Smart casts in when and if in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of smart cast in if condition

What is the output of this Kotlin code using smart casts in an if statement?

Kotlin

fun main() {
    val obj: Any = "Hello"
    if (obj is String) {
        println(obj.length)
    } else {
        println("Not a string")
    }
}

A 5

B Not a string

C Compilation error

D Runtime exception

💡 Hint

The variable obj is checked to be a String before accessing length.

The if condition checks if obj is a String. Inside the if block, Kotlin smart casts obj to String, so obj.length is valid and prints 5.

❓ Predict Output

intermediate

Smart cast behavior in when expression

What will be printed by this Kotlin code using smart casts in a when expression?

Kotlin

fun main() {
    val x: Any = 42
    when (x) {
        is String -> println("String of length ${x.length}")
        is Int -> println("Int doubled is ${x * 2}")
        else -> println("Unknown type")
    }
}

A Compilation error

B Int doubled is 84

C Unknown type

D String of length 2

💡 Hint

Check the type of x and how smart casts work in when branches.

The variable x is an Int with value 42. The when expression smart casts x to Int in the second branch, so it prints 84.

🔧 Debug

advanced

Why does this smart cast fail?

Consider this Kotlin code snippet. Why does it fail to compile?

Kotlin

fun main() {
    var obj: Any? = "Kotlin"
    if (obj is String && obj.length > 3) {
        println(obj.length)
    }
}

A Because length property is not accessible on String

B Because obj is not declared as val

C Because && operator cannot be used with smart casts

D Because obj is nullable, smart cast is not possible without explicit null check

💡 Hint

Think about nullability and smart casts combined with logical operators.

Smart casts do not work when the variable is nullable and used with && without explicit null check. Kotlin cannot guarantee obj is not null after the check.

📝 Syntax

advanced

Identify the syntax error with smart casts in when

Which option contains a syntax error related to smart casts in this Kotlin when expression?

Kotlin

fun main() {
    val y: Any = 10
    when (y) {
        is String -> println(y.length)
        is Int -> println(y + 5)
        else -> println("Unknown")
    }
}

A Replacing 'else -> println("Unknown")' with 'else println("Unknown")'

B Replacing 'is String -> println(y.length)' with 'is String -> println(y.toUpperCase())'

C Replacing 'when (y)' with 'when y'

D Replacing 'is Int -> println(y + 5)' with 'is Int -> println(y.length)'

💡 Hint

Check the syntax of when branches and the use of arrows.

In Kotlin, each when branch must use '->' before the expression. Omitting '->' causes a syntax error.

🚀 Application

expert

Determine the number of items in the resulting map

What is the number of entries in the map produced by this Kotlin code using smart casts in when?

Kotlin

fun main() {
    val items: List<Any> = listOf("apple", 10, "banana", 3.14)
    val result = items.mapNotNull {
        when (it) {
            is String -> it.uppercase()
            is Int -> (it * 2).toString()
            else -> null
        }
    }.associateWith { it.length }
    println(result.size)
}

A 2

B 5

C 3

D 4

💡 Hint

Count how many items are transformed and not filtered out.

Only Strings and Ints are transformed; the Double is filtered out by returning null. So 3 items remain in the map.