Property-based testing concept in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this property-based test snippet?

Consider this Kotlin code using the property-based testing library Kotest. What will be printed when the test runs?

Kotlin

import io.kotest.property.checkAll
import io.kotest.property.arbitrary.int

fun main() {
    checkAll<Int> { a ->
        println(a * 2 >= a)
    }
}

A Prints 'true' for all generated integers

B Compilation error due to missing import

C Prints 'false' for some integers

D Runtime exception due to overflow

💡 Hint

Consider negative integers: for a = -1, is -2 >= -1?

The property 'a * 2 >= a' is true for non-negative integers but false for negative ones (e.g., -1 * 2 = -2 which is not >= -1). Therefore, 'false' will be printed for some generated integers.

🧠 Conceptual

intermediate

What is the main advantage of property-based testing?

Choose the best description of the main advantage of property-based testing compared to example-based testing.

A It tests many random inputs automatically to find edge cases

B It replaces the need for unit tests entirely

C It only tests the user interface of applications

D It requires writing fewer tests because it uses examples

💡 Hint

Think about how property-based testing generates inputs.

Property-based testing automatically generates many inputs to test properties, helping find edge cases that example-based tests might miss.

🔧 Debug

advanced

Why does this property-based test fail with a false negative?

This Kotlin property-based test is supposed to check if reversing a list twice returns the original list. Why might it fail unexpectedly?

Kotlin

import io.kotest.property.checkAll

fun main() {
    checkAll<List<Int>> { list ->
        assert(list.reversed().reversed() == list)
    }
}

A The test fails because reversed() changes the original list in place

B The test fails because reversed() does not return a new list

C The test fails because assert throws an exception instead of reporting failure

D The test fails because List<Int> is not a valid generator type without specifying an arbitrary

💡 Hint

Consider how property-based testing generates inputs for generic types.

Kotest requires specifying an arbitrary generator for generic types like List. Without it, the test cannot generate inputs and fails.

📝 Syntax

advanced

Which option correctly defines a property test for reversing a string twice?

Select the Kotlin code snippet that correctly tests if reversing a string twice returns the original string using Kotest property testing.

A checkAll<String> { s -> assertEquals(s, s.reversed().reversed()) }

B checkAll<String> { s -> assert(s.reversed().reversed() == s) }

C checkAll<String> { s -> assert(s == s.reversed().reversed()) }

D checkAll<String> { s -> assert(s.reversed() == s) }

💡 Hint

Consider the best assertion function to use in Kotlin tests.

Option A uses assertEquals which is the proper way to compare expected and actual values in tests. Options A and B use assert which is less descriptive, and D tests a different property.

🚀 Application

expert

How many test cases will be generated by this property test?

Given this Kotlin property-based test code, how many test cases will Kotest generate by default?

Kotlin

import io.kotest.property.checkAll

fun main() {
    checkAll<Int> { a ->
        println(a)
    }
}

A 50 test cases

B 100 test cases

C 10 test cases

D Unlimited test cases until failure

💡 Hint

Check Kotest default number of generated cases for checkAll.

Kotest's checkAll function generates 100 test cases by default unless configured otherwise.