Reified type parameters with inline in Kotlin - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare an inline function with a reified type parameter.

Kotlin

inline fun <[1]> printTypeName() {
    println("Type: ${T::class.simpleName}")
}

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

AT

Binline T

Creified T

Dval T

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting the reified keyword causes a compilation error.

Using inline inside the angle brackets instead of before the function.

The reified keyword allows access to the type T at runtime inside an inline function.

2fill in blank

medium

Complete the code to check if an object is of type T using a reified type parameter.

Kotlin

inline fun <reified T> isType(value: Any): Boolean {
    return value is [1]
}

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

AT

BAny

CUnit

DNothing

Attempts:

3 left

💡 Hint

Common Mistakes

Trying to use a non-reified type parameter in an is check causes an error.

Using a concrete type like Any instead of T.

Using value is T inside an inline function with a reified type parameter allows type checking at runtime.

3fill in blank

hard

Fix the error in the function to get the class name of type T using reified type parameters.

Kotlin

inline fun <reified T> getClassName(): String {
    return [1]::class.simpleName ?: "Unknown"
}

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

AT.class

BT

CT::class

DT::class.java

Attempts:

3 left

💡 Hint

Common Mistakes

Using T.class which is Java syntax and invalid in Kotlin.

Using T::class.java which returns a Java class, not Kotlin class.

Inside an inline function with a reified type parameter, T::class is valid, but here the code uses T::class.simpleName. The blank expects T to use T::class.simpleName.

4fill in blank

hard

Fill both blanks to create a function that returns a list of elements filtered by type T using reified type parameters.

Kotlin

inline fun <reified T> filterByType(list: List<Any>): List<[1]> {
    return list.filterIsInstance<[2]>()
}

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

AT

BAny

CString

DInt

Attempts:

3 left

💡 Hint

Common Mistakes

Using different types in the return type and filter function causes type mismatch.

Using concrete types like String or Int instead of the generic T.

The function uses the reified type parameter T to filter elements of that type from the list.

5fill in blank

hard

Fill all three blanks to create an inline function that casts a value to type T or returns null if the cast fails.

Kotlin

inline fun <reified T> safeCast(value: Any): [1]? {
    return value as? [2] ?: [3]
}

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

AT

BAny

Cnull

DUnit

Attempts:

3 left

💡 Hint

Common Mistakes

Using non-nullable return type causes compilation errors.

Returning Unit instead of null on cast failure.

The function returns the value cast to T or null if the cast fails, using the safe cast operator as?.

Practice

1. What is the main benefit of using reified type parameters in Kotlin inline functions?

easy

A. They improve performance by avoiding inline expansion.

B. They allow access to the actual type at runtime inside the function.

C. They enable functions to be called without specifying type arguments.

D. They automatically convert types to strings.

Solution

1. Step 1: Understand reified type parameters

   Reified type parameters let you use the real type inside an inline function at runtime, unlike normal generics which erase type info.

2. Step 2: Identify the main benefit

   This means you can do type checks and casts safely inside the function using the actual type.

3. Final Answer:

   They allow access to the actual type at runtime inside the function. -> Option B

4. Quick Check:

   Reified = real type access [OK]

Hint: Reified means real type info available at runtime [OK]

Common Mistakes:

• Thinking reified improves performance by avoiding inline
• Believing reified removes need for type arguments
• Confusing reified with automatic type conversion

2. Which of the following is the correct syntax to declare an inline function with a reified type parameter T in Kotlin?

easy

A. inline fun myFunction() { }

B. inline fun <T> myFunction() { }

C. fun inline myFunction() { }

D. fun inline myFunction() { }

Solution

1. Step 1: Recall syntax for inline with reified

   The correct syntax places inline before the function and reified before the type parameter inside angle brackets.

2. Step 2: Check each option

   inline fun myFunction() { } correctly uses inline fun <reified T> myFunction(). Others have wrong order or missing keywords.

3. Final Answer:

   inline fun <reified T> myFunction() { } -> Option A

4. Quick Check:

   inline + reified before T = correct syntax [OK]

Hint: Put 'inline' before fun and 'reified' before type param [OK]

Common Mistakes:

• Placing 'reified' after the type parameter
• Writing 'fun inline' instead of 'inline fun'
• Omitting 'inline' keyword

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

inline fun  isString(value: Any): Boolean {
    return value is T
}

fun main() {
    println(isString<String>("hello"))
    println(isString<Int>(123))
    println(isString<Double>(123.0))
}

medium

A. false\nfalse\nfalse

B. true\nfalse\ntrue

C. false\ntrue\nfalse

D. true\ntrue\ntrue

Solution

1. Step 1: Understand the function behavior

   The function checks if the given value is of type T using the reified type parameter for runtime type check.

2. Step 2: Evaluate each println call

   First call: isString<String>("hello") -- "hello" is String -> true.
   Second call: isString<Int>(123) -- 123 is Int -> true.
   Third call: isString<Double>(123.0) -- 123.0 is Double -> true.
   Output: true\ntrue\ntrue.

3. Final Answer:

   true\ntrue\ntrue -> Option D

4. Quick Check:

   All type checks match = true true true [OK]

Hint: Reified lets 'is' check real type at runtime [OK]

Common Mistakes:

• Assuming type erasure causes false results
• Mixing up types in the is check
• Ignoring that reified allows runtime type check

4. Identify the error in the following Kotlin code snippet:

fun  printTypeName() {
    println(T::class.simpleName)
}

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

medium

A. Missing type argument when calling printTypeName.

B. T::class.simpleName is not a valid expression.

C. Cannot use reified type parameter in a non-inline function.

D. Function printTypeName must return a value.

Solution

1. Step 1: Check function declaration

   The function uses reified type parameter but is not marked inline. This is not allowed in Kotlin.

2. Step 2: Understand Kotlin rules for reified

   Reified type parameters require the function to be inline to preserve type info at runtime.

3. Final Answer:

   Cannot use reified type parameter in a non-inline function. -> Option C

4. Quick Check:

   Reified requires inline function [OK]

Hint: Reified always needs inline keyword [OK]

Common Mistakes:

• Forgetting to mark function as inline
• Thinking reified works in normal functions
• Misunderstanding T::class usage

5. You want to create a generic inline function createInstance that returns a new instance of type T using its no-argument constructor. Which of the following implementations correctly uses reified type parameters to achieve this?

hard

A. inline fun createInstance(): T = T::class.java.newInstance()

B. fun createInstance(): T = T::class.java.getDeclaredConstructor().newInstance()

C. inline fun <T> createInstance(): T = T::class.java.newInstance()

D. inline fun createInstance(): T = T()

Solution

1. Step 1: Understand how to create instance with reified

   Using reified lets us access T::class.java at runtime to call newInstance() for no-arg constructor.

2. Step 2: Check each option

   inline fun createInstance(): T = T::class.java.newInstance() uses inline and reified correctly and calls newInstance().
   fun createInstance(): T = T::class.java.getDeclaredConstructor().newInstance() is not inline, so reified is invalid.
   inline fun <T> createInstance(): T = T::class.java.newInstance() lacks reified, so T::class is invalid.
   inline fun createInstance(): T = T() tries to call T() which is invalid syntax in Kotlin.

3. Final Answer:

   inline fun <reified T> createInstance(): T = T::class.java.newInstance() -> Option A

4. Quick Check:

   Inline + reified + newInstance() = correct instance creation [OK]

Hint: Use inline + reified + T::class.java.newInstance() [OK]

Common Mistakes:

• Omitting inline with reified
• Trying to call T() directly
• Using reified without inline