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.

Practice

1.

What is the main idea behind property-based testing in Kotlin?

easy

A. Writing tests only for fixed input values

B. Manually verifying outputs for each test case

C. Checking if the code compiles without errors

D. Testing that a property or rule holds true for many random inputs

Solution

1. Step 1: Understand property-based testing concept

   Property-based testing means checking if a rule or property is true for many inputs, not just a few fixed ones.

2. Step 2: Compare options with concept

   Only Testing that a property or rule holds true for many random inputs describes testing many random inputs against a property, which matches the concept.

3. Final Answer:

   Testing that a property or rule holds true for many random inputs -> Option D

4. Quick Check:

   Property-based testing = many inputs tested [OK]

Hint: Look for 'many inputs' and 'rules' in the answer [OK]

Common Mistakes:

• Confusing property-based testing with example-based testing
• Thinking it only tests fixed inputs
• Assuming it checks compilation only

2.

Which Kotlin library is commonly used for property-based testing?

easy

A. JUnit

B. Mockito

C. Kotest

D. Retrofit

Solution

1. Step 1: Identify Kotlin testing libraries

   JUnit is for unit testing, Mockito for mocking, Retrofit for networking, Kotest supports property-based testing.

2. Step 2: Match library to property-based testing

   Kotest provides generators and property testing features, making it the correct choice.

3. Final Answer:

   Kotest -> Option C

4. Quick Check:

   Kotest = property-based testing library [OK]

Hint: Remember Kotest for property-based tests in Kotlin [OK]

Common Mistakes:

• Confusing JUnit with property-based testing support
• Choosing Mockito which is for mocking
• Selecting Retrofit which is unrelated

3.

Consider this Kotlin property-based test snippet using Kotest:

import io.kotest.property.checkAll

fun isSorted(list: List<Int>): Boolean = list.zipWithNext().all { it.first <= it.second }

checkAll { list ->
    assert(isSorted(list.sorted()))
}

What will this test do?

medium

A. Check that the list contains only positive numbers

B. Check that sorting any list results in a sorted list

C. Check that the list is empty

D. Check that any list is already sorted

Solution

1. Step 1: Understand the test code

   The test generates many random lists of integers. For each list, it sorts the list and checks if the sorted list is sorted using isSorted.

2. Step 2: Interpret the assertion

   The assertion verifies that sorting produces a sorted list, which is a property that should always hold.

3. Final Answer:

   Check that sorting any list results in a sorted list -> Option B

4. Quick Check:

   Sorting list => sorted list [OK]

Hint: Look for sorting and isSorted check in the code [OK]

Common Mistakes:

• Thinking it checks if original list is sorted
• Assuming it checks list emptiness
• Confusing list contents with positivity

4.

What is wrong with this Kotlin property-based test using Kotest?

checkAll<Int> { n ->
    assert(n > 0)
}

Choose the best explanation.

medium

A. It will fail because it tests all Ints, including zero and negatives

B. It will pass because all Ints are positive

C. It has a syntax error in the lambda

D. It only tests positive numbers, so it's correct

Solution

1. Step 1: Analyze the test input range

   The test uses checkAll<Int> which generates all Int values, including negatives and zero.

2. Step 2: Understand the assertion

   The assertion assert(n > 0) expects all numbers to be positive, which is false for many generated values.

3. Final Answer:

   It will fail because it tests all Ints, including zero and negatives -> Option A

4. Quick Check:

   All Ints include negatives, so assert fails [OK]

Hint: Remember Int includes negatives and zero [OK]

Common Mistakes:

• Assuming all Ints are positive
• Thinking lambda syntax is wrong
• Believing test only runs on positive numbers

5.

You want to test a function reverseTwice(list: List<Int>): List<Int> that reverses a list twice. Which property-based test best checks that reverseTwice returns the original list?

fun reverseTwice(list: List<Int>): List<Int> = list.reversed().reversed()

hard

A. checkAll { list -> assert(reverseTwice(list) == list) }

B. checkAll { list -> assert(reverseTwice(list) == list.reversed()) }

C. checkAll { list -> assert(reverseTwice(list).isEmpty()) }

D. checkAll { list -> assert(reverseTwice(list).size == 0) }

Solution

1. Step 1: Understand the function behavior

   Reversing a list twice should return the original list unchanged.

2. Step 2: Match the property test to the behavior

   checkAll { list -> assert(reverseTwice(list) == list) } asserts that reverseTwice(list) equals the original list, which correctly tests the property.

3. Final Answer:

   checkAll<List<Int>> { list -> assert(reverseTwice(list) == list) } -> Option A

4. Quick Check:

   Reverse twice = original list [OK]

Hint: Check if test compares to original list [OK]

Common Mistakes:

• Comparing to reversed list instead of original
• Checking if result is empty incorrectly
• Testing size equals zero wrongly