Mutation testing concept (PIT) in JUnit - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is mutation testing?

Mutation testing is a way to check how good your tests are by making small changes (mutations) to your code and seeing if the tests catch those changes.

beginner

What does PIT stand for in mutation testing?

PIT stands for "PIT" or "Pitest", a popular mutation testing tool for Java that works well with JUnit tests.

intermediate

How does PIT help improve test quality?

PIT changes parts of your code and runs tests to see if tests fail. If tests don't fail, it means tests might miss bugs, so you can improve them.

beginner

What is a 'mutant' in mutation testing?

A mutant is a version of your code with a small change introduced by the mutation testing tool to check if tests detect it.

intermediate

What does it mean if a mutant survives in PIT testing?

If a mutant survives, it means the tests did not catch the change, showing a possible weakness in your test coverage.

What is the main goal of mutation testing?

A To check if tests can find small code changes

B To speed up test execution

C To write new features automatically

D To remove bugs from production code

Mutation testing checks if your tests can detect small changes (mutations) in the code.

In PIT, what is a 'mutant'?

A A test case

B A bug report

C A changed version of the code

D A test runner

A mutant is a version of your code with a small change introduced to test your tests.

If a mutant survives, what does it mean?

A Tests caught the change

B Tests ran faster

C Code is error-free

D Tests missed the change

A surviving mutant means tests did not detect the change, indicating weak test coverage.

Which tool is commonly used for mutation testing in Java with JUnit?

A PIT

B JUnit5

C Selenium

D Mockito

PIT is a popular mutation testing tool for Java that works with JUnit.

What should you do if mutation testing shows many surviving mutants?

A Ignore them

B Write better tests to catch those mutants

C Delete the code

D Run tests less often

Surviving mutants mean tests miss bugs, so improving tests is needed.

Explain how mutation testing with PIT helps improve your test suite.

Think about how changing code and checking tests can show test quality.

Describe what a 'mutant' is and what it means when it survives mutation testing.

Focus on the mutant concept and test detection.

Practice

1. What is the main purpose of mutation testing using PIT in JUnit projects?

easy

A. To check if tests can detect small changes (mutations) in the code

B. To improve the performance of the application

C. To generate new test cases automatically

D. To refactor the code for better readability

Solution

1. Step 1: Understand mutation testing concept

   Mutation testing changes code slightly to see if tests catch the changes.

2. Step 2: Identify PIT's role in JUnit projects

   PIT automates mutation testing to check test effectiveness.

3. Final Answer:

   To check if tests can detect small changes (mutations) in the code -> Option A

4. Quick Check:

   Mutation testing = detect code changes [OK]

Hint: Mutation testing checks test strength by code changes [OK]

Common Mistakes:

• Thinking mutation testing improves app speed
• Confusing mutation testing with test generation
• Assuming mutation testing refactors code

2. Which of the following is the correct way to run PIT mutation testing with Maven in a JUnit project?

easy

A. mvn test:pitest

B. mvn pitest:mutationCoverage

C. mvn mutation:test

D. mvn pit:run

Solution

1. Step 1: Recall PIT Maven command

   The standard Maven command to run PIT is mvn pitest:mutationCoverage.

2. Step 2: Check other options for correctness

   Other options are invalid or do not match PIT's official command syntax.

3. Final Answer:

   mvn pitest:mutationCoverage -> Option B

4. Quick Check:

   PIT Maven command = mvn pitest:mutationCoverage [OK]

Hint: PIT Maven command always starts with 'mvn pitest:' [OK]

Common Mistakes:

• Using incorrect Maven goals like test:pitest
• Confusing PIT commands with other plugins
• Missing the 'mutationCoverage' goal

3. Given the following code snippet and its test, what will PIT report if the test does NOT fail after mutation?

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

Mutation changes '+' to '-'. The test asserts add(2,3) == 5.

medium

A. Mutation killed (test failed)

B. Syntax error in mutation

C. Mutation survived (test passed)

D. Test skipped due to mutation

Solution

1. Step 1: Understand mutation effect

   The '+' operator is changed to '-', so add(2,3) returns -1 instead of 5.

2. Step 2: Analyze test result

   If the test still passes, it means it did not detect the mutation, so mutation survived.

3. Final Answer:

   Mutation survived (test passed) -> Option C

4. Quick Check:

   Mutation survived = test missed bug [OK]

Hint: If test passes despite mutation, mutation survived [OK]

Common Mistakes:

• Assuming mutation killed when test passes
• Confusing mutation survival with syntax errors
• Thinking mutation causes test to skip

4. You run PIT on a JUnit project but notice no mutations are detected. Which of the following is the MOST likely cause?

medium

A. Tests are missing or not executed during mutation testing

B. PIT does not support JUnit tests

C. Mutation testing always reports zero mutations

D. Code has no executable statements

Solution

1. Step 1: Consider PIT and JUnit compatibility

   PIT supports JUnit tests, so PIT does not support JUnit tests is incorrect.

2. Step 2: Analyze why no mutations detected

   If tests are missing or not run, PIT cannot detect mutations, causing zero mutations reported.

3. Final Answer:

   Tests are missing or not executed during mutation testing -> Option A

4. Quick Check:

   No tests run = no mutations detected [OK]

Hint: No tests run means no mutation detection [OK]

Common Mistakes:

• Believing PIT doesn't support JUnit
• Thinking mutation testing always reports mutations
• Assuming code has no executable statements

5. In a JUnit project using PIT, you notice that mutation coverage is low even though your tests pass. What is the BEST way to improve mutation coverage?

hard

A. Ignore mutation coverage and focus on code coverage

B. Increase the number of mutations PIT generates

C. Disable mutation testing for complex classes

D. Add more tests that fail when code is mutated

Solution

1. Step 1: Understand mutation coverage meaning

   Low mutation coverage means tests do not fail when code is changed (mutated).

2. Step 2: Identify how to improve mutation coverage

   Adding tests that detect mutations (fail on changes) improves mutation coverage.

3. Final Answer:

   Add more tests that fail when code is mutated -> Option D

4. Quick Check:

   Better tests = higher mutation coverage [OK]

Hint: Stronger tests catch mutations, raising coverage [OK]

Common Mistakes:

• Trying to increase mutations instead of test quality
• Disabling mutation testing to hide low coverage
• Ignoring mutation coverage in favor of code coverage only