Property-based testing concept in Kotlin - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is property-based testing?

Property-based testing checks that a program works correctly for many inputs by testing general rules (properties) instead of specific examples.

beginner

How does property-based testing differ from example-based testing?

Example-based testing uses fixed inputs and expected outputs, while property-based testing generates many random inputs to check if general properties always hold.

intermediate

In Kotlin, which library is commonly used for property-based testing?

The Kotest library supports property-based testing with easy syntax to define properties and generate test data.

beginner

What is a 'property' in property-based testing?

A property is a rule or condition that should always be true for all valid inputs of the program.

beginner

Why is property-based testing useful?

It finds edge cases and bugs that example tests might miss by testing many random inputs automatically.

What does property-based testing focus on?

A Testing only one fixed input

B Testing general rules for many inputs

C Testing user interface layout

D Testing database connections

Property-based testing checks if general properties hold true for many inputs.

Which Kotlin library supports property-based testing?

A Kotest

B JUnit

C Mockito

D Spring Boot

Kotest provides built-in support for property-based testing in Kotlin.

What is a key benefit of property-based testing?

A Runs tests faster than unit tests

B Replaces all manual testing

C Finds bugs with many random inputs

D Only tests user interface

Property-based testing finds bugs by checking many random inputs automatically.

In property-based testing, what is a 'property'?

A A rule that should always be true

B A fixed input value

C A test case name

D A bug report

A property is a rule or condition that must hold for all inputs.

How does property-based testing generate inputs?

A From a database

B Manually by the tester

C From user input only

D Randomly, based on generators

Property-based testing uses generators to create many random inputs automatically.

Explain property-based testing and how it helps find bugs.

Think about testing rules instead of fixed examples.

Describe how you would write a simple property-based test in Kotlin.

Focus on the steps to create a property test.

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