Multiple type parameters in Kotlin - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a generic class with two type parameters.

Kotlin

class Pair<[1], B>(val first: [1], val second: B)

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

AK

BA

CX

DT

Attempts:

3 left

💡 Hint

Common Mistakes

Using lowercase letters for type parameters.

Using non-letter characters.

The first type parameter is commonly named T, so we use T for the first generic type.

2fill in blank

medium

Complete the function signature to use two generic type parameters.

Kotlin

fun <[1], [2]> createPair(first: [1], second: [2]): Pair<[1], [2]> = Pair(first, second)

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

AT

BU

CV

DW

Attempts:

3 left

💡 Hint

Common Mistakes

Using the same type parameter twice.

Not declaring generic parameters before the function name.

We use T and U as the two generic type parameters consistently in the function signature.

3fill in blank

hard

Fix the error in the generic class declaration with two type parameters.

Kotlin

class Box<[1], [2]>(val content1: [1], val content2: [2])

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

AT

BT, U

CT U

DU

Attempts:

3 left

💡 Hint

Common Mistakes

Separating type parameters with spaces or semicolons.

Using only one type parameter when two are needed.

Type parameters must be separated by commas inside angle brackets. So use T and U separated by a comma.

4fill in blank

hard

Fill both blanks to create a generic function that swaps two values of different types.

Kotlin

fun <[1], [2]> swap(pair: Pair<[1], [2]>): Pair<[2], [1]> = Pair(pair.second, pair.first)

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

AT

BU

CV

DW

Attempts:

3 left

💡 Hint

Common Mistakes

Using the same type parameter for both elements.

Not swapping the type parameters in the return type.

We use T and U as the two generic type parameters consistently to swap the pair elements.

5fill in blank

hard

Fill all three blanks to define a generic class with three type parameters and a method returning a Triple.

Kotlin

class TripleHolder<[1], [2], [3]>(val first: [1], val second: [2], val third: [3]) {
    fun toTriple(): Triple<[1], [2], [3]> = Triple(first, second, third)
}

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

AA

BB

CC

DD

Attempts:

3 left

💡 Hint

Common Mistakes

Using fewer than three type parameters.

Not matching the type parameters in the method return type.

We use A, B, and C as the three generic type parameters consistently in the class and method.

Practice

1. What does it mean when a Kotlin function has <T, U> before its name?

easy

A. The function is limited to only one type parameter named T or U.

B. The function can only accept types T or U but not both at the same time.

C. The function uses two different type parameters, T and U, to work with multiple types.

D. The function automatically converts types T and U to String.

Solution

1. Step 1: Understand type parameters in Kotlin

   Type parameters like <T, U> allow a function to accept multiple types, making it flexible.

2. Step 2: Interpret the meaning of <T, U>

   Having <T, U> means the function can work with two different types, T and U, independently.

3. Final Answer:

   The function uses two different type parameters, T and U, to work with multiple types. -> Option C

4. Quick Check:

   Multiple type parameters = flexible function types [OK]

Hint: Multiple type parameters mean multiple flexible types [OK]

Common Mistakes:

• Thinking it limits to only one type at a time
• Assuming automatic type conversion happens
• Confusing type parameters with actual types

2. Which of the following is the correct syntax to declare a Kotlin function with two type parameters named T and U?

easy

A. fun myFunction(param1: T, param2: U) {}

B. fun myFunction(param1: T, param2: U) {}

C. fun myFunction(param1: T, param2: U) {}

D. fun myFunction(param1: T, param2: U) {}

Solution

1. Step 1: Recall Kotlin generic function syntax

   Type parameters are declared immediately after the fun keyword inside angle brackets, separated by commas.

2. Step 2: Check each option's syntax

   fun myFunction(param1: T, param2: U) {} correctly uses <T, U> after fun and before the function name, matching Kotlin syntax.

3. Final Answer:

   fun <T, U> myFunction(param1: T, param2: U) {} -> Option A

4. Quick Check:

   Type parameters go after fun with commas [OK]

Hint: Type parameters come right after fun, separated by commas [OK]

Common Mistakes:

• Missing comma between type parameters
• Placing type parameters after function name
• Putting type parameters after parameter list

3. What is the output of this Kotlin code?

fun <T, U> pairToString(pair: Pair<T, U>): String {
    return "${pair.first} and ${pair.second}"
}

fun main() {
    val result = pairToString(Pair(10, "cats"))
    println(result)
}

medium

A. Compilation error

B. Pair(10, cats)

C. cats and 10

D. 10 and cats

Solution

1. Step 1: Understand the function behavior

   The function takes a Pair with types T and U, then returns a string combining first and second values.

2. Step 2: Trace the main function call

   pairToString is called with Pair(10, "cats"), so pair.first = 10 and pair.second = "cats".

3. Final Answer:

   10 and cats -> Option D

4. Quick Check:

   pair.first + " and " + pair.second = "10 and cats" [OK]

Hint: Read pair.first and pair.second values carefully [OK]

Common Mistakes:

• Swapping first and second values
• Expecting Pair's default toString output
• Thinking it causes a compilation error

4. Identify the error in this Kotlin function declaration:

fun <T, U> combine(a: T, b: U): T {
    return b
}

medium

A. Return type T does not match returned value b of type U.

B. Missing comma between T and U in type parameters.

C. Function must return type U instead of T.

D. No error, the function is correct.

Solution

1. Step 1: Check function return type and return statement

   The function declares return type T but returns b which is of type U.

2. Step 2: Understand type mismatch error

   Returning U where T is expected causes a type mismatch compilation error.

3. Final Answer:

   Return type T does not match returned value b of type U. -> Option A

4. Quick Check:

   Return type must match returned value type [OK]

Hint: Return type must match the returned value type [OK]

Common Mistakes:

• Ignoring type mismatch between T and U
• Assuming Kotlin auto-converts types
• Missing comma in type parameters (not the case here)

5. You want to create a Kotlin generic class Box that can hold two different types of items, T and U. Which of the following class declarations correctly uses multiple type parameters and allows storing and retrieving both items?

hard

A. class Box<T>(val first: T, val second: T) {
fun getFirst(): T = first
fun getSecond(): T = second
}

B. class Box(val first: T, val second: U) {
fun getFirst(): T = first
fun getSecond(): U = second
}

C. class Box {
val first: T
val second: U
}

D. class Box(val first: T, val second: U) {
fun getFirst(): T = first
fun getSecond(): U = second
}

Solution

1. Step 1: Check correct multiple type parameter declaration

   class Box(val first: T, val second: U) { fun getFirst(): T = first fun getSecond(): U = second } declares class Box with <T, U> and constructor parameters of types T and U, which is correct.

2. Step 2: Verify methods to retrieve both items

   class Box(val first: T, val second: U) { fun getFirst(): T = first fun getSecond(): U = second } provides getFirst() returning T and getSecond() returning U, matching stored types.

3. Final Answer:

   class Box<T, U>(val first: T, val second: U) { fun getFirst(): T = first fun getSecond(): U = second } -> Option B

4. Quick Check:

   Multiple type parameters with matching properties and methods [OK]

Hint: Use and matching properties for both types [OK]

Common Mistakes:

• Using only one type parameter for both items
• Not declaring type parameters in class header
• Using undefined types without generics