Property-based testing concept in Kotlin - Step-by-Step Execution

Concept Flow - Property-based testing concept

Define properties

↓

Generate random inputs

↓

Run test with inputs

↓

Check if property holds

↓

Next input

↓

Repeat until inputs exhausted or failure

Property-based testing runs many random inputs through a property to check if it always holds true.

Execution Sample

Kotlin

import io.kotest.property.checkAll

checkAll<Int> { x ->
  (x + 0) == x
}

This test checks that adding zero to any integer x returns x, using many random x values.

Execution Table

Step	Input x	Property Checked	Result	Action
1	5	(5 + 0) == 5	True	Continue testing
2	-3	(-3 + 0) == -3	True	Continue testing
3	0	(0 + 0) == 0	True	Continue testing
4	999	(999 + 0) == 999	True	Continue testing
5	-1000	(-1000 + 0) == -1000	True	Continue testing
...	...	...	...	...
N	random x	(x + 0) == x	True	All tests passed

💡 All random inputs tested, property holds for all, test passes

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4	After 5	...final
x	undefined	5	-3	0	999	-1000	various random ints
propertyResult	undefined	true	true	true	true	true	true

Key Moments - 3 Insights

Why do we test many random inputs instead of just one?

Property-based testing checks that a property holds for all inputs, not just one example. The execution_table shows multiple inputs tested to catch edge cases.

What happens if the property fails for one input?

If the property fails (result false), testing stops and failure is reported immediately, as shown in the 'No' branch in concept_flow.

Are the inputs truly random or fixed?

Inputs are randomly generated each run to explore many cases. The variable_tracker shows different values of x for each test step.

Visual Quiz - 3 Questions

Look at the execution_table, what is the property result at step 3?

A False

B True

C Undefined

D Error

💡 Hint

Check the 'Result' column at step 3 in execution_table

At which step does the testing stop if the property fails?

A After all inputs tested

B After 5 inputs

C Immediately at failure

D Never stops

💡 Hint

Refer to concept_flow where 'No' branch leads to 'Report failure'

If we change the property to (x + 1) == x, how would the execution_table change?

A Some results become False

B All results remain True

C No inputs tested

D Property always passes

💡 Hint

Think about propertyResult values in variable_tracker and execution_table for a false property

Concept Snapshot

Property-based testing runs many random inputs to check a property.
Define property, generate inputs, test property, report failures.
Stops on first failure or passes if all hold.
Good for catching edge cases beyond example tests.

Full Transcript

Property-based testing means writing a rule (property) that should always be true for many inputs. The test runs many random inputs through this rule. Each input is checked if the property holds. If it fails for any input, testing stops and reports failure. If all inputs pass, the test passes. This helps find bugs that example-based tests might miss. The execution table shows inputs and results step by step. The variable tracker shows how inputs and results change. Key moments explain why many inputs are tested and what happens on failure. The quiz checks understanding of these steps.

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