assertNull and assertNotNull in JUnit - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ assertion

intermediate

Using assertNull to check a null object

Given the following JUnit test snippet, what will be the test result when executed?

JUnit

String testString = null;
assertNull(testString);

A Test fails because testString is null

B Test passes because testString is null

C Test fails with NullPointerException

D Test passes but warns about null usage

💡 Hint

assertNull passes only if the object is null.

assertNull checks if the given object is null. Since testString is null, the assertion passes and the test passes.

❓ assertion

intermediate

Using assertNotNull to check a non-null object

What happens when the following JUnit assertion runs?

JUnit

String testString = "hello";
assertNotNull(testString);

A Test passes but logs a warning

B Test fails because testString is not null

C Test fails with NullPointerException

D Test passes because testString is not null

💡 Hint

assertNotNull passes only if the object is not null.

assertNotNull checks that the object is not null. Since testString contains "hello", the assertion passes and the test passes.

❓ Predict Output

advanced

What error occurs with assertNull on a non-null object?

Consider this JUnit test code snippet. What is the test result?

JUnit

Integer number = 5;
assertNull(number);

A Test fails because number is not null

B Test passes because 5 is a valid Integer

C Test fails with NullPointerException

D Test passes but warns about non-null value

💡 Hint

assertNull expects the object to be null to pass.

assertNull fails if the object is not null. Since number is 5, the assertion fails and the test fails.

❓ Predict Output

advanced

What happens when assertNotNull is used on a null object?

Analyze this JUnit test snippet and select the correct test result.

JUnit

Object obj = null;
assertNotNull(obj);

A Test fails because obj is null

B Test passes because obj is declared

C Test throws NullPointerException

D Test passes but logs a warning

💡 Hint

assertNotNull fails if the object is null.

assertNotNull requires the object to be not null. Since obj is null, the assertion fails and the test fails.

🧠 Conceptual

expert

Choosing correct assertion for null checks in JUnit

You want to write a test that confirms a method returns null when no data is found. Which assertion is the best choice?

A assertNotNull because it confirms the object exists

B assertTrue with a null check inside

C assertNull because it confirms the object is null

D assertEquals with an empty string

💡 Hint

Use assertNull to check if an object is null directly.

assertNull is designed to check if an object is null. It is the clearest and simplest way to confirm a method returns null.