Multiple type parameters in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a generic function with two type parameters

What is the output of this Kotlin code?

Kotlin

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

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

A Pair: apples and 10

B 10 apples

C Pair: 10 and apples

D Compilation error

💡 Hint

Look at the order of parameters and how they are used in the string.

The function takes two parameters of any types A and B, then returns a string showing them in the order they were passed. So it prints "Pair: 10 and apples".

🧠 Conceptual

intermediate

Understanding multiple type parameters in Kotlin classes

Which statement correctly describes the following Kotlin class declaration? class Box(val first: A, val second: B)

A Box can hold two values of any types specified when creating an instance.

B Box can only hold values of the same type for both first and second.

C Box requires A and B to be subclasses of the same parent class.

D Box can only hold primitive types like Int and String.

💡 Hint

Think about what type parameters mean in Kotlin generics.

The class Box has two type parameters A and B, so it can hold two values of any types independently specified when creating an instance.

❓ Predict Output

advanced

Output of a generic function swapping two values

What is the output of this Kotlin code?

Kotlin

fun <X, Y> swap(pair: Pair<X, Y>): Pair<Y, X> {
    return Pair(pair.second, pair.first)
}

fun main() {
    val original = Pair("hello", 42)
    val swapped = swap(original)
    println(swapped)
}

A (42, hello)

B (hello, 42)

C Compilation error due to type mismatch

D ("42", "hello")

💡 Hint

The function swaps the first and second values in the pair.

The swap function returns a new Pair with the second and first values swapped, so the output is (42, hello).

🔧 Debug

advanced

Identify the error in this generic function with multiple type parameters

What error does this Kotlin code produce when compiled? fun combine(a: T, b: U): T { return b } fun main() { println(combine(5, "test")) }

A Type inference failed for T and U

B Unresolved reference: combine

C No error, prints "test"

D Type mismatch: inferred type is U but T was expected

💡 Hint

Look at the return type and the returned value types.

The function declares it returns type T but tries to return b of type U, causing a type mismatch error.

📝 Syntax

expert

Correct syntax for a Kotlin function with multiple type parameters and constraints

Which option shows the correct syntax for a Kotlin function with two type parameters where T must be a Number and U must be a Comparable of U?

A fun <T : Number, U : Comparable> compareValues(a: T, b: U): Int { return b.compareTo(b) }

B fun <T : Number, U : Comparable<U>> compareValues(a: T, b: U): Int { return b.compareTo(b) }

C fun <T Number, U Comparable<U>> compareValues(a: T, b: U): Int { return b.compareTo(b) }

D fun <T, U> compareValues(a: T : Number, b: U : Comparable<U>): Int { return b.compareTo(b) }

💡 Hint

Look for correct use of colon and angle brackets for constraints.

Option B correctly uses colon to specify constraints on type parameters inside angle brackets.