Generic function declaration in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a simple generic function

What is the output of this Kotlin code that uses a generic function to return the input value?

Kotlin

fun <T> identity(value: T): T {
    return value
}

fun main() {
    println(identity(42))
}

A null

B identity(42)

C 42

D Compilation error

💡 Hint

Think about what the function returns when given an integer.

The generic function identity returns the same value it receives. Here, it returns 42, so the output is 42.

🧠 Conceptual

intermediate

Understanding generic function type parameters

In Kotlin, what does the in the function declaration fun echo(item: T): T mean?

A It declares a generic type parameter T that can be any type.

B It specifies that T is always an Int type.

C It restricts the function to only accept String types.

D It is a syntax error and should be removed.

💡 Hint

Generic type parameters allow flexibility in the function input and output types.

The declares a generic type parameter named T. This means the function can accept and return any type specified when called.

❓ Predict Output

advanced

Output of generic function with multiple types

What is the output of this Kotlin code that uses a generic function with two type parameters?

Kotlin

fun <A, B> pairToString(a: A, b: B): String {
    return "$a and $b"
}

fun main() {
    println(pairToString(10, "apples"))
}

A 10 and apples

B 10 and 10

C apples and 10

D Compilation error

💡 Hint

Look at how the function combines the two parameters into a string.

The function takes two generic parameters and returns a string combining them. Here it prints "10 and apples".

🔧 Debug

advanced

Identify the error in this generic function declaration

What error does this Kotlin code produce? fun printTwice(value: T) { println(value) println(value) } fun main() { printTwice() }

Kotlin

fun <T> printTwice(value: T) {
    println(value)
    println(value)
}

fun main() {
    printTwice()
}

A Compilation error: Generic type T not specified

B Compilation error: No value passed for parameter 'value'

C Prints two blank lines

D Runtime error: NullPointerException

💡 Hint

Check the function call and its parameters.

The function requires one argument of type T, but the call provides none, causing a compilation error.

🚀 Application

expert

Result of generic function with nullable type and default value

What is the output of this Kotlin code that uses a generic function with a nullable type and a default value?

Kotlin

fun <T> defaultIfNull(value: T?, default: T): T {
    return value ?: default
}

fun main() {
    println(defaultIfNull(null, "Hello"))
    println(defaultIfNull("World", "Hello"))
}

A Compilation error

B null\nWorld

C Hello\nHello

D Hello\nWorld

💡 Hint

The Elvis operator ?: returns the left value if not null, otherwise the right value.

The function returns the default value if the input is null. So first line prints "Hello", second prints "World".