Kotlin test assertions - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of assertEquals(expected, actual) in Kotlin tests?

It checks if the actual value is equal to the expected value. If not, the test fails.

beginner

How does assertTrue(condition) work in Kotlin test assertions?

It verifies that the condition is true. If the condition is false, the test fails.

beginner

What does assertNotNull(value) check in Kotlin tests?

It checks that the value is not null. If it is null, the test fails.

intermediate

Explain the use of assertFailsWith<ExceptionType> { ... } in Kotlin tests.

It verifies that the code inside the block throws an exception of type ExceptionType. If no exception or a different type is thrown, the test fails.

intermediate

What is the difference between assertEquals and assertSame in Kotlin?

assertEquals checks if two objects are equal in value, while assertSame checks if they are the exact same object (reference equality).

Which assertion checks if a condition is true in Kotlin tests?

A assertTrue(condition)

B assertEquals(expected, actual)

C assertNotNull(value)

D assertFailsWith<Exception> { }

assertTrue(condition) verifies that the condition is true.

What happens if assertEquals(expected, actual) fails?

A The test fails and reports a mismatch

B The test passes silently

C The test throws a runtime exception unrelated to assertion

D Nothing happens

If the values are not equal, the test fails and reports the difference.

Which assertion would you use to check that a variable is not null?

A assertNull(value)

B assertTrue(value)

C assertNotNull(value)

D assertEquals(null, value)

assertNotNull(value) ensures the value is not null.

How do you test that a block of code throws a specific exception in Kotlin?

A assertEquals<Exception> { }

B assertFailsWith<Exception> { }

C assertTrue { }

D assertNotNull { }

assertFailsWith<Exception> { } checks for a specific exception.

What does assertSame(obj1, obj2) check?

A If obj1 and obj2 have the same value

B If obj2 is not null

C If obj1 is null

D If obj1 and obj2 are the same object in memory

assertSame checks if both references point to the exact same object.

Describe how you would use Kotlin test assertions to verify that a function returns the expected value and does not return null.

Think about checking value equality and null separately.

Explain how to test that a Kotlin function throws a specific exception when given invalid input.

Focus on the assertion that expects an exception.

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