Generic function declaration in Kotlin - Mini Project: Build & Apply

Generic Function Declaration in Kotlin

📖 Scenario: Imagine you want to create a simple function that can work with different types of data, like numbers or words, without writing separate functions for each type.

🎯 Goal: You will build a generic function in Kotlin that takes one input and returns it unchanged. This helps you understand how to write functions that work with any type.

📋 What You'll Learn

Create a generic function called identity with a type parameter T

The function should take one parameter named value of type T

The function should return the same value without changes

Call the identity function with an Int and a String

Print the results of both calls

💡 Why This Matters

🌍 Real World

Generic functions let you write flexible code that works with many data types, saving time and avoiding repetition.

💼 Career

Understanding generics is important for Kotlin developers to build reusable libraries and clean code.

1

Create a generic function declaration

Write a generic function called identity with a type parameter T that takes one parameter named value of type T and returns value.

Kotlin

# Your code here
// Define the generic function identity(value: T): T

Hint

Use fun <T> identity(value: T): T to declare a generic function.

2

Call the generic function with an Int

Call the identity function with the integer 42 and store the result in a variable called intResult.

Kotlin

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

// Call identity with 42 and store in intResult
# Your code here

Hint

Use val intResult = identity(42) to call the function with an Int.

3

Call the generic function with a String

Call the identity function with the string "Hello" and store the result in a variable called stringResult.

Kotlin

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

val intResult = identity(42)

// Call identity with "Hello" and store in stringResult
# Your code here

Hint

Use val stringResult = identity("Hello") to call the function with a String.

4

Print the results

Print the values of intResult and stringResult using two separate println statements.

Kotlin

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

val intResult = identity(42)
val stringResult = identity("Hello")

// Print intResult and stringResult
# Your code here

Hint

Use println(intResult) and println(stringResult) to show the results.

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