Kotlin test assertions - Mini Project: Build & Apply

Kotlin test assertions

📖 Scenario: You are writing simple tests to check if your Kotlin functions work correctly. Testing helps you catch mistakes early, like checking if a calculator gives the right answer.

🎯 Goal: Build a Kotlin program that uses test assertions to verify the results of simple functions.

📋 What You'll Learn

Create a function that returns the sum of two numbers

Create a function that returns the product of two numbers

Write assertions to check if these functions return expected results

Print a message if all tests pass

💡 Why This Matters

🌍 Real World

Testing small functions helps ensure your code works correctly before using it in bigger programs.

💼 Career

Writing and understanding test assertions is a key skill for software developers to maintain code quality.

1

Create sum and product functions

Write a Kotlin function called sum that takes two Int parameters named a and b and returns their sum. Also write a function called product that takes two Int parameters named a and b and returns their product.

Kotlin

// Your code here

Hint

Define two functions with the exact names sum and product. Each should take two integers and return an integer.

2

Set expected test results

Create two val variables named expectedSum and expectedProduct. Set expectedSum to 7 and expectedProduct to 12.

Kotlin

fun sum(a: Int, b: Int): Int {
    return a + b
}

fun product(a: Int, b: Int): Int {
    return a * b
}

// Your code here

Hint

Use val to create constants for the expected results.

3

Write assertions to test functions

Write two assert statements. The first should check if sum(3, 4) equals expectedSum. The second should check if product(3, 4) equals expectedProduct.

Kotlin

fun sum(a: Int, b: Int): Int {
    return a + b
}

fun product(a: Int, b: Int): Int {
    return a * b
}

val expectedSum = 7
val expectedProduct = 12

// Your code here

Hint

Use assert(condition) to check if the function results match the expected values.

4

Print success message

Write a println statement that prints "All tests passed!".

Kotlin

fun sum(a: Int, b: Int): Int {
    return a + b
}

fun product(a: Int, b: Int): Int {
    return a * b
}

val expectedSum = 7
val expectedProduct = 12

assert(sum(3, 4) == expectedSum)
assert(product(3, 4) == expectedProduct)

// Your code here

Hint

Use println("All tests passed!") to show the success message.

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