assertNull and assertNotNull in JUnit - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to check if the object is null using JUnit.

JUnit

[1](myObject);

Drag options to blanks, or click blank then click option'

AassertEquals

BassertNotNull

CassertNull

DassertTrue

Attempts:

3 left

💡 Hint

Common Mistakes

Using assertNotNull instead of assertNull.

Using assertEquals which requires two parameters.

The assertNull method checks if the given object is null. It passes if the object is null.

2fill in blank

medium

Complete the code to check if the object is not null using JUnit.

JUnit

[1](user);

Drag options to blanks, or click blank then click option'

AassertNotNull

BassertFalse

CassertNull

DassertSame

Attempts:

3 left

💡 Hint

Common Mistakes

Using assertNull instead of assertNotNull.

Using assertFalse which expects a boolean condition.

The assertNotNull method checks if the object is not null. It passes if the object exists.

3fill in blank

hard

Fix the error in the assertion to check that the variable is null.

JUnit

[1](expectedValue, actualValue);

Drag options to blanks, or click blank then click option'

AassertNull

BassertNotNull

CassertEquals

DassertTrue

Attempts:

3 left

💡 Hint

Common Mistakes

Using assertNull with two parameters causes a compile error.

Confusing assertNull with assertEquals.

The assertNull method takes only one argument. Using two arguments causes an error. Replace with assertNull and pass only the object to check.

4fill in blank

hard

Fill both blanks to assert that the object is not null and provide a failure message.

JUnit

[1]("Object should not be null", [2]);

Drag options to blanks, or click blank then click option'

AassertNotNull

BassertNull

CmyObject

Dnull

Attempts:

3 left

💡 Hint

Common Mistakes

Using assertNull instead of assertNotNull.

Passing null instead of the object variable.

The assertNotNull method can take a message and the object to check. The message shows if the test fails. The object must be the second argument.

5fill in blank

hard

Fill all three blanks to assert that the user object is null with a custom message.

JUnit

[1]("User must be null", [2]); // Check user
// Next, assert that the session is not null
[3](session);

Drag options to blanks, or click blank then click option'

AassertNull

Buser

CassertNotNull

Dsession

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up assertNull and assertNotNull.

Passing wrong variables to assertions.

Use assertNull with a message and the user object to check it is null. Then use assertNotNull to check the session is not 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