Kotlin test assertions

Introduction

Test assertions help you check if your code works as expected by comparing actual results with what you want.

When you want to check if a function returns the correct value.

When you want to verify that a variable has the expected content.

When you want to make sure an error or exception happens as planned.

When you want to confirm that two objects are equal or not equal.

When you want to test if a condition is true or false during your program.

Syntax

Kotlin

assertEquals(expected, actual)
assertTrue(condition)
assertFalse(condition)
assertNotNull(value)
assertNull(value)
assertFailsWith<ExceptionType> { code }

These functions come from Kotlin's test libraries like kotlin.test or JUnit.

You usually write these inside test functions to check your code automatically.

Examples

Check if the sum of 2 and 3 equals 5.

Kotlin

assertEquals(5, sum(2, 3))

Check if the variable isValid is true.

Kotlin

assertTrue(isValid)

Check if calling parseNumber with "abc" throws an IllegalArgumentException.

Kotlin

assertFailsWith<IllegalArgumentException> {
    parseNumber("abc")
}

Sample Program

This program tests the add function with different assertions. It also checks that converting "abc" to an integer throws a NumberFormatException.

Kotlin

import kotlin.test.*

fun add(a: Int, b: Int): Int = a + b

fun main() {
    assertEquals(7, add(3, 4))
    assertTrue(add(1, 1) == 2)
    assertFalse(add(2, 2) == 5)
    assertNotNull(add(0, 0))
    assertNull(null)

    val exception = assertFailsWith<NumberFormatException> {
        "abc".toInt()
    }
    println("Caught exception: ${exception.message}")
}

Important Notes

Use clear messages in assertions to understand failures better.

Assertions help catch bugs early by testing small parts of your code.

Run tests often to keep your code reliable.

Summary

Assertions check if your code behaves as expected.

Use different assertions for equality, truth, null checks, and exceptions.

Write assertions inside test functions to automate checking your code.

Practice

1. Which Kotlin test assertion checks if two values are equal?

easy

A. assertEquals(expected, actual)

B. assertTrue(condition)

C. assertNull(value)

D. assertFails { block }

Solution

1. Step 1: Understand assertion types

   assertEquals is used to check if two values are equal.

2. Step 2: Match assertion to purpose

   assertTrue checks if a condition is true, assertNull checks for null, and assertFails checks for exceptions.

3. Final Answer:

   assertEquals(expected, actual) -> Option A

4. Quick Check:

   Equality check = assertEquals [OK]

Hint: Use assertEquals to compare expected and actual values [OK]

Common Mistakes:

• Confusing assertTrue with assertEquals
• Using assertNull for equality checks
• Using assertFails to check values

2. Which of the following is the correct syntax to assert a condition is true in Kotlin tests?

easy

A. assertTrue { condition }

B. assertTrue condition

C. assertTrue(condition)

D. assertTrue: condition

Solution

1. Step 1: Recall Kotlin assertion syntax

   assertTrue takes a Boolean condition as a parameter inside parentheses.

2. Step 2: Identify correct syntax

   Only assertTrue(condition) is valid Kotlin syntax; others are incorrect.

3. Final Answer:

   assertTrue(condition) -> Option C

4. Quick Check:

   assertTrue uses parentheses for condition [OK]

Hint: Remember assertions use parentheses for arguments [OK]

Common Mistakes:

• Using curly braces instead of parentheses
• Omitting parentheses
• Using colon instead of parentheses

3. What will be the result of this Kotlin test assertion?

val result = 5
assertEquals(5, result)

medium

A. Test passes without error

B. Test fails with AssertionError

C. Compilation error

D. Runtime exception other than AssertionError

Solution

1. Step 1: Check values compared

   The expected value is 5 and actual value stored in result is also 5.

2. Step 2: Understand assertEquals behavior

   Since both values are equal, assertEquals passes without throwing an error.

3. Final Answer:

   Test passes without error -> Option A

4. Quick Check:

   Equal values = test passes [OK]

Hint: Equal expected and actual means test passes [OK]

Common Mistakes:

• Assuming assertEquals throws error on equal values
• Confusing expected and actual order
• Thinking assertEquals returns a value

4. Identify the error in this Kotlin test assertion code:

assertEquals(10, 5 + 3)

medium

A. Runtime exception due to invalid operation

B. No error, test passes

C. Syntax error due to missing parentheses

D. Test fails because expected and actual differ

Solution

1. Step 1: Calculate actual value

   The expression 5 + 3 equals 8.

2. Step 2: Compare expected and actual

   Expected is 10 but actual is 8, so assertEquals fails and throws AssertionError.

3. Final Answer:

   Test fails because expected and actual differ -> Option D

4. Quick Check:

   Different values = test fails [OK]

Hint: Check actual calculation matches expected value [OK]

Common Mistakes:

• Assuming test passes despite value mismatch
• Thinking syntax error occurs
• Confusing runtime exceptions with assertion failures

5. You want to test that a function throws an exception when given invalid input. Which Kotlin assertion should you use?

hard

A. assertNull(function(invalidInput))

B. assertFails { function(invalidInput) }

C. assertEquals(exception, function(invalidInput))

D. assertTrue(function(invalidInput))

Solution

1. Step 1: Understand exception testing

   To check if a function throws an exception, use assertFails with a lambda calling the function.

2. Step 2: Evaluate other options

   assertEquals compares values, not exceptions; assertNull checks for null; assertTrue checks a Boolean condition.

3. Final Answer:

   assertFails { function(invalidInput) } -> Option B

4. Quick Check:

   Exception test = assertFails [OK]

Hint: Use assertFails with lambda to test exceptions [OK]

Common Mistakes:

• Using assertEquals to check exceptions
• Using assertNull expecting exception
• Using assertTrue for exception testing