Why testing matters in Kotlin - See It in Action

Why testing matters

📖 Scenario: Imagine you are building a simple calculator app that adds two numbers. You want to make sure your addition works correctly every time, even if you change your code later.

🎯 Goal: You will create a small Kotlin program that adds two numbers and write a simple test function to check if the addition works correctly. This will show why testing your code is important.

📋 What You'll Learn

Create two variables with exact values

Create a function to add two numbers

Create a test function to check the addition

Print the test result

💡 Why This Matters

🌍 Real World

Testing helps developers make sure their apps work correctly before users see them. It saves time and avoids bugs.

💼 Career

Writing tests is a key skill for software developers. It shows you can write reliable and maintainable code.

1

DATA SETUP: Create two variables with exact values

Create two variables called num1 and num2 with values 5 and 7 respectively.

Kotlin

# Create two variables num1 and num2 with values 5 and 7
// Your code here

Hint

Use val to create variables and assign the numbers 5 and 7.

2

CONFIGURATION: Create a function to add two numbers

Create a function called add that takes two Int parameters named a and b and returns their sum.

Kotlin

val num1 = 5
val num2 = 7

// Create a function add(a: Int, b: Int): Int that returns a + b
// Your code here

Hint

Define a function with fun keyword and return the sum of a and b.

3

CORE LOGIC: Create a test function to check the addition

Create a function called testAdd that calls add(num1, num2) and checks if the result equals 12. Store the check result in a variable called testPassed.

Kotlin

val num1 = 5
val num2 = 7

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

// Create a function testAdd() that checks if add(num1, num2) == 12
// Your code here

Hint

Inside testAdd, call add(num1, num2) and compare it to 12 using ==.

4

OUTPUT: Print the test result

Inside the testAdd function, add a line to print "Test passed!" if testPassed is true, otherwise print "Test failed!". Then call testAdd().

Kotlin

val num1 = 5
val num2 = 7

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

fun testAdd() {
    val testPassed = add(num1, num2) == 12
    // Print "Test passed!" if testPassed is true, else "Test failed!"
}

// Call testAdd()
// Your code here

Hint

Use an if statement to print the correct message based on testPassed. Then call testAdd() to run the test.

Practice

1. Why is testing important when writing Kotlin programs?

easy

A. It makes the program run faster automatically.

B. It allows the program to work without any user input.

C. It removes the need to write comments in code.

D. It helps find mistakes early and keeps the program working well.

Solution

1. Step 1: Understand the purpose of testing

   Testing is used to find mistakes and ensure the program behaves as expected.

2. Step 2: Compare options with testing goals

   Only It helps find mistakes early and keeps the program working well. correctly states that testing finds mistakes early and keeps the program working well.

3. Final Answer:

   It helps find mistakes early and keeps the program working well. -> Option D

4. Quick Check:

   Testing finds mistakes early = B [OK]

Hint: Testing finds bugs early and keeps code reliable [OK]

Common Mistakes:

• Thinking testing makes code faster
• Believing testing replaces comments
• Assuming testing removes user input

2. Which Kotlin syntax correctly defines a simple test function named testSum?

easy

A. fun testSum() { /* test code here */ }

B. def testSum() { /* test code here */ }

C. function testSum() { /* test code here */ }

D. testSum fun() { /* test code here */ }

Solution

1. Step 1: Recall Kotlin function syntax

   Kotlin functions start with the keyword fun, followed by the function name and parentheses.

2. Step 2: Match options to Kotlin syntax

   Only fun testSum() { /* test code here */ } uses the correct Kotlin syntax: fun testSum() { ... }.

3. Final Answer:

   fun testSum() { /* test code here */ } -> Option A

4. Quick Check:

   Kotlin functions start with fun = A [OK]

Hint: Kotlin functions always start with 'fun' keyword [OK]

Common Mistakes:

• Using 'def' like in Python
• Using 'function' like in JavaScript
• Placing 'fun' after the function name

3. What will be the output of this Kotlin test function?

fun testMultiply() {
    val result = 3 * 4
    if (result == 12) {
        println("Test passed")
    } else {
        println("Test failed")
    }
}
testMultiply()

medium

A. Test passed

B. No output

C. Test failed

D. Compilation error

Solution

1. Step 1: Calculate the multiplication

   The variable result is assigned 3 * 4, which equals 12.

2. Step 2: Check the if condition

   The condition result == 12 is true, so it prints "Test passed".

3. Final Answer:

   Test passed -> Option A

4. Quick Check:

   3 * 4 = 12, prints 'Test passed' = C [OK]

Hint: Calculate expressions before checking conditions [OK]

Common Mistakes:

• Assuming multiplication is wrong
• Ignoring the if condition
• Thinking no output occurs

4. This Kotlin test function is supposed to check if a number is even. What is wrong?

fun testIsEven() {
    val number = 5
    if (number % 2 = 0) {
        println("Even")
    } else {
        println("Odd")
    }
}
testIsEven()

medium

A. The variable 'number' should be a String

B. Missing parentheses around the if condition

C. Using single '=' instead of '==' in the if condition

D. The function name should start with 'is'

Solution

1. Step 1: Identify the if condition syntax error

   The condition uses a single '=' which is assignment, not comparison.

2. Step 2: Correct the comparison operator

   It should use '==' to compare if number % 2 equals 0.

3. Final Answer:

   Using single '=' instead of '==' in the if condition -> Option C

4. Quick Check:

   Comparison needs '==' not '=' = A [OK]

Hint: Use '==' for comparison, '=' is assignment [OK]

Common Mistakes:

• Confusing '=' with '=='
• Thinking parentheses are missing
• Believing variable type is wrong

5. You want to write a Kotlin test that checks if a function calculateDiscount returns the correct discount for a price. Which approach best shows why testing matters?

hard

A. Write a test that changes the price variable but does not check the output.

B. Write a test that calls calculateDiscount(100) and checks if the result is 10.

C. Write a test that always returns true without calling the function.

D. Write a test that prints the price without checking the result.

Solution

1. Step 1: Understand the goal of testing functions

   Tests should call the function and verify if the output matches expected results.

2. Step 2: Evaluate each option

   Only Write a test that calls calculateDiscount(100) and checks if the result is 10. calls the function and checks the result, which helps find mistakes and trust the code.

3. Final Answer:

   Write a test that calls calculateDiscount(100) and checks if the result is 10. -> Option B

4. Quick Check:

   Test calls function and checks output = D [OK]

Hint: Tests must check function output against expected values [OK]

Common Mistakes:

• Writing tests that don't check results
• Returning true without testing
• Changing variables without verification