Inner class access to outer members in Kotlin - Mini Project: Build & Apply

Inner class access to outer members

📖 Scenario: Imagine you are creating a simple app to manage a library. You want to keep track of a Book and its Details like title and author. The Details class is inside the Book class as an inner class.

🎯 Goal: You will create a Book class with an inner class Details. The inner class will access the outer class's properties to print the full book information.

📋 What You'll Learn

Create a class called Book with a property name of type String

Inside Book, create an inner class Details with a property author of type String

In Details, write a function printInfo() that prints the book name and author using the outer class's name property

Create an instance of Book and its inner class Details, then call printInfo() to show the output

💡 Why This Matters

🌍 Real World

Inner classes help organize related code together, like keeping book details inside a book class, making code easier to read and maintain.

💼 Career

Understanding inner classes is useful for Kotlin developers working on Android apps or backend services where encapsulation and clear structure are important.

1

Create the outer class with a property

Create a class called Book with a property name of type String and set it to "Kotlin Programming".

Kotlin

# Your code here

Hint

Use class Book { val name: String = "Kotlin Programming" } to create the class and property.

2

Add an inner class with a property

Inside the Book class, create an inner class Details with a property author of type String set to "John Doe".

Kotlin

class Book {
    val name: String = "Kotlin Programming"
    
    # Your code here
}

Hint

Use inner class Details { val author: String = "John Doe" } inside Book.

3

Add a function in inner class to print book info

In the Details inner class, add a function printInfo() that prints the book name and author using println. Access the outer class's name property with [e-mail].

Kotlin

class Book {
    val name: String = "Kotlin Programming"
    inner class Details {
        val author: String = "John Doe"
        
        # Your code here
    }
}

Hint

Use fun printInfo() { println("Book: ${[e-mail]}, Author: $author") } inside Details.

4

Create instances and call the function

Create an instance of Book called book. Then create an instance of the inner class Details called details from book. Finally, call details.printInfo() to display the book information.

Kotlin

class Book {
    val name: String = "Kotlin Programming"
    inner class Details {
        val author: String = "John Doe"
        fun printInfo() {
            println("Book: ${[e-mail]}, Author: $author")
        }
    }
}

# Your code here

Hint

Create val book = Book(), then val details = book.Details(), then call details.printInfo().

Practice

1. In Kotlin, what keyword must you use to allow an inner class to access members of its outer class?

easy

A. inner

B. outer

C. this

D. nested

Solution

1. Step 1: Understand inner class behavior

   By default, nested classes in Kotlin cannot access outer class members directly.

2. Step 2: Identify the keyword for access

   The keyword inner allows the nested class to hold a reference to the outer class and access its members.

3. Final Answer:

   inner -> Option A

4. Quick Check:

   Use inner for access [OK]

Hint: Remember: only 'inner' classes access outer members directly [OK]

Common Mistakes:

• Confusing 'inner' with 'nested' keyword
• Using 'this' instead of 'inner'
• Assuming all nested classes access outer members

2. Which of the following Kotlin class declarations correctly allows the inner class to access the outer class's property name?

easy

A. class Outer { val name = \"Kotlin\"; inner class Inner { fun printName() = println(name) } }

B. class Outer { val name = \"Kotlin\"; class Inner { fun printName() = println(name) } }

C. class Outer { val name = \"Kotlin\"; nested class Inner { fun printName() = println(name) } }

D. class Outer { val name = \"Kotlin\"; inner class Inner { fun printName() = println(this.name) } }

Solution

1. Step 1: Check class declaration syntax

   Only the class declared with inner keyword can access outer class properties directly.

2. Step 2: Analyze each option

   class Outer { val name = \"Kotlin\"; inner class Inner { fun printName() = println(name) } } uses inner and accesses name correctly. class Outer { val name = \"Kotlin\"; class Inner { fun printName() = println(name) } } lacks inner. class Outer { val name = \"Kotlin\"; nested class Inner { fun printName() = println(name) } } uses invalid nested keyword. class Outer { val name = \"Kotlin\"; inner class Inner { fun printName() = println(this.name) } } tries to access this.name which refers to inner class, not outer.

3. Final Answer:

   B -> Option A

4. Quick Check:

   Use inner and direct property name [OK]

Hint: Use 'inner' keyword and access outer property by name [OK]

Common Mistakes:

• Omitting 'inner' keyword
• Using 'nested' instead of 'inner'
• Using 'this.name' inside inner class incorrectly

3. What is the output of the following Kotlin code?

class Outer(val x: Int) {
    inner class Inner {
        fun printX() = println(x)
    }
}

fun main() {
    val outer = Outer(10)
    val inner = outer.Inner()
    inner.printX()
}

medium

A. Runtime exception

B. 0

C. Compilation error

D. 10

Solution

1. Step 1: Understand inner class access

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

2. Step 2: Trace the code execution

   An Outer object is created with x=10. The inner class instance calls printX() which prints x value 10.

3. Final Answer:

   10 -> Option D

4. Quick Check:

   Inner class prints outer property value [OK]

Hint: Inner class prints outer property directly if 'inner' keyword used [OK]

Common Mistakes:

• Assuming inner class cannot access outer property
• Expecting 0 instead of 10
• Thinking code causes error

4. Identify the error in this Kotlin code snippet:

class Outer {
    val message = "Hello"
    class Inner {
        fun greet() = println(message)
    }
}

medium

A. Incorrect println syntax

B. Missing semicolon after 'message' declaration

C. Inner class cannot access 'message' without 'inner' keyword

D. Outer class must be declared as 'inner'

Solution

1. Step 1: Check nested class access rules

   Nested classes without inner keyword cannot access outer class properties.

2. Step 2: Identify cause of error

   The Inner class tries to print message but cannot see it, causing a compilation error.

3. Final Answer:

   Inner class cannot access 'message' without 'inner' keyword -> Option C

4. Quick Check:

   Use 'inner' to access outer members [OK]

Hint: Add 'inner' keyword to nested class to fix access error [OK]

Common Mistakes:

• Thinking semicolon is required in Kotlin
• Assuming println syntax is wrong
• Confusing outer class declaration with inner

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

class Outer(val number: Int) {
    inner class Inner {
        fun multiply(factor: Int) = number * factor
    }
}

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

hard

A. 53

B. 15

C. Compilation error due to access

D. Runtime exception

Solution

1. Step 1: Understand inner class access and method

   The inner class can access the outer class property number because it is declared with inner. The method multiplies number by the given factor.

2. Step 2: Calculate the output

   Outer number is 5, factor is 3, so 5 * 3 = 15 is printed.

3. Final Answer:

   15 -> Option B

4. Quick Check:

   Inner class method uses outer property correctly [OK]

Hint: Inner class can use outer properties in calculations [OK]

Common Mistakes:

• Concatenating instead of multiplying
• Expecting access error without 'inner'
• Confusing output with string '53'