Trailing lambda convention in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of trailing lambda usage in Kotlin

What is the output of this Kotlin code using the trailing lambda convention?

Kotlin

fun greet(name: String, action: () -> Unit) {
    println("Hello, $name!")
    action()
}

fun main() {
    greet("Alice") {
        println("Welcome to Kotlin.")
    }
}

A Hello, Alice!\nWelcome to Kotlin.

B Hello, Alice!\n() -> Unit

C Hello, Alice!\nWelcome to Kotlin.\n() -> Unit

D Compilation error due to lambda syntax

💡 Hint

Remember that trailing lambda syntax allows the lambda to be placed outside the parentheses.

The function greet prints a greeting and then calls the lambda passed as the second argument. Using trailing lambda syntax, the lambda is placed outside the parentheses and executed, printing "Welcome to Kotlin." after the greeting.

❓ Predict Output

intermediate

Trailing lambda with multiple parameters

What will this Kotlin program print when using trailing lambda syntax with multiple parameters?

Kotlin

fun operate(x: Int, y: Int, operation: (Int, Int) -> Int) {
    val result = operation(x, y)
    println("Result: $result")
}

fun main() {
    operate(5, 3) { a, b -> a * b }
}

A Result: 15

B Result: 8

C Result: 53

D Compilation error due to lambda parameters

💡 Hint

The lambda multiplies the two integers passed to it.

The operate function takes two integers and a lambda that operates on them. The trailing lambda multiplies 5 and 3, resulting in 15.

🔧 Debug

advanced

Identify the error with trailing lambda usage

What error does this Kotlin code produce?

Kotlin

fun printMessage(message: String, printer: () -> Unit) {
    printer()
    println(message)
}

fun main() {
    printMessage("Hi") print { println("Printing...") }
}

A No error, prints 'Printing...' then 'Hi'

B SyntaxError: Unexpected token 'print'

C RuntimeException: Null pointer exception

D Compilation error: Lambda outside parentheses must be last argument

💡 Hint

Trailing lambda must be the last argument and placed immediately after parentheses.

The code tries to call printMessage with a trailing lambda but places 'print' keyword incorrectly. The compiler expects the lambda immediately after the parentheses as the last argument. This causes a compilation error.

🧠 Conceptual

advanced

Understanding trailing lambda with multiple lambdas

Given a Kotlin function with two lambda parameters, which call correctly uses the trailing lambda convention?

Kotlin

fun process(a: Int, onSuccess: () -> Unit, onError: () -> Unit) {
    if (a > 0) onSuccess() else onError()
}

// Which call is correct?

A process(5) { println("Success") } { println("Error") }

B process(5) { println("Success") }, { println("Error") }

C process(5, { println("Success") }) { println("Error") }

D process(5, { println("Success") }, { println("Error") })

💡 Hint

Only one lambda can be passed outside parentheses as trailing lambda.

Kotlin allows only one trailing lambda, which must be the last argument. Option C correctly passes the first lambda inside parentheses and uses trailing syntax for the second lambda.

🚀 Application

expert

Refactor code using trailing lambda convention

Which option correctly refactors this Kotlin call to use the trailing lambda convention?

Kotlin

fun calculate(x: Int, y: Int, operation: (Int, Int) -> Int): Int {
    return operation(x, y)
}

val result = calculate(10, 20, { a, b -> a + b })

A val result = calculate(10, 20) (a, b) -> a + b

B val result = calculate(10, 20) { a, b -> a + b }

C val result = calculate(10, 20) { a, b -> a + b }()

D val result = calculate(10, 20, a, b -> a + b)

💡 Hint

Trailing lambda goes outside the parentheses as the last argument.

Option B correctly moves the lambda outside the parentheses as a trailing lambda. Other options have syntax errors or invalid lambda placement.