Mutation testing concept (PIT) in JUnit - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

What is the main goal of mutation testing?

Mutation testing is a technique used in software testing. What is its primary purpose?

A To check if the test suite can detect small changes (mutations) in the code

B To find syntax errors in the source code before compilation

C To automatically generate user interface tests

D To measure the performance of the application under load

💡 Hint

Think about how mutation testing challenges your tests.

Mutation testing introduces small changes to code and checks if tests fail, ensuring tests are effective.

❓ Predict Output

intermediate

What is the mutation score for this test result?

Given a mutation testing run with 100 mutants generated, 80 mutants killed by tests, and 20 mutants survived, what is the mutation score percentage?

A 80%

B 20%

C 100%

D 0%

💡 Hint

Mutation score = (killed mutants / total mutants) * 100

Mutation score is the percentage of mutants detected (killed) by tests out of total mutants.

❓ assertion

advanced

Which assertion correctly verifies a mutation testing report's killed mutants count?

You have a variable killedMutants = 45 from a mutation testing report. Which JUnit assertion correctly checks that killedMutants equals 45?

A assertNull(killedMutants);

B assertEquals(45, killedMutants);

C assertFalse(killedMutants != 45);

D assertTrue(killedMutants == 45);

💡 Hint

Use the assertion that compares expected and actual values directly.

assertEquals(expected, actual) is the standard way to check equality in JUnit.

🔧 Debug

advanced

Why does PIT report 0% mutation coverage despite tests passing?

You run PIT mutation testing on your Java project. All unit tests pass, but PIT reports 0% mutation coverage. What is the most likely cause?

A The tests have syntax errors and do not run

B The code has no mutations generated by PIT

C The tests do not execute the mutated code paths at all

D PIT only works with integration tests, not unit tests

💡 Hint

Think about what it means if mutants survive despite tests passing.

If tests never run the mutated code, mutants survive and coverage is 0%.

❓ framework

expert

Which PIT configuration option excludes test classes from mutation analysis?

In PIT configuration for a Maven project, which option correctly excludes test classes from mutation testing?

A <targetTests>**/*Test*</targetTests>

B <targetClasses>**/*Test*</targetClasses>

C <excludedMethods>test*</excludedMethods>

D <excludedClasses>**/*Test*</excludedClasses>

💡 Hint

Look for the option that excludes classes by pattern.

excludedClasses tells PIT which classes to skip; using **/*Test* excludes test classes.