Mutation testing concept (PIT) in JUnit - Test Execution Trace

Test Overview

This test runs a simple JUnit test on a Calculator class to verify addition. Mutation testing with PIT will modify the code to check if the test detects errors, ensuring test quality.

Test Code - JUnit 5

JUnit

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

class Calculator {
    int add(int a, int b) {
        return a + b;
    }
}

public class CalculatorTest {
    @Test
    void testAdd() {
        Calculator calc = new Calculator();
        assertEquals(5, calc.add(2, 3));
    }
}

Execution Trace - 9 Steps

Step	Action	System State	Assertion	Result
1	Test starts	JUnit test runner initialized	-	PASS
2	CalculatorTest.testAdd() method invoked	Calculator instance created	-	PASS
3	calc.add(2, 3) called, returns 5	Method returns sum of 2 and 3	-	PASS
4	assertEquals checks if returned value equals 5	Expected=5, Actual=5	assertEquals(5, 5)	PASS
5	Test completes successfully	Test passed	-	PASS
6	PIT mutation testing runs, changes '+' to '-' in add method	Calculator.add method mutated to return a - b	-	PASS
7	Mutated testAdd() runs, calc.add(2, 3) returns -1	Method returns 2 - 3 = -1	-	FAIL
8	assertEquals checks if returned value equals 5	Expected=5, Actual=-1	assertEquals(5, -1)	FAIL
9	Mutation detected by test failure	Test failed due to mutation	-	PASS

Failure Scenario

Failing Condition: Test does not fail when mutation changes '+' to '-'

Execution Trace Quiz - 3 Questions

What does the original test verify?

A That subtracting 2 and 3 returns -1

B That adding 2 and 3 returns 5

C That multiplication works correctly

D That the Calculator class exists

Key Result

Mutation testing helps check if your tests can catch small code changes that introduce errors, improving test quality.