Unit testing with JUnit in Android Kotlin - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a basic JUnit test function in Kotlin.

Android Kotlin

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

class CalculatorTest {
  @Test
  fun testAddition() {
    val result = 2 + 3
    assertEquals(5, result[1])
  }
}

Drag options to blanks, or click blank then click option'

A)

B]

C}

D;

Attempts:

3 left

💡 Hint

Common Mistakes

Using a bracket or brace instead of a parenthesis to close the function call.

Adding a semicolon which is not needed in Kotlin.

The assertEquals function call must be properly closed with a parenthesis ")".

2fill in blank

medium

Complete the code to import the JUnit Assert class correctly.

Android Kotlin

import org.junit.[1].*

Drag options to blanks, or click blank then click option'

AAssert

BTest

CAssertions

DRule

Attempts:

3 left

💡 Hint

Common Mistakes

Importing Test instead of Assert, which only provides the @Test annotation.

Using Assertions which is from JUnit 5, but here we use JUnit 4 style.

The Assert class contains the assertion methods like assertEquals used in tests.

3fill in blank

hard

Fix the error in the test function annotation to make it a valid JUnit test.

Android Kotlin

class SampleTest {
  @[1]
  fun sampleTest() {
    assertTrue(true)
  }
}

Drag options to blanks, or click blank then click option'

ATestCase

Btest

CTest

DTestFunction

Attempts:

3 left

💡 Hint

Common Mistakes

Using lowercase @test which is not recognized by JUnit.

Using @TestCase or @TestFunction which do not exist.

The correct annotation to mark a function as a JUnit test is @Test with capital T.

4fill in blank

hard

Complete the code to write an assertion that checks if two strings are equal.

Android Kotlin

class StringTest {
  @Test
  fun testStrings() {
    val expected = "hello"
    val actual = "hello"
    assert[1](expected, actual)
  }
}

Drag options to blanks, or click blank then click option'

AEquals

BEqualsIgnoreCase

C)

D]

Attempts:

3 left

💡 Hint

Common Mistakes

Using assertEqualsIgnoreCase which does not exist in JUnit.

Closing the call with a bracket instead of a parenthesis.

The assertion method is assertEquals and it must be closed with a parenthesis.

5fill in blank

hard

Fill all three blanks to write a test that checks if a list contains a specific element.

Android Kotlin

class ListTest {
  @Test
  fun testContains() {
    val items = listOf("apple", "banana", "cherry")
    assertTrue(items.[1]("banana") [2] [3])
  }
}

Drag options to blanks, or click blank then click option'

Acontains

B==

Ctrue

Dequals

Attempts:

3 left

💡 Hint

Common Mistakes

Using equals instead of contains to check list membership.

Not comparing the result to true, which is needed for assertTrue.

The list method contains checks if an element is present, and assertTrue expects a boolean expression, so we check if items.contains("banana") == true.

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