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.