Unit testing with JUnit in Android Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this JUnit test?

Consider the following Kotlin function and its JUnit test. What will be the test result when running this test?

Android Kotlin

import org.junit.jupiter.api.Test
import org.junit.jupiter.api.Assertions.assertEquals

fun multiply(a: Int, b: Int): Int {
    return a * b
}

@Test
fun testMultiply() {
    assertEquals(20, multiply(4, 5))
}

A Test fails with AssertionError

B Test passes successfully

C Test fails with NullPointerException

D Test does not compile due to missing import

💡 Hint

Check if the expected value matches the actual multiplication result.

The multiply function returns 4 * 5 = 20, which matches the expected value in assertEquals, so the test passes.

❓ assertion

intermediate

Which assertion correctly tests that a list contains exactly 3 items?

You have a Kotlin list: val items = listOf("a", "b", "c"). Which JUnit assertion correctly verifies the list size is 3?

Android Kotlin

val items = listOf("a", "b", "c")

A assertEquals(3, items.size)

B assertTrue(items.count() == 3)

C assertFalse(items.isEmpty())

D assertNull(items[3])

💡 Hint

Check which assertion directly compares the size to 3.

assertEquals(3, items.size) directly checks that the list size is exactly 3. Other options either check different conditions or cause errors.

🔧 Debug

advanced

Why does this JUnit test fail with NullPointerException?

Examine the Kotlin test code below. Why does it throw NullPointerException during execution?

Android Kotlin

import org.junit.jupiter.api.Test
import org.junit.jupiter.api.Assertions.assertEquals

class Calculator {
    fun divide(a: Int, b: Int): Int? {
        return if (b == 0) null else a / b
    }
}

@Test
fun testDivide() {
    val calc = Calculator()
    val result = calc.divide(10, 0)
    assertEquals(0, result!!)
}

A assertEquals expects non-null but result is nullable

B divide function throws ArithmeticException for division by zero

C result is null and using !! causes NullPointerException

D Calculator class is not instantiated properly

💡 Hint

Check what happens when divide returns null and !! is used.

The divide function returns null when dividing by zero. Using result!! forces a non-null assertion on null, causing NullPointerException.

🧠 Conceptual

advanced

What is the purpose of the @BeforeEach annotation in JUnit 5?

In JUnit 5, what does the @BeforeEach annotation do in a test class?

A Runs the annotated method before each test method executes

B Runs the annotated method once before all tests in the class

C Marks a test method to be ignored during execution

D Specifies the order in which test methods run

💡 Hint

Think about setup tasks needed before every test.

@BeforeEach runs the annotated method before every test method to set up test conditions.

❓ framework

expert

Which JUnit 5 feature allows parameterized tests with multiple input values?

You want to run the same test method multiple times with different input values in JUnit 5. Which feature supports this?

A @Test with multiple assert statements

B @RepeatedTest with @Test annotation

C @TestFactory with dynamic tests

D @ParameterizedTest with @ValueSource or @CsvSource annotations

💡 Hint

Look for annotations that provide input data to tests.

@ParameterizedTest combined with @ValueSource or @CsvSource runs the test multiple times with different inputs.