Nested class independence from outer in Kotlin - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a nested class inside Outer.

Kotlin

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

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

ANested

BInner

COuterNested

DCompanion

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'Inner' instead of 'Nested' which changes the class type.

Using 'Companion' which is for companion objects, not nested classes.

The keyword class Nested declares a nested class inside Outer. Nested classes do not have access to the outer class instance.

2fill in blank

medium

Complete the code to create an instance of the nested class.

Kotlin

val nested = Outer.[1]()

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

AInner

BCompanion

CNested

DOuterNested

Attempts:

3 left

💡 Hint

Common Mistakes

Trying to create an instance of 'Inner' which requires an outer instance.

Using 'Companion' which is not a class.

To create an instance of a nested class, use Outer.Nested(). Nested classes do not require an outer instance.

3fill in blank

hard

Fix the error in accessing the outer class property from the nested class.

Kotlin

class Outer(val name: String) {
    class Nested {
        fun printName() = [1]
    }
}

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

Aname

B"Cannot access outer property"

[e-mail]

DOuter().name

Attempts:

3 left

💡 Hint

Common Mistakes

Trying to use '[e-mail]' inside a nested class which is invalid.

Creating a new Outer instance to access name which is not the same as the outer instance.

Nested classes do not have access to the outer class instance or its properties. So you cannot access name directly. The correct approach is to pass the value explicitly if needed.

4fill in blank

hard

Fill both blanks to create an inner class that can access the outer class property.

Kotlin

class Outer(val name: String) {
    inner class [1] {
        fun printName() = [2]
    }
}

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

AInner

BNested

[e-mail]

Dname

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'Nested' instead of 'Inner' which does not allow access to outer properties.

Using just 'name' without 'this@Outer' which causes ambiguity.

Using inner class Inner allows access to the outer class instance. Inside the inner class, [e-mail] accesses the outer property.

5fill in blank

hard

Fill all three blanks to create a nested class with a function that returns a greeting including a passed outer class name.

Kotlin

class Outer {
    class [1] {
        fun greet(name: String) = "Hello, [2]!"
    }
}

val nested = Outer.[3]()
println(nested.greet("Alice"))

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

ANested

B$name

DInner

Attempts:

3 left

💡 Hint

Common Mistakes

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

Using '\$name' instead of '$name' which prevents interpolation.

Trying to create an instance of 'Inner' without an outer instance.

The nested class is named 'Nested'. The greet function uses string interpolation with $name to include the passed name. The instance is created with Outer.Nested().

Practice

1. What is true about a nested class in Kotlin compared to an inner class?

easy

A. A nested class can access all members of the outer class directly.

B. A nested class does not have access to the outer class's members.

C. A nested class is the same as an inner class.

D. A nested class must be declared with the inner keyword.

Solution

1. Step 1: Understand nested class independence

   In Kotlin, a nested class is static by default and cannot access outer class members.

2. Step 2: Compare with inner class

   Only inner classes (declared with inner) can access outer class properties and functions.

3. Final Answer:

   A nested class does not have access to the outer class's members. -> Option B

4. Quick Check:

   Nested class independence = A [OK]

Hint: Remember: nested = no outer access, inner = has outer access [OK]

Common Mistakes:

• Confusing nested and inner classes
• Thinking nested classes access outer members
• Assuming nested classes require 'inner' keyword

2. Which of the following is the correct syntax to declare a nested class inside an outer class in Kotlin?

easy

A. class Outer { inner class Nested { } }

B. class Outer { nested class Nested { } }

C. class Outer { static class Nested { } }

D. class Outer { class Nested { } }

Solution

1. Step 1: Recall Kotlin nested class syntax

   In Kotlin, nested classes are declared simply inside the outer class without any keyword.

2. Step 2: Identify incorrect options

   inner declares an inner class, static and nested keywords do not exist in Kotlin.

3. Final Answer:

   class Outer { class Nested { } } -> Option D

4. Quick Check:

   Nested class syntax = B [OK]

Hint: Nested class = just class inside class, no keywords needed [OK]

Common Mistakes:

• Using 'static' keyword (Java style)
• Using 'nested' keyword (not valid Kotlin)
• Confusing inner and nested class syntax

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

class Outer {
    val x = 10
    class Nested {
        fun printX() = "Cannot access x"
    }
}
fun main() {
    val nested = Outer.Nested()
    println(nested.printX())
}

medium

A. Cannot access x

B. NullPointerException

C. Compilation error

D. 10

Solution

1. Step 1: Analyze nested class access

   The nested class cannot access the outer class property x because it is not an inner class.

2. Step 2: Check printX() function

   It returns the string "Cannot access x" explicitly, so that will be printed.

3. Final Answer:

   Cannot access x -> Option A

4. Quick Check:

   Nested class method output = D [OK]

Hint: Nested classes can't access outer properties directly [OK]

Common Mistakes:

• Expecting nested class to print 10
• Assuming compilation error due to access
• Confusing inner and nested class behavior

4. Identify the error in this Kotlin code snippet:

class Outer {
    val y = 5
    class Nested {
        fun printY() = y
    }
}

medium

A. Nested class cannot access 'y' directly; must be inner class.

B. Missing 'inner' keyword before Nested class declaration.

C. Property 'y' must be static to be accessed.

D. No error; code is correct.

Solution

1. Step 1: Check nested class access rules

   A nested class cannot access outer class properties directly because it is static by default.

2. Step 2: Understand how to fix

   To access outer properties, the class must be declared as inner, but here it is not.

3. Final Answer:

   Nested class cannot access 'y' directly; must be inner class. -> Option A

4. Quick Check:

   Nested class access error = A [OK]

Hint: Nested class can't access outer vars; use inner class instead [OK]

Common Mistakes:

• Thinking 'inner' keyword is optional
• Assuming 'y' can be static in Kotlin
• Ignoring nested class static nature

5. Given this Kotlin code, what is the correct way to allow Nested to access outerValue from Outer class?

class Outer(val outerValue: Int) {
    class Nested {
        fun printValue() = "Cannot access outerValue"
    }
}

hard

A. Pass outerValue as a parameter to Nested's constructor.

B. Make outerValue static to access from Nested.

C. Change Nested to an inner class and access outerValue directly.

D. Use a companion object inside Nested to access outerValue.

Solution

1. Step 1: Understand nested vs inner class access

   Nested classes cannot access outer class properties unless declared inner.

2. Step 2: Apply inner class to access outerValue

   Changing Nested to inner class Nested allows direct access to outerValue.

3. Final Answer:

   Change Nested to an inner class and access outerValue directly. -> Option C

4. Quick Check:

   Use inner class for outer access = C [OK]

Hint: Use 'inner' keyword to access outer class properties [OK]

Common Mistakes:

• Trying to make properties static (not Kotlin style)
• Passing outerValue manually instead of inner class
• Using companion object incorrectly for instance data