Test independence in JUnit - Test Execution Trace

Test Overview

This test checks that two JUnit test methods run independently without sharing state. It verifies that the counter variable resets for each test method, ensuring test independence.

Test Code - JUnit 5

JUnit

import static org.junit.jupiter.api.Assertions.assertEquals;
import org.junit.jupiter.api.BeforeEach;
import org.junit.jupiter.api.Test;

public class CounterTest {
    private int counter;

    @BeforeEach
    void setUp() {
        counter = 0;
    }

    @Test
    void testIncrementOnce() {
        counter++;
        assertEquals(1, counter);
    }

    @Test
    void testIncrementTwice() {
        counter++;
        counter++;
        assertEquals(2, counter);
    }
}

Execution Trace - 4 Steps

Step	Action	System State	Assertion	Result
1	JUnit framework initializes test class instance and calls @BeforeEach method	counter variable is set to 0	-	PASS
2	testIncrementOnce method runs and increments counter by 1	counter value is 1	assertEquals(1, counter) verifies counter is 1	PASS
3	JUnit framework initializes new test class instance and calls @BeforeEach method again	counter variable is reset to 0	-	PASS
4	testIncrementTwice method runs and increments counter by 2	counter value is 2	assertEquals(2, counter) verifies counter is 2	PASS

Failure Scenario

Failing Condition: If the counter variable is shared between tests and not reset, the second test will fail because counter will not start at 0.

Execution Trace Quiz - 3 Questions

Why does the counter variable reset to 0 before each test method?

A Because the tests run in parallel

B Because the @BeforeEach method runs before every test to reset state

C Because the counter variable is static and shared

D Because the counter is initialized only once

Key Result

Always reset shared state before each test method using setup methods like @BeforeEach to ensure tests run independently and do not affect each other.