Kotlin test assertions - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Kotlin test assertion code?

Consider the following Kotlin test code using JUnit assertions. What will be the output when this test runs?

Kotlin

import org.junit.jupiter.api.Assertions.*
import org.junit.jupiter.api.Test

class SampleTest {
    @Test
    fun testSum() {
        val result = 2 + 3
        assertEquals(5, result)
        assertTrue(result > 4)
        assertFalse(result < 0)
    }
}

A Test passes with no errors

B Test fails with AssertionError on assertEquals

C Test fails with AssertionError on assertTrue

D Test fails with AssertionError on assertFalse

💡 Hint

Check the values compared in the assertions carefully.

The sum 2 + 3 equals 5, so assertEquals(5, result) passes. result > 4 is true, so assertTrue passes. result < 0 is false, so assertFalse passes. All assertions succeed, so the test passes.

❓ Predict Output

intermediate

What error does this Kotlin test assertion code raise?

What error will be raised when running this Kotlin test code?

Kotlin

import org.junit.jupiter.api.Assertions.*
import org.junit.jupiter.api.Test

class SampleTest {
    @Test
    fun testString() {
        val actual = "Kotlin"
        assertEquals("kotlin", actual)
    }
}

A No error, test passes

B NullPointerException

C AssertionError due to case mismatch

D Compilation error

💡 Hint

assertEquals is case sensitive for strings.

The expected string "kotlin" is lowercase, but actual is "Kotlin" with uppercase K. assertEquals fails and throws AssertionError.

🔧 Debug

advanced

Why does this Kotlin test assertion fail unexpectedly?

This Kotlin test code fails with an AssertionError. Identify the reason.

Kotlin

import org.junit.jupiter.api.Assertions.*
import org.junit.jupiter.api.Test

class SampleTest {
    @Test
    fun testList() {
        val list1 = listOf(1, 2, 3)
        val list2 = listOf(3, 2, 1)
        assertEquals(list1, list2)
    }
}

A Lists have same elements but different order

B Lists have different sizes

C assertEquals cannot compare lists

D Compilation error due to missing import

💡 Hint

assertEquals compares lists element by element in order.

Although both lists contain the same numbers, their order differs. assertEquals compares elements in order, so the test fails.

📝 Syntax

advanced

Which option correctly asserts that a nullable variable is null in Kotlin test?

Given a nullable variable val name: String? = null, which assertion correctly checks it is null?

A assertNull(name)

B assertEquals(null, name)

C assertTrue(name == null)

D All of the above

💡 Hint

Check which assertions are valid and equivalent for null checks.

assertNull(name) explicitly checks for null. assertEquals(null, name) compares to null. assertTrue(name == null) checks condition. All are valid ways to assert null.

🚀 Application

expert

What is the output of this Kotlin test assertion with exception checking?

What will happen when running this Kotlin test code that checks for an exception?

Kotlin

import org.junit.jupiter.api.Assertions.*
import org.junit.jupiter.api.Test

class SampleTest {
    @Test
    fun testException() {
        val exception = assertThrows<IllegalArgumentException> {
            throw IllegalArgumentException("Invalid argument")
        }
        assertEquals("Invalid argument", exception.message)
    }
}

A Test fails due to message mismatch

B Test passes with no errors

C Test fails due to wrong exception type

D Test fails because no exception is thrown

💡 Hint

assertThrows expects the specified exception type to be thrown inside the lambda.

The code throws IllegalArgumentException with message "Invalid argument". assertThrows catches it and returns the exception. The message matches, so all assertions pass.

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