@BeforeEach method in JUnit - Test Execution Trace

Test Overview

This test uses the @BeforeEach method to set up a fresh list before each test runs. It verifies that the list starts empty and can add an item correctly.

Test Code - JUnit 5

JUnit

import org.junit.jupiter.api.BeforeEach;
import org.junit.jupiter.api.Test;
import static org.junit.jupiter.api.Assertions.*;
import java.util.ArrayList;
import java.util.List;

public class ListTest {
    private List<String> list;

    @BeforeEach
    void setUp() {
        list = new ArrayList<>();
    }

    @Test
    void testListStartsEmpty() {
        assertTrue(list.isEmpty(), "List should be empty before each test");
    }

    @Test
    void testAddItem() {
        list.add("item1");
        assertEquals(1, list.size(), "List size should be 1 after adding an item");
        assertEquals("item1", list.get(0), "First item should be 'item1'");
    }
}

Execution Trace - 10 Steps

Step	Action	System State	Assertion	Result
1	JUnit starts test run and prepares to run testListStartsEmpty	No list instance exists yet	-	PASS
2	JUnit calls @BeforeEach setUp() method to initialize list as empty ArrayList	list is a new empty ArrayList	-	PASS
3	JUnit runs testListStartsEmpty method	list is empty	assertTrue(list.isEmpty()) verifies list is empty	PASS
4	JUnit finishes testListStartsEmpty and prepares to run testAddItem	Previous list discarded	-	PASS
5	JUnit calls @BeforeEach setUp() method again to initialize a new empty list	list is a new empty ArrayList	-	PASS
6	JUnit runs testAddItem method	list is empty at start of testAddItem	-	PASS
7	testAddItem adds 'item1' to list	list contains one element: 'item1'	-	PASS
8	testAddItem asserts list size is 1	list size is 1	assertEquals(1, list.size()) verifies list size is 1	PASS
9	testAddItem asserts first item is 'item1'	list.get(0) returns 'item1'	assertEquals('item1', list.get(0)) verifies first item	PASS
10	JUnit finishes testAddItem and completes test run	All tests passed	-	PASS

Failure Scenario

Failing Condition: If @BeforeEach setUp() method does not initialize the list, list will be null causing NullPointerException

Execution Trace Quiz - 3 Questions

What does the @BeforeEach method do in this test?

A It runs only once before all tests

B It runs after all tests to clean up resources

C It runs before each test to set up a fresh list

D It runs only if a test fails

Key Result

Using @BeforeEach ensures each test starts with a clean state, preventing tests from affecting each other and making tests reliable and independent.