Preconditions (require, check, error) in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Kotlin code using require?

Consider this Kotlin function that uses require to check input. What will be printed when calling checkAge(15)?

Kotlin

fun checkAge(age: Int) {
    require(age >= 18) { "Age must be at least 18" }
    println("Age is valid: $age")
}

fun main() {
    checkAge(15)
}

A Exception in thread "main" java.lang.IllegalArgumentException: Age must be at least 18

B No output, program ends silently

C Exception in thread "main" java.lang.IllegalStateException: Age must be at least 18

D Age is valid: 15

💡 Hint

Remember that require throws IllegalArgumentException if the condition is false.

The require function checks the condition and throws IllegalArgumentException with the given message if the condition is false. Since 15 is less than 18, the exception is thrown and the println line is not executed.

❓ Predict Output

intermediate

What error does this Kotlin code raise using check?

Look at this Kotlin function using check. What error will be raised when calling validateNumber(-5)?

Kotlin

fun validateNumber(num: Int) {
    check(num > 0) { "Number must be positive" }
    println("Number is valid: $num")
}

fun main() {
    validateNumber(-5)
}

A Exception in thread "main" java.lang.IllegalArgumentException: Number must be positive

B Exception in thread "main" java.lang.IllegalStateException: Number must be positive

C Number is valid: -5

D No output, program ends silently

💡 Hint

Check throws IllegalStateException if the condition is false.

The check function throws IllegalStateException with the given message if the condition is false. Since -5 is not greater than 0, the exception is thrown.

🔧 Debug

advanced

Why does this Kotlin code using error() crash with this message?

This Kotlin function calls error("Invalid state"). What is the type of exception thrown and why?

Kotlin

fun fail() {
    error("Invalid state")
}

fun main() {
    fail()
}

A Throws java.lang.IllegalStateException with message "Invalid state" because error() always throws this exception.

B Throws java.lang.IllegalArgumentException with message "Invalid state" because error() throws this exception.

C Throws java.lang.RuntimeException with message "Invalid state" because error() throws a generic runtime exception.

D No exception thrown, error() just logs the message.

💡 Hint

Check the Kotlin documentation for error() function behavior.

The error() function in Kotlin throws an IllegalStateException with the provided message. It is used to indicate that the program has reached an invalid state.

📝 Syntax

advanced

Which Kotlin code snippet correctly uses require with a message lambda?

Select the code that compiles and runs without syntax errors.

A require(x > 0) -> { "x must be positive" }

B require(x > 0, "x must be positive")

C require(x > 0) { "x must be positive" }

D require(x > 0) "x must be positive"

💡 Hint

require can take a lambda that returns the message.

Option C uses the correct syntax: require(condition) { "message" }. Option C is invalid because require does not take two parameters like that. Options A, B, and C have syntax errors.

🚀 Application

expert

How many items are in the list after filtering with require in this Kotlin code?

This Kotlin code filters a list using require inside a map. How many items remain after running processList()?

Kotlin

fun processList(): List<Int> {
    val numbers = listOf(1, 2, 3, 4, 5)
    return numbers.map {
        require(it % 2 == 1) { "Only odd numbers allowed" }
        it * 2
    }
}

fun main() {
    try {
        val result = processList()
        println(result.size)
    } catch (e: IllegalArgumentException) {
        println("Exception caught")
    }
}

A 0

B 3

C 5

D Exception caught

💡 Hint

require throws exception on first even number, stopping the map.

The require inside map throws an exception when it encounters the first even number (2). This stops the mapping and the exception is caught, printing "Exception caught".