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.

Practice

1. What does the JUnit method assertNull(object) verify in a test?

easy

A. That the object is not null (has a value)

B. That the object is null (has no value)

C. That the object is an instance of a class

D. That the object equals another object

Solution

1. Step 1: Understand assertNull purpose

   The method assertNull checks if the given object is null, meaning it has no value assigned.

2. Step 2: Compare options with definition

   That the object is null (has no value) matches the definition exactly, while others describe different checks.

3. Final Answer:

   That the object is null (has no value) -> Option B

4. Quick Check:

   assertNull = checks null [OK]

Hint: assertNull means value must be null to pass [OK]

Common Mistakes:

• Confusing assertNull with assertNotNull
• Thinking assertNull checks object type
• Assuming assertNull compares equality

2. Which of the following is the correct syntax to assert that an object user is NOT null in JUnit?

easy

A. assertNotNull(user);

B. assertNull(user);

C. assertNotNull user;

D. assertNotNull(user == null);

Solution

1. Step 1: Identify correct method usage

   The correct syntax calls assertNotNull as a method with parentheses and the object as argument.

2. Step 2: Check each option

   assertNotNull(user); uses correct method call syntax. assertNull(user); uses assertNull which is opposite. assertNotNull user; misses parentheses. assertNotNull(user == null); passes a boolean expression, which is incorrect.

3. Final Answer:

   assertNotNull(user); -> Option A

4. Quick Check:

   assertNotNull(object) needs parentheses [OK]

Hint: assertNotNull requires parentheses around the object [OK]

Common Mistakes:

• Omitting parentheses in method call
• Using assertNull instead of assertNotNull
• Passing boolean expressions instead of objects

3. Consider the following JUnit test snippet:

String name = null;
assertNull(name);
name = "Alice";
assertNotNull(name);

What will be the result of running this test?

medium

A. The test will fail at assertNotNull(name)

B. The test will fail at assertNull(name)

C. The test will pass successfully

D. The test will throw a NullPointerException

Solution

1. Step 1: Analyze first assertion

   Initially, name is null, so assertNull(name) passes.

2. Step 2: Analyze second assertion

   Then name is assigned "Alice", which is not null, so assertNotNull(name) passes.

3. Final Answer:

   The test will pass successfully -> Option C

4. Quick Check:

   assertNull(null) and assertNotNull(non-null) pass [OK]

Hint: assertNull passes if null; assertNotNull passes if not null [OK]

Common Mistakes:

• Assuming assertNull fails if variable is null
• Thinking assertNotNull fails with non-null value
• Expecting exceptions from these assertions

4. Identify the error in this JUnit test code:

Object obj = new Object();
assertNull(obj);

medium

A. No error; test will pass

B. Runtime error: NullPointerException

C. Syntax error: missing semicolon

D. assertNull fails because obj is not null

Solution

1. Step 1: Understand object initialization

   The variable obj is assigned a new Object instance, so it is not null.

2. Step 2: Check assertion behavior

   assertNull(obj) expects null, but obj is not null, so the assertion fails.

3. Final Answer:

   assertNull fails because obj is not null -> Option D

4. Quick Check:

   assertNull(non-null) fails test [OK]

Hint: assertNull fails if object is created (not null) [OK]

Common Mistakes:

• Assuming assertNull passes with any object
• Confusing assertion failure with syntax error
• Expecting exceptions instead of assertion failure

5. You have a method getUserEmail() that returns a String email or null if no email is set. Which JUnit assertions correctly test that the email is present and not empty?

hard

A. assertNotNull(email); assertTrue(!email.isEmpty());

B. assertNull(email); assertTrue(email.isEmpty());

C. assertNotNull(email); assertNull(email.isEmpty());

D. assertNull(email); assertFalse(email.isEmpty());

Solution

1. Step 1: Check for non-null email

   Use assertNotNull(email) to ensure email is not null (exists).

2. Step 2: Check email is not empty string

   Use assertTrue(!email.isEmpty()) to confirm email has characters.

3. Final Answer:

   assertNotNull(email); assertTrue(!email.isEmpty()); -> Option A

4. Quick Check:

   Not null and not empty = assertNotNull(email); assertTrue(!email.isEmpty()); [OK]

Hint: Check not null first, then check string is not empty [OK]

Common Mistakes:

• Using assertNull when expecting a value
• Checking isEmpty on null causing exceptions
• Mixing assertNull with isEmpty checks incorrectly