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.