Inner classes and nested classes in Kotlin - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a nested class inside the outer class.

Kotlin

class Outer {
    class [1] {
        fun greet() = "Hello from nested class"
    }
}

Drag options to blanks, or click blank then click option'

ANested

BCompanion

CInner

DStatic

Attempts:

3 left

💡 Hint

Common Mistakes

Using the keyword 'Inner' instead of 'Nested' for a nested class.

Trying to use 'Static' keyword which does not exist in Kotlin.

The keyword class Nested declares a nested class inside Outer. Nested classes do not hold a reference to the outer class.

2fill in blank

medium

Complete the code to declare an inner class that can access the outer class's property.

Kotlin

class Outer {
    val greeting = "Hello"
    inner class [1] {
        fun greet() = greeting
    }
}

Drag options to blanks, or click blank then click option'

AInner

BNested

CCompanion

DStatic

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'Nested' instead of 'Inner' when access to outer class properties is needed.

Forgetting the 'inner' keyword causes no access to outer class members.

The inner keyword declares an inner class that holds a reference to the outer class instance, allowing access to its properties.

3fill in blank

hard

Fix the error in the code to correctly create an instance of the inner class.

Kotlin

class Outer {
    inner class Inner {
        fun greet() = "Hi"
    }
}

fun main() {
    val inner = Outer().[1]()
    println(inner.greet())
}

Drag options to blanks, or click blank then click option'

AOuter

Bnew

Ccreate

DInner

Attempts:

3 left

💡 Hint

Common Mistakes

Trying to instantiate inner class without an outer class instance.

Using keywords like 'new' which are not used in Kotlin.

To create an instance of an inner class, you call it on an instance of the outer class like Outer().Inner().

4fill in blank

hard

Fill both blanks to create a nested class and call its method.

Kotlin

class Outer {
    class [1] {
        fun greet() = "Hello from nested"
    }
}

fun main() {
    val nested = Outer.[2]()
    println(nested.greet())
}

Drag options to blanks, or click blank then click option'

ANested

BInner

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'Inner' keyword for nested class declaration.

Trying to instantiate nested class like an inner class.

The nested class is declared with class Nested and called with Outer.Nested().

5fill in blank

hard

Fill all three blanks to create an inner class, instantiate it, and call its method.

Kotlin

class Outer {
    val message = "Hi from outer"
    inner class [1] {
        fun greet() = message
    }
}

fun main() {
    val outer = Outer()
    val inner = outer.[2]()
    println(inner.[3]())
}

Drag options to blanks, or click blank then click option'

AInner

Cgreet

DNested

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'Nested' instead of 'Inner' for the inner class.

Calling the method incorrectly or forgetting parentheses.

The inner class is declared with inner class Inner, instantiated with outer.Inner(), and its method greet() is called.

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