Coverage reports in JUnit

Introduction

Coverage reports show which parts of your code were tested. They help find untested code so you can improve your tests.

After writing tests to see how much code they check.

Before releasing software to ensure important parts are tested.

When fixing bugs to confirm the fix is covered by tests.

During code reviews to verify test completeness.

When adding new features to track test coverage.

Syntax

JUnit

Use a coverage tool like JaCoCo with JUnit tests.

Example Maven plugin configuration:

<plugin>
  <groupId>org.jacoco</groupId>
  <artifactId>jacoco-maven-plugin</artifactId>
  <version>0.8.8</version>
  <executions>
    <execution>
      <goals>
        <goal>prepare-agent</goal>
      </goals>
    </execution>
    <execution>
      <id>report</id>
      <phase>verify</phase>
      <goals>
        <goal>report</goal>
      </goals>
    </execution>
  </executions>
</plugin>

Coverage tools run alongside your tests to collect data.

Reports can be in HTML, XML, or other formats for easy reading.

Examples

Run tests and then generate a coverage report using Maven commands.

JUnit

mvn clean test
mvn jacoco:report

Maven plugin setup to collect coverage data during tests and generate reports automatically.

JUnit

<plugin>
  <groupId>org.jacoco</groupId>
  <artifactId>jacoco-maven-plugin</artifactId>
  <version>0.8.8</version>
  <executions>
    <execution>
      <goals>
        <goal>prepare-agent</goal>
      </goals>
    </execution>
    <execution>
      <id>report</id>
      <phase>verify</phase>
      <goals>
        <goal>report</goal>
      </goals>
    </execution>
  </executions>
</plugin>

Sample Program

This test covers only the add method of Calculator. The subtract method is not tested, so coverage report will show partial coverage.

JUnit

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

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

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

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

Important Notes

Coverage does not guarantee bug-free code but helps find untested parts.

High coverage is good, but tests must also check correct behavior.

Use coverage reports regularly to improve test quality over time.

Summary

Coverage reports show which code is tested and which is not.

Use tools like JaCoCo with JUnit to generate coverage reports.

Review reports to improve your tests and software quality.

Practice

1. What is the main purpose of a coverage report in JUnit testing?

easy

A. To show which parts of the code were executed during tests

B. To list all test cases in the project

C. To measure the speed of test execution

D. To generate test data automatically

Solution

1. Step 1: Understand coverage report purpose

   A coverage report shows which lines or branches of code were run during testing.

2. Step 2: Compare options to definition

   Only To show which parts of the code were executed during tests matches this purpose; others describe unrelated features.

3. Final Answer:

   To show which parts of the code were executed during tests -> Option A

4. Quick Check:

   Coverage report = executed code lines [OK]

Hint: Coverage reports highlight tested code lines quickly [OK]

Common Mistakes:

• Confusing coverage with test case listing
• Thinking coverage measures test speed
• Assuming coverage generates test data

2. Which tool is commonly used with JUnit to generate coverage reports?

easy

A. JaCoCo

B. JUnitParams

C. Mockito

D. Selenium

Solution

1. Step 1: Identify coverage tools for JUnit

   JaCoCo is a popular Java code coverage tool that integrates with JUnit.

2. Step 2: Eliminate unrelated tools

   Selenium is for UI testing, Mockito for mocking, JUnitParams for parameterized tests, not coverage.

3. Final Answer:

   JaCoCo -> Option A

4. Quick Check:

   Coverage tool with JUnit = JaCoCo [OK]

Hint: JaCoCo is the go-to coverage tool for Java tests [OK]

Common Mistakes:

• Confusing Selenium with coverage tools
• Mixing mocking frameworks with coverage
• Assuming JUnitParams generates coverage

3. Given this JaCoCo coverage summary:
Lines covered: 80
Lines missed: 20
What is the coverage percentage?

medium

A. 100%

B. 20%

C. 80%

D. 60%

Solution

1. Step 1: Calculate total lines

   Total lines = covered + missed = 80 + 20 = 100 lines.

2. Step 2: Calculate coverage percentage

   Coverage % = (covered / total) * 100 = (80 / 100) * 100 = 80%.

3. Final Answer:

   80% -> Option C

4. Quick Check:

   80 covered of 100 total = 80% [OK]

Hint: Coverage % = covered lines ÷ total lines x 100 [OK]

Common Mistakes:

• Using missed lines as coverage
• Forgetting to add covered and missed for total
• Mixing up numerator and denominator

4. You ran tests with JaCoCo but the coverage report shows 0% coverage. What is the most likely cause?

medium

A. Tests did not execute any code

B. JaCoCo was not properly attached to the test run

C. All tests passed successfully

D. The source code is empty

Solution

1. Step 1: Analyze 0% coverage meaning

   0% coverage means no code was recorded as executed during tests.

2. Step 2: Identify common causes

   If tests ran but coverage is zero, JaCoCo likely was not attached or configured correctly.

3. Final Answer:

   JaCoCo was not properly attached to the test run -> Option B

4. Quick Check:

   0% coverage usually means missing JaCoCo setup [OK]

Hint: Check JaCoCo setup if coverage is zero [OK]

Common Mistakes:

• Assuming tests passed means coverage is full
• Thinking empty source code is common
• Ignoring JaCoCo configuration errors

5. You want to improve your test coverage report by including branch coverage in JaCoCo. Which configuration change is needed?

hard

A. Increase the number of test classes

B. Add @BranchCoverage annotation to test methods

C. Use a different testing framework instead of JUnit

D. Enable branchCoverage option in JaCoCo plugin settings

Solution

1. Step 1: Understand branch coverage

   Branch coverage measures if all decision branches (like if/else) are tested.

2. Step 2: Configure JaCoCo for branch coverage

   JaCoCo supports branch coverage via plugin settings; enabling branchCoverage activates it.

3. Step 3: Eliminate incorrect options

   No annotation @BranchCoverage exists; changing framework or test count doesn't enable branch coverage reporting.

4. Final Answer:

   Enable branchCoverage option in JaCoCo plugin settings -> Option D

5. Quick Check:

   Branch coverage = enable in JaCoCo config [OK]

Hint: Enable branchCoverage in JaCoCo config to track branches [OK]

Common Mistakes:

• Looking for annotations to enable coverage
• Thinking more tests auto-enable branch coverage
• Switching frameworks unnecessarily