Generic function declaration in Kotlin - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a generic function that returns the input value.

Kotlin

fun <T> identity(value: [1]): T {
    return value
}

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

AT

BAny

CInt

DString

Attempts:

3 left

💡 Hint

Common Mistakes

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

Using Any instead of the generic type T.

The generic type T is used as the parameter type to make the function generic.

2fill in blank

medium

Complete the code to declare a generic function that swaps two values in a Pair.

Kotlin

fun <T> swap(pair: Pair<T, T>): Pair<T, T> {
    return Pair(pair.second, [1])
}

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

Apair

Bpair.first

Cpair.second

Dpair.swap

Attempts:

3 left

💡 Hint

Common Mistakes

Using pair.second twice instead of pair.first for the second element.

Trying to call a non-existent swap method on pair.

The function returns a new Pair with the first and second values swapped. So the second value is paired with the original first value.

3fill in blank

hard

Fix the error in the generic function declaration that returns the first element of a list.

Kotlin

fun <T> firstElement(list: List<[1]>): T? {
    return list.firstOrNull()
}

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

AAny

BList

CT

DInt

Attempts:

3 left

💡 Hint

Common Mistakes

Using a concrete type like Int instead of the generic type T.

Using List without a type parameter.

The generic type parameter T must be used inside the List type to match the function's return type.

4fill in blank

hard

Fill both blanks to declare a generic function that returns the size of a collection.

Kotlin

fun <[1]> collectionSize(collection: Collection<[2]>): Int {
    return collection.size
}

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

AT

BInt

DE

Attempts:

3 left

💡 Hint

Common Mistakes

Using different generic type names in declaration and parameter.

Using a concrete type like Int instead of a generic type.

The generic type parameter is declared as T and used as the element type of the collection.

5fill in blank

hard

Fill all three blanks to declare a generic function that returns a map from keys to values filtered by a predicate.

Kotlin

fun <[1], [2]> filterMap(
    map: Map<[1], [2]>,
    predicate: ([3], [2]) -> Boolean
): Map<[1], [2]> {
    return map.filter { (key, value) -> predicate(key, value) }
}

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

AK

BV

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up the order of generic types.

Using inconsistent generic type names.

The generic types are declared as K and V for key and value respectively. They are used consistently in the Map and predicate types.

Practice

1. What is the purpose of declaring a generic function in Kotlin using <T> before the function name?

easy

A. To declare a function that returns a string only

B. To restrict the function to only work with integers

C. To make the function run faster

D. To allow the function to work with any type specified when called

Solution

1. Step 1: Understand generic type declaration

   Declaring <T> before a function name means the function can accept any type as input.

2. Step 2: Purpose of generic functions

   This makes the function reusable for different data types without rewriting it for each type.

3. Final Answer:

   To allow the function to work with any type specified when called -> Option D

4. Quick Check:

   Generic functions = reusable for any type [OK]

Hint: Generic <T> means any type can be used [OK]

Common Mistakes:

• Thinking generic restricts to one type
• Confusing generic with function overloading
• Assuming generic improves speed automatically

2. Which of the following is the correct syntax to declare a generic function named printItem that takes a parameter of any type T in Kotlin?

easy

A. fun <T> printItem(item: T) { println(item) }

B. fun printItem<T>(item) { println(item) }

C. fun printItem(T item) { println(item) }

D. fun printItem(item: <T>) { println(item) }

Solution

1. Step 1: Identify correct generic syntax

   In Kotlin, generic type parameters are declared before the function name using <T>.

2. Step 2: Check parameter declaration

   The parameter must specify its type as T, so item: T is correct.

3. Final Answer:

   fun <T> printItem(item: T) { println(item) } -> Option A

4. Quick Check:

   Generic syntax = <T> before function name [OK]

Hint: Generic type <T> goes before function name [OK]

Common Mistakes:

• Placing <T> after function name
• Not specifying parameter type as T
• Using parentheses incorrectly around <T>

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

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

fun main() {
    println(identity(123))
    println(identity("Hello"))
}

medium

A. 123
123

B. 123
Hello

C. Hello
Hello

D. Compilation error

Solution

1. Step 1: Understand the generic function behavior

   The function identity returns the same value it receives, regardless of type.

2. Step 2: Trace the calls in main()

   Calling identity(123) returns 123, and identity("Hello") returns "Hello".

3. Final Answer:

   123 Hello -> Option B

4. Quick Check:

   Generic function returns input value [OK]

Hint: Generic function returns input as is [OK]

Common Mistakes:

• Expecting type conversion
• Thinking output repeats first call
• Assuming compilation error due to mixed types

4. Identify the error in this Kotlin generic function declaration:

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

medium

A. Generic type parameter <T> must be declared before the function name

B. Parameter type cannot be generic

C. Function must return a value

D. println cannot print generic types

Solution

1. Step 1: Check generic syntax rules

   In Kotlin, generic type parameters must be declared before the function name, not after.

2. Step 2: Analyze the given code

   The code declares printValue<T>, which is incorrect syntax.

3. Final Answer:

   Generic type parameter <T> must be declared before the function name -> Option A

4. Quick Check:

   Generic <T> goes before function name [OK]

Hint: Place <T> before function name, not after [OK]

Common Mistakes:

• Putting <T> after function name
• Assuming generic parameters go in parentheses
• Thinking generic functions must return a value

5. You want to write a generic function swap that swaps two elements in a mutable list at given indices. Which of the following is the correct Kotlin function declaration and implementation?

hard

A. fun swap<T>(list: MutableList<T>, i: Int, j: Int) {
val temp = list[i]
list[i] = list[j]
list[j] = temp
}

B. fun <T> swap(list: List<T>, i: Int, j: Int) {
val temp = list[i]
list[i] = list[j]
list[j] = temp
}

C. fun <T> swap(list: MutableList<T>, i: Int, j: Int) {
val temp = list[i]
list[i] = list[j]
list[j] = temp
}

D. fun swap(list: MutableList<Any>, i: Int, j: Int) {
val temp = list[i]
list[i] = list[j]
list[j] = temp
}

Solution

1. Step 1: Correct generic syntax and parameter types

   The generic type parameter <T> must be declared before the function name. The list must be mutable to allow swapping.

2. Step 2: Verify implementation correctness

   The function swaps elements at indices i and j using a temporary variable, which is correct.

3. Final Answer:

   fun <T> swap(list: MutableList<T>, i: Int, j: Int) { val temp = list[i]; list[i] = list[j]; list[j] = temp } -> Option C

4. Quick Check:

   Generic swap needs <T> before name and MutableList<T> [OK]

Hint: Use <T> before function and MutableList<T> for mutability [OK]

Common Mistakes:

• Putting <T> after function name
• Using immutable List instead of MutableList
• Using MutableList<Any> losing type safety