First JUnit test - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a simple JUnit test

What is the result of running this JUnit test class?

JUnit

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

public class SimpleTest {
    @Test
    void testSum() {
        int result = 2 + 3;
        assertEquals(5, result);
    }
}

A Compilation error due to missing import.

B Test fails with an AssertionError.

C Test passes successfully with no errors.

D Runtime exception due to null pointer.

💡 Hint

Check the assertion and the calculation inside the test method.

The test method calculates 2 + 3 which equals 5, and asserts that the result equals 5. Since this is true, the test passes without errors.

❓ assertion

intermediate

Choosing the correct assertion for null check

Which assertion correctly verifies that an object reference is null in a JUnit test?

A assertNull(myObject);

B assertEquals(null, myObject);

C assertTrue(myObject == null);

D assertNotNull(myObject);

💡 Hint

JUnit provides a specific assertion method for null checks.

assertNull(myObject) is the dedicated JUnit assertion to check if myObject is null. While B and C might work, A is the best practice and most readable.

🔧 Debug

advanced

Identify the cause of test failure

Why does this JUnit test fail when run?

JUnit

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

public class DebugTest {
    @Test
    void testDivide() {
        int result = divide(10, 0);
        assertEquals(0, result);
    }

    int divide(int a, int b) {
        return a / b;
    }
}

A The test throws an ArithmeticException due to division by zero.

B The test fails because the assertion expects 0 but gets 10.

C The test fails because divide method returns null.

D The test passes because 10 divided by 0 is 0.

💡 Hint

Think about what happens when dividing by zero in Java.

Dividing by zero in Java causes an ArithmeticException at runtime, so the test fails with an exception before the assertion.

❓ framework

advanced

JUnit 5 annotation for setup before each test

Which annotation runs a method before each test method in a JUnit 5 test class?

A @AfterAll

B @BeforeAll

C @AfterEach

D @BeforeEach

💡 Hint

This annotation runs before every test method, not just once.

@BeforeEach runs the annotated method before each test method. @BeforeAll runs once before all tests.

🧠 Conceptual

expert

Understanding test isolation in JUnit

In JUnit, why is it important that each test method runs in isolation with a fresh instance of the test class?

A To speed up test execution by reusing the same instance for all tests.

B To prevent side effects from one test affecting another, ensuring reliable and independent tests.

C To allow tests to share state easily between methods.

D To reduce memory usage by creating fewer objects.

💡 Hint

Think about what happens if tests share data or state.

Running each test in a fresh instance prevents tests from interfering with each other, making tests reliable and easier to debug.