Property-based testing concept in Kotlin - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define a property test function using Kotest.

Kotlin

fun testAddition() = property({ forAll<Int, Int> { a, b -> (a + b) [1] (b + a) } })

Drag options to blanks, or click blank then click option'

A==

B=

C!=

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using assignment operator '=' instead of equality '=='

Using inequality '!=' which would invert the test logic

The property test checks if addition is commutative by asserting a + b == b + a.

2fill in blank

medium

Complete the code to generate random strings for property testing.

Kotlin

val stringGen = Gen.[1](5..10)

Drag options to blanks, or click blank then click option'

Alist

Bint

Cboolean

Dstring

Attempts:

3 left

💡 Hint

Common Mistakes

Using Gen.int or Gen.boolean which generate numbers or booleans, not strings

Using Gen.list which generates lists, not strings

We use Gen.string to generate random strings of length between 5 and 10.

3fill in blank

hard

Fix the error in the property test that checks list reversal.

Kotlin

property({ forAll<List<Int>> { list -> list.reversed().reversed() [1] list } })

Drag options to blanks, or click blank then click option'

A!=

B=

C==

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using assignment '=' instead of equality '=='

Using inequality '!=' which would fail the test

The test asserts that reversing a list twice returns the original list, so we use == to check equality.

4fill in blank

hard

Fill both blanks to create a property test that checks if sorting a list keeps its size unchanged.

Kotlin

property({ forAll<List<Int>> { list -> list.sorted().[1] list.size && list.size [2] list.sorted().size } })

Drag options to blanks, or click blank then click option'

Asize ==

B==

C!=

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using inequality '!=' which would fail the test

Using '<' which is incorrect for size comparison

The test checks that the size of the sorted list equals the original list size, so we use size == and == operators.

5fill in blank

hard

Fill all three blanks to create a property test that checks if concatenating two strings has length equal to the sum of their lengths.

Kotlin

property({ forAll<String, String> { s1, s2 -> (s1 + s2).[1] == s1.[2] + s2.[3] } })

Drag options to blanks, or click blank then click option'

Alength

Bcount

Dsize

Attempts:

3 left

💡 Hint

Common Mistakes

Using count or size which are not properties of String in Kotlin

Mixing different properties for length comparison

The property test checks that the length of the concatenated string equals the sum of the lengths of the two strings. The correct property to use is length.

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