Nested class independence from outer in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of nested class accessing outer class members

What is the output of this Kotlin code?

Kotlin

class Outer {
    val x = 10
    class Nested {
        fun printX() {
            // println(x) // Can this access x?
            println("Nested class cannot access outer class property directly")
        }
    }
}

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

A Compilation error: Cannot access 'x' from nested class

B Nested class cannot access outer class property directly

C Runtime error

D 10

💡 Hint

Remember that Kotlin nested classes do not have access to outer class instance members unless marked inner.

In Kotlin, a nested class is static by default and cannot access members of the outer class instance. To access outer class members, the nested class must be marked with the 'inner' keyword.

❓ Predict Output

intermediate

Output of inner class accessing outer class property

What will this Kotlin program print?

Kotlin

class Outer {
    val x = 5
    inner class Inner {
        fun printX() {
            println(x)
        }
    }
}

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

A 0

B Compilation error: Cannot access 'x' from inner class

C 5

D Runtime error

💡 Hint

Inner classes hold a reference to the outer class instance.

The 'inner' keyword allows the nested class to access members of the outer class instance. So 'x' is accessible and prints 5.

🔧 Debug

advanced

Why does this nested class code fail to compile?

Identify the reason for the compilation error in this Kotlin code.

Kotlin

class Outer {
    val y = 20
    class Nested {
        fun printY() {
            println(y)
        }
    }
}

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

A Nested class cannot access outer class property 'y' because it is not marked 'inner'.

B Property 'y' is private and inaccessible.

C Nested class must be instantiated with an outer class instance.

D Missing override keyword for printY() function.

💡 Hint

Check the difference between nested and inner classes in Kotlin.

A nested class in Kotlin is static by default and cannot access instance members of the outer class. Marking it 'inner' allows access.

📝 Syntax

advanced

Correct syntax to make nested class access outer class property

Which option correctly modifies the nested class to access the outer class property 'z'?

Kotlin

class Outer {
    val z = 100
    // Modify the nested class below
    class Nested {
        fun printZ() {
            println(z)
        }
    }
}

A Add 'inner' keyword before 'class Nested' and instantiate with outer instance

B Make 'z' a companion object property

C Declare 'Nested' as a top-level class outside 'Outer'

D Add 'static' keyword before 'class Nested'

💡 Hint

Only inner classes hold a reference to the outer class instance.

Marking the nested class as 'inner' allows it to access instance properties of the outer class. Companion object properties are accessed differently.

🚀 Application

expert

How many instances are created and why?

Consider this Kotlin code. How many instances of Outer and Inner are created, and why? class Outer { val a = 1 inner class Inner { val b = 2 } } fun main() { val outer1 = Outer() val inner1 = outer1.Inner() val outer2 = Outer() val inner2 = outer2.Inner() }

A One Outer instance and one Inner instance.

B One Outer instance and two Inner instances sharing it.

C Two Outer instances and one Inner instance shared between them.

D Two Outer instances and two Inner instances; each Inner is tied to its Outer instance.

💡 Hint

Inner class instances hold a reference to their specific outer instance.

Each 'Outer()' call creates a new Outer instance. Each Inner instance is tied to the Outer instance it was created from, so two Outer and two Inner instances exist.