@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.

Practice

1. What is the main purpose of the @BeforeEach annotation in JUnit?

easy

A. To clean up resources after all tests have run

B. To run a method only once before all tests in the class

C. To mark a method as a test case

D. To run a method before each test method to set up test conditions

Solution

1. Step 1: Understand the role of @BeforeEach

   The @BeforeEach annotation marks a method to run before every test method, preparing the test environment.

2. Step 2: Compare with other annotations

   @BeforeAll runs once before all tests, @Test marks test methods, and cleanup is done with @AfterAll or @AfterEach.

3. Final Answer:

   To run a method before each test method to set up test conditions -> Option D

4. Quick Check:

   @BeforeEach = setup before every test [OK]

Hint: Runs setup before each test method to avoid repetition [OK]

Common Mistakes:

• Confusing @BeforeEach with @BeforeAll
• Thinking it runs after tests
• Using it to mark test methods

2. Which of the following is the correct way to declare a @BeforeEach method in JUnit 5?

easy

A. public void setup() { }

B. @BeforeEach public void setup() { }

C. void @BeforeEach setup() { }

D. public @BeforeEach void setup() { }

Solution

1. Step 1: Recall correct annotation syntax

   Annotations like @BeforeEach must be placed before the method signature with proper access modifier and return type.

2. Step 2: Check each option

   @BeforeEach public void setup() { } correctly places @BeforeEach before a public void method. public void setup() { } lacks the annotation. public @BeforeEach void setup() { } incorrectly places the access modifier before the annotation (syntax error). void @BeforeEach setup() { } misplaces the annotation after the return type.

3. Final Answer:

   @BeforeEach public void setup() { } -> Option B

4. Quick Check:

   Annotation before method signature = correct [OK]

Hint: Annotation goes immediately before method declaration [OK]

Common Mistakes:

• Placing annotation after method name
• Omitting public access modifier
• Missing parentheses in method declaration

3. Given the following JUnit 5 test class, what will be the output when running the tests?

import org.junit.jupiter.api.*;

class SampleTest {
  int count = 0;

  @BeforeEach
  void setup() {
    count++;
    System.out.println("Setup count: " + count);
  }

  @Test
  void testOne() {
    System.out.println("Test One count: " + count);
  }

  @Test
  void testTwo() {
    System.out.println("Test Two count: " + count);
  }
}

medium

A. Setup count: 1
Test One count: 1
Setup count: 1
Test Two count: 1

B. Setup count: 1
Test One count: 1
Test Two count: 1
Setup count: 2

C. Setup count: 1
Test One count: 1
Setup count: 2
Test Two count: 2

D. Setup count: 0
Test One count: 0
Setup count: 0
Test Two count: 0

Solution

1. Step 1: Understand @BeforeEach execution

   The setup() method runs before each test, incrementing count and printing it.

2. Step 2: Trace the output for each test

   Before testOne(), count becomes 1 and prints "Setup count: 1" then "Test One count: 1". Before testTwo(), count increments to 2 and prints "Setup count: 2" then "Test Two count: 2".

3. Final Answer:

   Setup count: 1 Test One count: 1 Setup count: 2 Test Two count: 2 -> Option C

4. Quick Check:

   @BeforeEach runs before each test, incrementing count [OK]

Hint: Remember @BeforeEach runs before every test method [OK]

Common Mistakes:

• Assuming count resets before each test
• Thinking @BeforeEach runs only once
• Mixing order of print statements

4. Identify the error in the following JUnit 5 test class using @BeforeEach:

import org.junit.jupiter.api.*;

class TestExample {
  int value;

  @BeforeEach
  void initialize() {
    value = 10;
  }

  @Test
  void testValue() {
    Assertions.assertEquals(10, value);
  }

  @BeforeEach
  void reset() {
    value = 20;
  }
}

medium

A. Multiple @BeforeEach methods cause both to run before each test

B. Only the first @BeforeEach method runs, so value is 10 in test

C. JUnit throws an error because multiple @BeforeEach methods are not allowed

D. The test fails because value is never initialized

Solution

1. Step 1: Understand multiple @BeforeEach methods behavior

   JUnit allows multiple methods annotated with @BeforeEach. All run before each test, but order is not guaranteed.

2. Step 2: Analyze effect on variable value

   Both initialize() and reset() run before testValue(). Since reset() sets value last to 20, the assertion expecting 10 will fail.

3. Final Answer:

   Multiple @BeforeEach methods cause both to run before each test -> Option A

4. Quick Check:

   Multiple @BeforeEach methods all run before tests [OK]

Hint: Multiple @BeforeEach methods all run; last one may override setup [OK]

Common Mistakes:

• Assuming only one @BeforeEach method runs
• Expecting JUnit to throw error for multiple @BeforeEach
• Ignoring method execution order

5. You want to test a calculator class where each test needs a fresh calculator instance. Which is the best way to use @BeforeEach to achieve this?

hard

A. Use @BeforeEach to create a new calculator instance before every test

B. Initialize the calculator instance once in a static block

C. Create a new calculator instance inside each test method

D. Reuse the same calculator instance for all tests without resetting

Solution

1. Step 1: Understand test isolation needs

   Each test should have a clean calculator instance to avoid state carryover.

2. Step 2: Choose setup method for fresh instance

   Using @BeforeEach to create a new instance before every test ensures isolation and avoids repeating code inside each test.

3. Final Answer:

   Use @BeforeEach to create a new calculator instance before every test -> Option A

4. Quick Check:

   @BeforeEach creates fresh state for each test [OK]

Hint: Use @BeforeEach to reset objects before every test [OK]

Common Mistakes:

• Creating instances inside test methods causing repetition
• Using static initialization causing shared state
• Reusing instances without resetting state