Unit testing concept in JUnit - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

JUnit test method output

What will be the result of running this JUnit 5 test method?

JUnit

import static org.junit.jupiter.api.Assertions.*;
import org.junit.jupiter.api.Test;

class CalculatorTest {
    @Test
    void testAdd() {
        int result = 2 + 3;
        assertEquals(5, result);
    }
}

A Test fails with AssertionError

B Runtime NullPointerException

C Compilation error due to missing import

D Test passes successfully

💡 Hint

Check the assertion and the calculation result.

The test adds 2 and 3, expecting 5. The assertion matches the result, so the test passes.

❓ assertion

intermediate

Correct assertion for exception testing

Which JUnit 5 assertion correctly tests that a method throws a NullPointerException?

JUnit

void methodThatThrows() {
    throw new NullPointerException();
}

A assertThrows(NullPointerException.class, () -> methodThatThrows());

B assertEquals(NullPointerException.class, methodThatThrows());

C assertTrue(methodThatThrows() instanceof NullPointerException);

D assertNull(methodThatThrows());

💡 Hint

Look for the assertion designed to check exceptions.

assertThrows expects the exception class and a lambda that runs the method. It passes if the exception is thrown.

🔧 Debug

advanced

Identify the cause of test failure

Given this JUnit 5 test, why does it fail?

JUnit

import static org.junit.jupiter.api.Assertions.*;
import org.junit.jupiter.api.Test;

class StringTest {
    @Test
    void testLength() {
        String s = null;
        assertEquals(4, s.length());
    }
}

A Test passes because null length is treated as 0

B AssertionError because length is not 4

C NullPointerException because s is null when calling length()

D Compilation error due to missing import

💡 Hint

Think about calling a method on a null object.

Calling length() on a null String causes NullPointerException at runtime, failing the test.

🧠 Conceptual

advanced

Purpose of unit testing

What is the main purpose of unit testing in software development?

A To check the performance of the system under load

B To verify that individual parts of code work correctly in isolation

C To deploy the software to production automatically

D To test the entire application’s user interface

💡 Hint

Unit testing focuses on small pieces of code, not the whole system.

Unit testing checks small code units like methods or classes to ensure they behave as expected.

❓ framework

expert

JUnit 5 lifecycle method order

What is the correct order of JUnit 5 lifecycle methods when running multiple tests in a class?

A 1, 2, 3, 4, 5

B 1, 3, 2, 4, 5

C 2, 1, 3, 4, 5

D 2, 3, 4, 1, 5

💡 Hint

Remember which methods run once per class and which run before/after each test.

@BeforeAll runs once before all tests, @BeforeEach before each test, then @Test, @AfterEach after each test, and @AfterAll once after all tests.

Practice

1. What is the main purpose of a unit test in JUnit?

easy

A. To check small parts of code for errors early

B. To test the entire application at once

C. To deploy the application automatically

D. To write documentation for the code

Solution

1. Step 1: Understand unit test scope

   Unit tests focus on small parts or units of code, not the whole app.

2. Step 2: Identify main goal

   The goal is to find errors early by testing these small parts individually.

3. Final Answer:

   To check small parts of code for errors early -> Option A

4. Quick Check:

   Unit test purpose = check small code parts early [OK]

Hint: Unit tests check small code parts, not whole app [OK]

Common Mistakes:

• Confusing unit tests with integration or system tests
• Thinking unit tests deploy or document code
• Assuming unit tests cover the entire application

2. Which annotation is used in JUnit to mark a method as a test?

easy

A. @Run

B. @Assert

C. @Test

D. @Check

Solution

1. Step 1: Recall JUnit test annotation

   JUnit uses @Test to mark methods as tests.

2. Step 2: Verify other options

   @Run, @Check, and @Assert are not valid JUnit test annotations.

3. Final Answer:

   @Test -> Option C

4. Quick Check:

   JUnit test method = @Test annotation [OK]

Hint: JUnit test methods use @Test annotation [OK]

Common Mistakes:

• Using @Run instead of @Test
• Confusing @Assert with test annotation
• Assuming @Check is a JUnit annotation

3. What will be the result of this JUnit test?

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

public class CalcTest {
    @Test
    void testSum() {
        int 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 calculates 2 + 3 and stores 5 in result.

2. Step 2: Check assertion

   assertEquals(5, result) compares expected 5 with actual 5, so it matches.

3. Final Answer:

   Test passes successfully -> Option A

4. Quick Check:

   2+3=5 matches expected 5 [OK]

Hint: If expected equals actual, test passes [OK]

Common Mistakes:

• Assuming test fails when values match
• Thinking import is missing (it's present)
• Expecting runtime errors without cause

4. Identify the error in this JUnit test code:

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

public class SampleTest {
    @Test
    public void testMultiply() {
        int result = 3 * 4
        assertEquals(12, result);
    }
}

medium

A. Incorrect import statement

B. Wrong assertion method used

C. Test method missing @Test annotation

D. Missing semicolon after multiplication statement

Solution

1. Step 1: Check syntax in testMultiply method

   The line 'int result = 3 * 4' is missing a semicolon at the end.

2. Step 2: Verify other parts

   @Test annotation is present, assertion method is correct, imports are correct.

3. Final Answer:

   Missing semicolon after multiplication statement -> Option D

4. Quick Check:

   Java statements need semicolons [OK]

Hint: Look for missing semicolons in Java test methods [OK]

Common Mistakes:

• Ignoring missing semicolon errors
• Thinking @Test annotation is missing
• Assuming wrong assertion method

5. You want to write a unit test that checks if a method throws an exception when given invalid input. Which JUnit 5 feature should you use?

hard

A. Use @BeforeEach to catch exceptions

B. assertThrows to verify the exception is thrown

C. assertEquals to compare exception messages

D. Use @Disabled to skip the test

Solution

1. Step 1: Understand exception testing in JUnit 5

   JUnit 5 provides assertThrows to check if a specific exception is thrown by code.

2. Step 2: Evaluate other options

   assertEquals compares values, not exceptions; @BeforeEach runs before tests; @Disabled skips tests.

3. Final Answer:

   assertThrows to verify the exception is thrown -> Option B

4. Quick Check:

   Use assertThrows for exception tests [OK]

Hint: Use assertThrows to test exceptions in JUnit 5 [OK]

Common Mistakes:

• Using assertEquals to check exceptions
• Misusing @BeforeEach for exception handling
• Disabling tests instead of testing exceptions