCar. Then create an instance of Engine from myCar called myEngine and call start(). Also call the info() function from the CarInfo nested class. Finally, print both results.

Kotlin

class Car {
    val brand: String = "Toyota"

    inner class Engine {
        fun start(): String {
            return "Engine of $brand started"
        }
    }

    class CarInfo {
        fun info(): String {
            return "Cars are vehicles"
        }
    }
}

// Your code here

Hint

Remember to create Engine instance from myCar and call start(). Call info() from Car.CarInfo().

Practice

1. What is the main difference between a nested class and an inner class in Kotlin?

easy

A. An inner class can access members of the outer class, but a nested class cannot.

B. A nested class can access members of the outer class, but an inner class cannot.

C. Both inner and nested classes can access outer class members equally.

D. Inner classes are static by default, nested classes are not.

Solution

1. Step 1: Understand nested class behavior

   Nested classes are static by default and cannot access outer class members directly.

2. Step 2: Understand inner class behavior

   Inner classes use the inner keyword and can access outer class members.

3. Final Answer:

   An inner class can access members of the outer class, but a nested class cannot. -> Option A

4. Quick Check:

   Inner class = access outer members [OK]

Hint: Inner classes use 'inner' keyword to access outer class [OK]

Common Mistakes:

• Thinking nested classes can access outer members
• Confusing inner classes as static
• Assuming both classes behave the same

2. Which of the following is the correct syntax to declare an inner class in Kotlin?

easy

A. class Outer { class Inner { } }

B. class Outer { open class Inner { } }

C. class Outer { inner class Inner { } }

D. class Outer { static class Inner { } }

Solution

1. Step 1: Recall inner class syntax

   In Kotlin, inner classes require the inner keyword to access outer class members.

2. Step 2: Check options for correct keyword

   class Outer { inner class Inner { } } uses inner class Inner, which is the correct syntax.

3. Final Answer:

   class Outer { inner class Inner { } } -> Option C

4. Quick Check:

   Use 'inner' keyword for inner classes [OK]

Hint: Use 'inner' keyword to declare inner classes [OK]

Common Mistakes:

• Omitting 'inner' keyword for inner classes
• Using 'static' keyword which is invalid in Kotlin
• Confusing 'open' with 'inner'

3. What will be the output of the following Kotlin code?

class Outer(val name: String) {
    inner class Inner {
        fun greet() = "Hello, $name"
    }
}

fun main() {
    val outer = Outer("Kotlin")
    val inner = outer.Inner()
    println(inner.greet())
}

medium

A. Hello, null

B. Hello, Kotlin

C. Compilation error

D. Hello,

Solution

1. Step 1: Understand inner class access

   The inner class can access the outer class property name because it is declared with inner.

2. Step 2: Trace the code execution

   Creating outer with name "Kotlin" and then inner instance calls greet() which returns "Hello, Kotlin".

3. Final Answer:

   Hello, Kotlin -> Option B

4. Quick Check:

   Inner class accesses outer property correctly [OK]

Hint: Inner classes access outer properties directly [OK]

Common Mistakes:

• Assuming inner class cannot access outer properties
• Expecting null or empty output
• Thinking code causes compilation error

4. Identify the error in the following Kotlin code snippet:

class Outer {
    class Nested {
        fun printMessage() = println("Hello from Nested")
    }
}

fun main() {
    val nested = Outer.Nested()
    nested.printMessage()
    println(nested.printMessage())
}

medium

A. Calling println on a function that returns Unit causes an error.

B. Nested class cannot be instantiated without an Outer instance.

C. Nested class must be declared with 'inner' keyword to access printMessage.

D. No errors; code runs and prints correctly.

Solution

1. Step 1: Check nested class instantiation

   Nested classes are static by default and can be instantiated without an Outer instance, so no error here.

2. Step 2: Analyze println usage

   The function printMessage() prints a message and returns Unit. Calling println(nested.printMessage()) prints the message and then prints kotlin.Unit, which is allowed.

3. Step 3: Confirm no errors

   There is no compilation or runtime error; the code runs and produces output as written.

4. Final Answer:

   No errors; code runs and prints correctly. -> Option D

5. Quick Check:

   Nested class instantiation and Unit printing both OK [OK]

Hint: Nested classes don't require outer instance [OK]

Common Mistakes:

• Thinking nested class needs Outer instance
• Expecting compilation error on nested class
• Ignoring Unit return type printing

5. Given the following Kotlin code, what will be the output?

class Outer(val x: Int) {
    inner class Inner {
        fun multiply(y: Int) = x * y
    }
    class Nested {
        fun multiply(y: Int) = y * 2
    }
}

fun main() {
    val outer = Outer(5)
    val inner = outer.Inner()
    val nested = Outer.Nested()
    println(inner.multiply(3))
    println(nested.multiply(3))
}

hard

A. 15
6

B. 8
6

C. 15
9

D. Compilation error due to nested class

Solution

1. Step 1: Calculate inner class multiply

   Inner class accesses outer property x=5, so multiply(3) returns 5*3=15.

2. Step 2: Calculate nested class multiply

   Nested class method multiplies input by 2, so multiply(3) returns 6.

3. Step 3: Confirm output lines

   Printing inner.multiply(3) prints 15, printing nested.multiply(3) prints 6 on next line.

4. Final Answer:

   15 6 -> Option A

5. Quick Check:

   Inner uses outer x, nested doubles input [OK]

Hint: Inner uses outer data; nested does not [OK]

Common Mistakes:

• Assuming nested class can access outer x
• Mixing up multiply results
• Expecting compilation error