Coverage reports in JUnit - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

JUnit Coverage Report Output

Given the following JUnit test and code, what will the coverage report indicate for the method calculateSum?

JUnit

public class Calculator {
    public int calculateSum(int a, int b) {
        if (a > 0 && b > 0) {
            return a + b;
        } else {
            return 0;
        }
    }
}

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

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

A 50% method coverage, 50% branch coverage

B 100% method coverage, 100% branch coverage

C 100% method coverage, 50% branch coverage

D 50% method coverage, 100% branch coverage

💡 Hint

Think about which parts of the method are executed by the test inputs.

The test calls calculateSum with positive numbers, so the 'if' branch is taken, but the 'else' branch is not. The method is fully executed, so method coverage is 100%, but branch coverage is only 50% because one branch is not tested.

❓ assertion

intermediate

Assertion for Coverage Threshold

Which JUnit assertion correctly checks that the code coverage percentage is at least 80%?

JUnit

double coveragePercentage = 75.5;

A assertTrue(coveragePercentage >= 80);

B assertEquals(coveragePercentage, 80);

C assertFalse(coveragePercentage < 80);

D assertNotNull(coveragePercentage >= 80);

💡 Hint

Use an assertion that checks a condition is true.

assertTrue checks if the condition is true. Here, it verifies coveragePercentage is at least 80. assertEquals checks equality, which is not what we want. assertFalse would invert the condition, and assertNotNull is for objects, not booleans.

🧠 Conceptual

advanced

Understanding Coverage Report Metrics

Which statement best describes the difference between line coverage and branch coverage in a JUnit coverage report?

A Line coverage measures executed methods; branch coverage measures executed classes.

B Line coverage measures executed classes; branch coverage measures executed methods.

C Line coverage measures executed branches; branch coverage measures executed lines.

D Line coverage measures executed lines; branch coverage measures executed decision paths.

💡 Hint

Think about what a branch means in code.

Line coverage counts how many lines of code ran. Branch coverage counts how many possible paths (like if-else decisions) were taken during tests.

🔧 Debug

advanced

Debugging Missing Coverage

A JUnit coverage report shows 0% coverage for a method, but the test suite runs without errors. What is the most likely cause?

A The method is never called by any test.

B The test suite has syntax errors.

C The coverage tool is not installed.

D The method is private and cannot be tested.

💡 Hint

Think about what coverage means.

Coverage measures which code runs during tests. If a method is never called, coverage is zero even if tests pass. Syntax errors or missing tools would cause test failures or no report. Private methods can be covered if called indirectly.

❓ framework

expert

Configuring Coverage Thresholds in JUnit

Which configuration snippet correctly sets a minimum branch coverage threshold of 85% using JaCoCo in a Maven project for JUnit tests?

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>test</phase>
      <goals>
        <goal>report</goal>
      </goals>
    </execution>
    <execution>
      <id>check</id>
      <goals>
        <goal>check</goal>
      </goals>
      <configuration>
        <!-- Threshold config here -->
      </configuration>
    </execution>
  </executions>
</plugin>

A

<rules>
  <rule>
    <element>CLASS</element>
    <limits>
      <limit>
        <counter>LINE</counter>
        <value>COVEREDRATIO</value>
        <minimum>0.85</minimum>
      </limit>
    </limits>
  </rule>
</rules>

B

<rules>
  <rule>
    <element>BUNDLE</element>
    <limits>
      <limit>
        <counter>BRANCH</counter>
        <value>COVEREDRATIO</value>
        <minimum>0.85</minimum>
      </limit>
    </limits>
  </rule>
</rules>

C

<rules>
  <rule>
    <element>METHOD</element>
    <limits>
      <limit>
        <counter>INSTRUCTION</counter>
        <value>COVEREDRATIO</value>
        <minimum>0.85</minimum>
      </limit>
    </limits>
  </rule>
</rules>

D

<rules>
  <rule>
    <element>BUNDLE</element>
    <limits>
      <limit>
        <counter>LINE</counter>
        <value>COVEREDRATIO</value>
        <minimum>0.85</minimum>
      </limit>
    </limits>
  </rule>
</rules>

💡 Hint

Branch coverage uses the BRANCH counter and is usually set at the BUNDLE level.

To enforce branch coverage threshold, the counter must be BRANCH and element BUNDLE. Other options set thresholds for line or instruction coverage or at different element levels.

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