Reified type parameters with inline in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of inline function with reified type

What is the output of this Kotlin code snippet?

Kotlin

inline fun <reified T> printTypeName() {
    println(T::class.simpleName)
}

fun main() {
    printTypeName<String>()
    printTypeName<Int>()
}

A String\nInt

B java.lang.String\njava.lang.Integer

C T\nT

D Compilation error

💡 Hint

Reified type parameters allow access to the actual type at runtime inside inline functions.

The inline function with reified type parameter T can access T::class.simpleName, which returns the simple name of the type. So calling printTypeName() prints "String" and printTypeName() prints "Int".

🧠 Conceptual

intermediate

Why use reified type parameters with inline functions?

Which of the following best explains why Kotlin requires the inline modifier when using reified type parameters?

A Because reified types are only available in Java, not Kotlin.

B Because reified types can only be used with suspend functions.

C Because inline functions run slower and need reified types to optimize.

D Because reified types need to be replaced with actual types at compile time, which inline functions allow.

💡 Hint

Think about how Kotlin handles generics and type erasure.

Kotlin erases generic types at runtime. Using inline functions with reified type parameters allows the compiler to replace the generic type with the actual type during compilation, enabling type checks and casts inside the function.

🔧 Debug

advanced

Identify the error in this code using reified type parameter

What error does this Kotlin code produce?

Kotlin

fun <reified T> checkType(value: Any) {
    if (value is T) {
        println("Value is of type T")
    } else {
        println("Value is NOT of type T")
    }
}

fun main() {
    checkType<String>(123)
}

A Compilation error: 'reified' type parameters can only be used in inline functions

B Prints: Value is NOT of type T

C Prints: Value is of type T

D Runtime error: ClassCastException

💡 Hint

Check the function declaration for inline keyword.

Reified type parameters require the function to be marked as inline. Without 'inline', the compiler cannot access the actual type at runtime and will produce a compilation error.

📝 Syntax

advanced

Correct syntax to use reified type parameter in inline function

Which option shows the correct syntax to declare an inline function with a reified type parameter and check if an object is of that type?

A inline fun <T reified> isOfType(obj: Any) = obj is T

B inline fun <reified T> isOfType(obj: Any) = obj is T

C fun <reified T> inline isOfType(obj: Any) = obj is T

D fun <T> inline isOfType(obj: Any) = obj is reified T

💡 Hint

The order of keywords and placement of 'reified' matters.

The correct syntax places 'inline' before 'fun' and 'reified' before the type parameter T inside angle brackets. Option B follows this syntax correctly.

🚀 Application

expert

Using reified type parameters to filter a list

Given this Kotlin function, what is the output when filtering a mixed list for elements of type String?

Kotlin

inline fun <reified T> List<Any>.filterType(): List<T> {
    return this.filterIsInstance<T>()
}

fun main() {
    val mixedList = listOf(1, "apple", 2.5, "banana", 3)
    val strings = mixedList.filterType<String>()
    println(strings)
}

A Compilation error: filterIsInstance requires reified type

B [1, 2.5, 3]

C ["apple", "banana"]

D ["apple", 2.5, "banana"]

💡 Hint

filterIsInstance() returns elements of type T from the list.

The function filterType uses filterIsInstance() which returns a list of elements that are instances of T. Here, filtering for String returns only the string elements "apple" and "banana".