@Nested inner classes in JUnit - Test Execution Trace

Test Overview

This test demonstrates how to use @Nested inner classes in JUnit to group related test cases. It verifies that the outer and inner test methods run correctly and independently.

Test Code - JUnit 5

JUnit

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

public class CalculatorTest {

    @Test
    void testAddition() {
        assertEquals(5, 2 + 3);
    }

    @Nested
    class SubtractionTests {

        @Test
        void testSimpleSubtraction() {
            assertEquals(2, 5 - 3);
        }

        @Test
        void testNegativeResult() {
            assertEquals(-1, 2 - 3);
        }
    }
}

Execution Trace - 6 Steps

Step	Action	System State	Assertion	Result
1	Test runner starts and loads CalculatorTest class	JUnit test environment initialized	-	PASS
2	Runs outer test method testAddition()	CalculatorTest instance created	assertEquals(5, 2 + 3) verifies 5 equals 5	PASS
3	Discovers @Nested class SubtractionTests	JUnit prepares to run nested tests	-	PASS
4	Runs nested test method testSimpleSubtraction()	SubtractionTests instance created	assertEquals(2, 5 - 3) verifies 2 equals 2	PASS
5	Runs nested test method testNegativeResult()	SubtractionTests instance created	assertEquals(-1, 2 - 3) verifies -1 equals -1	PASS
6	Test runner completes all tests	All tests executed successfully	-	PASS

Failure Scenario

Failing Condition: An assertion in any test method fails, e.g., testSimpleSubtraction expects wrong value

Execution Trace Quiz - 3 Questions

What does the @Nested annotation do in JUnit tests?

A Groups related test methods inside an inner class

B Marks a test method to be ignored

C Runs tests in parallel

D Specifies a test method to run before all tests

Key Result

Using @Nested classes helps organize tests logically, making test suites easier to read and maintain by grouping related tests together.