Unit testing with JUnit in Android Kotlin - Mini App: Build & Ship

Build: Calculator Unit Test

This screen is for testing a simple Calculator class with JUnit unit tests in Kotlin. It checks basic math operations like addition and subtraction.

Target UI

---------------------
| Calculator Tests  |
|-------------------|
| Test Addition     |
| Test Subtraction  |
| Test Multiplication|
| Test Division     |
---------------------

Create a Calculator class with add, subtract, multiply, and divide methods.

Write JUnit test functions to verify each Calculator method returns correct results.

Use assertEquals to check expected vs actual values.

Include a test for division by zero that expects an exception.

Starter Code

Android Kotlin

package com.example.calculatortest

class Calculator {
    // TODO: Implement add, subtract, multiply, divide methods
}

// TODO: Create CalculatorTest class with JUnit tests

Task 1

Task 2

Task 3

Task 4

Task 5

Task 6

Task 7

Solution

Android Kotlin

package com.example.calculatortest

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

class Calculator {
    fun add(a: Int, b: Int): Int = a + b
    fun subtract(a: Int, b: Int): Int = a - b
    fun multiply(a: Int, b: Int): Int = a * b
    fun divide(a: Int, b: Int): Int {
        if (b == 0) throw IllegalArgumentException("Cannot divide by zero")
        return a / b
    }
}

class CalculatorTest {
    private val calculator = Calculator()

    @Test
    fun testAdd() {
        assertEquals(5, calculator.add(2, 3))
    }

    @Test
    fun testSubtract() {
        assertEquals(1, calculator.subtract(3, 2))
    }

    @Test
    fun testMultiply() {
        assertEquals(6, calculator.multiply(2, 3))
    }

    @Test
    fun testDivide() {
        assertEquals(2, calculator.divide(6, 3))
    }

    @Test(expected = IllegalArgumentException::class)
    fun testDivideByZero() {
        calculator.divide(5, 0)
    }
}

We created a simple Calculator class with four methods for basic math operations. Each method returns the expected result.

The divide method checks if the divisor is zero and throws an IllegalArgumentException to prevent division by zero.

The CalculatorTest class uses JUnit 4 annotations. Each test method calls a Calculator method and uses assertEquals to compare the expected and actual results.

The testDivideByZero method expects an exception, verifying that dividing by zero is handled properly.

This setup helps ensure the Calculator works correctly and safely.

Final Result

Completed Screen

---------------------
| Calculator Tests  |
|-------------------|
| ✔ Test Addition    |
| ✔ Test Subtraction |
| ✔ Test Multiplication|
| ✔ Test Division    |
| ✔ Test Divide by 0 |
---------------------

Run tests using Android Studio or command line.

Each test shows a green checkmark if passed.

If a test fails, it shows a red cross and error details.

The divide by zero test confirms exception handling.

Stretch Goal

Add tests for floating point operations with Double numbers in Calculator.

💡 Hint

Create new methods for Double parameters and write tests using assertEquals with delta for precision.

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