Coverage reports in JUnit - Build an Automation Script

Generate and verify code coverage report for Calculator class

Preconditions (3)

Step 1: Run the CalculatorTest JUnit test class using the build tool

Step 2: Locate the generated JaCoCo coverage report in the target or build directory

Step 3: Open the coverage report HTML file in a browser

Step 4: Verify that the coverage report shows 100% coverage for Calculator class methods add and subtract

✅ Expected Result: The coverage report shows 100% line and branch coverage for the Calculator class methods tested

Automation Requirements - JUnit 5 with JaCoCo

Assertions Needed:

Test methods execute without errors

Coverage report file exists after test run

Coverage report indicates 100% coverage for Calculator class methods

Best Practices:

Use JUnit 5 annotations and assertions

Configure JaCoCo plugin properly in build tool

Run tests and generate coverage report in a single build command

Verify coverage report file existence programmatically

Do not hardcode file paths; use relative paths or build tool properties

Automated Solution

JUnit

import static org.junit.jupiter.api.Assertions.*;
import org.junit.jupiter.api.Test;
import java.io.File;

public class CalculatorTest {

    @Test
    public void testAdd() {
        Calculator calc = new Calculator();
        assertEquals(5, calc.add(2, 3));
        assertEquals(-1, calc.add(2, -3));
    }

    @Test
    public void testSubtract() {
        Calculator calc = new Calculator();
        assertEquals(1, calc.subtract(3, 2));
        assertEquals(5, calc.subtract(2, -3));
    }

    @Test
    public void testCoverageReportExists() {
        // Assuming Maven default target directory and JaCoCo report path
        File report = new File("target/site/jacoco/index.html");
        assertTrue(report.exists(), "Coverage report should exist after tests run");
    }
}

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

    public int subtract(int a, int b) {
        return a - b;
    }
}

/*
Note: To generate coverage report, run:
  mvn clean test jacoco:report

Then open target/site/jacoco/index.html to verify coverage manually.
*/

The test class CalculatorTest contains three tests:

• testAdd() verifies the add method with two cases.
• testSubtract() verifies the subtract method with two cases.
• testCoverageReportExists() checks if the JaCoCo coverage report file exists after running tests.

The Calculator class is simple with two methods to test.

Running mvn clean test jacoco:report will execute tests and generate the coverage report.

The coverage report file existence is asserted programmatically to confirm report generation.

Manual verification of coverage percentage is recommended by opening the HTML report in a browser.

Common Mistakes - 4 Pitfalls

Not configuring JaCoCo plugin in the build tool

Hardcoding absolute file paths for coverage report

Not running the coverage report generation goal after tests

Not asserting coverage report existence in automation

Bonus Challenge

Now add data-driven testing for Calculator add method with 3 different input pairs

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