How to do Unit Testing in Android (Kotlin) using JUnit

Gone are the days when ‘it works on my machine’ was enough to ship a feature. Modern development requires us to be our own first line of defense. While we still lean on QA for the heavy lifting like regression and load testing, mastering basic unit tests is no longer optional — it’s part of the job description.

While new projects in Android Studio typically include JUnit 4 by default, you may need additional libraries depending on your requirements.

1. Essential JUnit Dependencies(add in build gradle file)

• JUnit 4 (Standard): Most default projects include testImplementation("junit:junit:4.13.2").
• JUnit 5 (Jupiter): To use modern features, you must manually add the JUnit Jupiter engine:

tasks.withType<Test> {
    useJUnitPlatform()
}

testImplementation("org.junit.jupiter:junit-jupiter:5.10.3")

📁 Project Structure

app/
 └── src/
     ├── main/   → your app code
     └── test/   → your test code

1. App Code

📄 File: **UserScoreManager**.kt

class UserScoreManager {
fun getUserLevel(score: Int): String {
        return when {
            score < 0 -> "Invalid score"
            score < 40 -> "Beginner"
            score < 70 -> "Intermediate"
            score < 90 -> "Advanced"
            else -> "Expert"
        }
    }
}

Logic

The function checks conditions from top to bottom:

• If score is negative → invalid
• If score < 40 → beginner
• If score < 70 → intermediate
• If score < 90 → advanced
• Otherwise → expert

Example:

score = 60 → Intermediate

2. Test Code

Go to:

app/src/test/

📄 File: UserScoreManagerTest.kt

class UserScoreManagerTest {

📄 File: UserScoreManagerTest.kt

private lateinit var manager: UserScoreManager

Why?

We will initialize it before each test.

📄 File: **UserScoreManagerTest**.kt

@Before
fun setup() {
    manager = UserScoreManager()
}

What this does:

Creates a fresh object before every test.

3. Test Cases

📄 File: **UserScoreManagerTest**.kt

@Test
fun getUserLevel_returnsInvalid_whenScoreNegative() {
val result = manager.getUserLevel(-5)
    assertEquals("Invalid score", result)
}

📄 File: **UserScoreManagerTest**.kt

@Test
fun getUserLevel_returnsBeginner_whenScoreLow() {
val result = manager.getUserLevel(20)
    assertEquals("Beginner", result)
}

📄 File: **UserScoreManagerTest**.kt

@Test
fun getUserLevel_returnsIntermediate_whenScoreMedium() {
val result = manager.getUserLevel(60)
    assertEquals("Intermediate", result)
}

📄 File: **UserScoreManagerTest**.kt

@Test
fun getUserLevel_returnsAdvanced_whenScoreHigh() {
val result = manager.getUserLevel(85)
    assertEquals("Advanced", result)
}

📄 File: **UserScoreManagerTest**.kt

@Test
fun getUserLevel_returnsExpert_whenScoreVeryHigh() {
val result = manager.getUserLevel(95)
    assertEquals("Expert", result)
}

4. How This Works

Each test follows the same flow:

Input → Function → Output → Check

Example:

60 → getUserLevel() → "Intermediate" → verified

5. Why This Matters

If someone changes logic incorrectly:

score <= 70 -> "Intermediate"

Your test will fail.

That’s the power of testing.

How to Run Tests and Read Results

You’ve written your test. Now the real question is:

“How do I actually run it… and know if it passed or failed?”

Let’s go step by step.

1. Where Your Test File Is

📄 File: UserScoreManagerTest.kt

Location:

app/src/test/java/your_package_name/UserScoreManagerTest.kt

Make sure your test file is inside the test folder, not androidTest.

2. How to Run the Test (Android Studio)

Option 1: Run Single Test File

• Open: UserScoreManagerTest.kt
• Right-click anywhere inside the file & Click:

Run 'UserScoreManagerTest'

Option 2: Run Specific Test Function

Inside the file:

@Test
fun getUserLevel_returnsBeginner_whenScoreLow() { ... }

• Click the small ▶️ icon next to the function name & Select:

Run 'getUserLevel_returnsBeginner_whenScoreLow'

Option 3: Run All Tests in Project

• Go to top menu & Click:

Run → Run 'All Tests'

3. What Happens After Running

Android Studio opens a panel at the bottom:

Run / Test Results Window

✅ If Test Passes

You’ll see:

✔ getUserLevel_returnsBeginner_whenScoreLow

• Green color
• No errors
• Means your logic is correct for that case

❌ If Test Fails

You’ll see:

✘ getUserLevel_returnsBeginner_whenScoreLow
Expected: Beginner
Actual: Intermediate

What This Means

• Your logic is wrong
• Or your expected value is wrong

Now you investigate.

4. Example: Understanding a Failure

📄 File: **UserScoreManager**.kt

score < 70 -> "Intermediate"

If someone changes it to:

score <= 70 -> "Intermediate"

Now test:

val result = manager.getUserLevel(70)

Output

Expected: Advanced
Actual: Intermediate

👉 Test catches the bug instantly

5. Reading the Test Results Panel

In the results window:

• Left side → list of tests
• Right side → error details

You can:

• Click any test → see output
• Expand stack trace → debug issue

6. Re-running Tests

After fixing code:

• Click Run again ▶️
• Or press:

Ctrl + Shift + F10  (Windows)

7. Quick Checklist

Before running tests, confirm:

✔ File is inside /test folder
✔ @Test annotation is present
✔ Dependencies are added
✔ Code compiles

Final Mental Model

Running a test is simple:

Write Test → Run → Check Result → Fix if needed → Run Again

Final Thought

Tests are not just for writing. They are for feedback.

Every time you run them, they tell you:

“Your logic is correct” or “Something is broken”

And that feedback is what makes you a better developer.

Final Thought

Testing becomes easy when:

• You know where code goes
• You understand the logic
• You verify each condition

No confusion. No guessing. Just clear behavior.

Optional Recommended Dependencies

For more comprehensive Kotlin testing, developers often add these popular libraries:

• Mocking Frameworks: Since unit tests should run in isolation, you use these to mock dependencies.
• MockK: testImplementation("io.mockk:mockk:1.13.10") (Highly recommended for Kotlin).
• Mockito: testImplementation("org.mockito.kotlin:mockito-kotlin:5.2.1").
• Coroutine Support: If your code uses suspend functions or Flows.
• testImplementation("org.jetbrains.kotlinx:kotlinx-coroutines-test:1.8.0").
• Architecture Components: If you are testing ViewModel or LiveData.
• testImplementation("androidx.arch.core:core-testing:2.2.0").
• Assertions: For more readable test results.
• Google Truth: testImplementation("com.google.truth:truth:1.4.2")

We will have separate tutorials on how to use these libraries to keep this tutorial simple. Try it out this simple one and move one step ahead.

Thank you.