assertNull and assertNotNull in JUnit - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does assertNull check in a JUnit test?

assertNull checks that the object being tested is null. If the object is not null, the test fails.

beginner

What is the purpose of assertNotNull in JUnit?

assertNotNull verifies that the object is not null. If the object is null, the test fails.

beginner

Write a simple example of assertNull usage in JUnit.

Example:

String result = null;
assertNull(result);

This test passes because result is null.

beginner

Write a simple example of assertNotNull usage in JUnit.

Example:

String result = "Hello";
assertNotNull(result);

This test passes because result is not null.

intermediate

Why is it important to use assertNull and assertNotNull in tests?

They help confirm that objects are in the expected state (null or not null), which prevents errors like NullPointerException in real use.

What happens if assertNull(object) is used but object is not null?

A The test fails

B The test passes

C The test is skipped

D The test throws an exception

assertNull expects the object to be null. If it is not, the test fails.

Which assertion would you use to check that a method returns a valid object and not null?

A assertTrue

B assertEquals

C assertNull

D assertNotNull

assertNotNull confirms the object is not null, meaning the method returned a valid object.

In JUnit, what is the return type of assertNull and assertNotNull methods?

A boolean

B Object

C void

D int

Both assertNull and assertNotNull are void methods that throw exceptions if the assertion fails.

Which of these is a correct way to use assertNotNull?

A assertNotNull("text");

B assertNotNull(null);

C assertNotNull();

D assertNotNull(5);

You must pass a non-null object to assertNotNull. "text" is a non-null String.

What message can you add to assertNull to help understand test failures?

A assertNull(object, "Object should be null");

B assertNull("Object should be null", object);

C assertNull(object); // Object should be null

D assertNull(object, null);

In JUnit, the message comes first: assertNull(String message, Object object).

Explain in your own words when and why you would use assertNull and assertNotNull in a test.

Think about checking if something exists or not in your code.

Describe a simple test scenario where assertNull would be useful.

Imagine a method that returns null if no user is found.

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