assertNull and assertNotNull in JUnit - Test Execution Trace

Test Overview

This test checks if a method returns null or a non-null object as expected. It uses assertNull to verify a null result and assertNotNull to verify a non-null result.

Test Code - JUnit 5

JUnit

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

class UserService {
    String findUserById(int id) {
        if (id == 0) return null;
        return "User" + id;
    }
}

public class UserServiceTest {
    UserService service = new UserService();

    @Test
    void testFindUserByIdReturnsNull() {
        String result = service.findUserById(0);
        assertNull(result, "Expected null when user ID is 0");
    }

    @Test
    void testFindUserByIdReturnsNotNull() {
        String result = service.findUserById(1);
        assertNotNull(result, "Expected non-null when user ID is 1");
    }
}

Execution Trace - 8 Steps

Step	Action	System State	Assertion	Result
1	Test starts for testFindUserByIdReturnsNull	JUnit test runner initialized	—	PASS
2	Calls findUserById with id=0	Method returns null	—	PASS
3	assertNull checks that result is null	Result is null	assertNull(result)	PASS
4	Test ends for testFindUserByIdReturnsNull	Test passed	—	PASS
5	Test starts for testFindUserByIdReturnsNotNull	JUnit test runner initialized	—	PASS
6	Calls findUserById with id=1	Method returns "User1"	—	PASS
7	assertNotNull checks that result is not null	Result is "User1"	assertNotNull(result)	PASS
8	Test ends for testFindUserByIdReturnsNotNull	Test passed	—	PASS

Failure Scenario

Failing Condition: assertNull fails if the method returns a non-null value when null is expected, or assertNotNull fails if the method returns null when a non-null value is expected.

Execution Trace Quiz - 3 Questions

What does assertNull verify in the test?

A That the returned value is null

B That the returned value is not null

C That the returned value equals "User1"

D That the method throws an exception

Key Result

Use assertNull to verify that a method returns null when expected, and assertNotNull to verify it returns a valid object. This helps catch unexpected null or non-null values early.

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