Unit testing with JUnit in Android Kotlin

Introduction

Unit testing with JUnit helps check small parts of your code work correctly. It finds mistakes early so your app runs better.

When you write a new function and want to make sure it returns the right result.

When you fix a bug and want to confirm it does not come back.

When you change code and want to check nothing else breaks.

When you want to automate testing so you don't test manually every time.

When you want to share your code with others and prove it works.

Syntax

Android Kotlin

import org.junit.Test
import org.junit.Assert.*

class ExampleUnitTest {
    @Test
    fun testFunction() {
        val expected = 4
        val actual = 2 + 2
        assertEquals(expected, actual)
    }
}

Use @Test annotation to mark a test method.

Use assertions like assertEquals to check expected vs actual results.

Examples

This test checks if 2 + 3 equals 5.

Android Kotlin

import org.junit.Test
import org.junit.Assert.*

class MathTest {
    @Test
    fun addition_isCorrect() {
        assertEquals(5, 2 + 3)
    }
}

This test checks if the string contains the word "Hello".

Android Kotlin

import org.junit.Test
import org.junit.Assert.*

class StringTest {
    @Test
    fun string_containsHello() {
        val text = "Hello World"
        assertTrue(text.contains("Hello"))
    }
}

This test checks that the object is not null.

Android Kotlin

import org.junit.Test
import org.junit.Assert.*

class NullTest {
    @Test
    fun object_isNotNull() {
        val obj: String? = "Kotlin"
        assertNotNull(obj)
    }
}

Sample App

This test class checks the add function of a simple calculator. It tests adding two positive numbers and adding a positive with a negative number.

Android Kotlin

import org.junit.Test
import org.junit.Assert.*

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

class CalculatorTest {
    private val calculator = Calculator()

    @Test
    fun add_twoPositiveNumbers_returnsCorrectSum() {
        val result = calculator.add(3, 7)
        assertEquals(10, result)
    }

    @Test
    fun add_positiveAndNegativeNumber_returnsCorrectSum() {
        val result = calculator.add(5, -2)
        assertEquals(3, result)
    }
}

Important Notes

Keep tests small and focused on one thing.

Name test methods clearly to describe what they check.

Run tests often to catch errors early.

Summary

Unit tests check small parts of code work as expected.

JUnit uses @Test and assertions to write tests.

Good tests help find bugs and keep code reliable.

Practice

1. What is the main purpose of a unit test in Android Kotlin development using JUnit?

easy

A. To deploy the app to the Play Store

B. To test the entire application UI

C. To check if a small part of code works correctly

D. To measure app performance under load

Solution

1. Step 1: Understand unit test scope

   Unit tests focus on small, isolated parts of code, like functions or classes.

2. Step 2: Differentiate from other tests

   UI tests and performance tests cover broader areas, not unit tests.

3. Final Answer:

   To check if a small part of code works correctly -> Option C

4. Quick Check:

   Unit test = small code check [OK]

Hint: Unit tests check small code parts only [OK]

Common Mistakes:

• Confusing unit tests with UI or integration tests
• Thinking unit tests deploy or measure performance
• Assuming unit tests cover the whole app

2. Which annotation is used in Kotlin with JUnit to mark a function as a test?

easy

A. @Test

B. @Before

C. @RunWith

D. @Ignore

Solution

1. Step 1: Identify JUnit test annotation

   The @Test annotation marks a function as a test method in JUnit.

2. Step 2: Understand other annotations

   @Before runs setup code, @RunWith specifies runner, @Ignore skips tests.

3. Final Answer:

   @Test -> Option A

4. Quick Check:

   @Test marks test functions [OK]

Hint: Use @Test to mark test functions [OK]

Common Mistakes:

• Using @Before instead of @Test for tests
• Confusing @RunWith with test marking
• Mistaking @Ignore as test annotation

3. Given this Kotlin JUnit test code, what will be the test result?

import org.junit.Test
import kotlin.test.assertEquals

class CalculatorTest {
    @Test
    fun testAdd() {
        val result = 2 + 3
        assertEquals(5, result)
    }
}

medium

A. Test passes successfully

B. Test fails with assertion error

C. Compilation error due to missing import

D. Runtime exception occurs

Solution

1. Step 1: Analyze the test logic

   The test adds 2 + 3 and stores in result, which equals 5.

2. Step 2: Check assertion correctness

   assertEquals expects 5 and result is 5, so assertion passes.

3. Final Answer:

   Test passes successfully -> Option A

4. Quick Check:

   2 + 3 = 5 matches assertEquals [OK]

Hint: Check if expected equals actual in assertEquals [OK]

Common Mistakes:

• Assuming test fails when values match
• Thinking imports are missing when they are correct
• Confusing runtime errors with assertion failures

4. Identify the error in this Kotlin JUnit test code:

import org.junit.Test
import kotlin.test.assertEquals

class CalculatorTest {
    @Test
    fun testSubtract() {
        val result = 5 - 2
        assertEquals(4, result)
    }
}

medium

A. Function name should start with 'test_'

B. Missing @Test annotation

C. Incorrect import statement

D. Wrong expected value in assertEquals

Solution

1. Step 1: Calculate the actual result

   5 - 2 equals 3, but expected value is 4 in assertEquals.

2. Step 2: Check annotations and imports

   @Test annotation and imports are correct; function name style is not enforced.

3. Final Answer:

   Wrong expected value in assertEquals -> Option D

4. Quick Check:

   5 - 2 = 3, expected 4 causes failure [OK]

Hint: Verify expected matches actual in assertions [OK]

Common Mistakes:

• Ignoring mismatch between expected and actual values
• Thinking function names must start with 'test_'
• Assuming missing annotations when present

5. You want to test a Kotlin function that returns null for invalid input. Which assertion correctly checks this behavior in JUnit?

hard

A. assertEquals(null, functionResult)

B. assertNull(functionResult)

C. assertTrue(functionResult == null)

D. assertNotNull(functionResult)

Solution

1. Step 1: Understand null checking assertions

   assertNull explicitly checks if a value is null, making tests clear and readable.

2. Step 2: Compare other options

   assertEquals(null, ...) works but is less clear; assertTrue(...) works but less direct; assertNotNull is opposite.

3. Final Answer:

   assertNull(functionResult) -> Option B

4. Quick Check:

   Use assertNull to check null values [OK]

Hint: Use assertNull to check if value is null [OK]

Common Mistakes:

• Using assertNotNull when expecting null
• Using assertEquals(null, ...) instead of assertNull
• Using assertTrue with complex conditions unnecessarily