10 Unit testing Interview Questions and Answers for qa engineers

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1. Can you describe your experience in creating and maintaining automated unit tests?

During my time as a software engineer at XYZ Company, I was responsible for creating and maintaining the automated unit test suite for our flagship product. I wrote over 500 unit tests and saw a 60% increase in test coverage over the course of six months.

  1. To create the unit tests, I used a combination of testing frameworks such as JUnit and Mockito, and also wrote custom assertion methods to cover specific edge cases.
  2. I was also able to reduce the time it took to run the entire test suite by implementing parallel testing using TestNG, resulting in a 45% decrease in the time it took to run the full test suite.
  3. Furthermore, I worked with the development team to integrate the unit test suite into our continuous integration pipeline, which helped catch bugs and regressions early in the development cycle.
  4. To maintain the test suite, I regularly reviewed and updated the tests to ensure they were still relevant and covering all possible scenarios.
  5. I also worked with the QA team to incorporate their feedback and bug reports into the unit tests, which helped us catch issues earlier in the development process and ultimately led to a higher quality product.
  6. Overall, I have a strong understanding of the importance of automated unit testing and have seen firsthand the benefits it can bring to a software development project.

2. Have you worked with any specific unit testing frameworks or tools? Which ones?

Sample answer:

Yes, I have experience working with several unit testing frameworks and tools. Some of the most important ones include:

  1. JUnit: I have used this popular unit testing framework extensively throughout my career, especially in Java projects. One notable project where I applied JUnit was a payment processing module for a fintech startup. By writing tests for each component, we were able to ensure accuracy and reliability, and catch any bugs or defects early in the development cycle.
  2. PyTest: I also have experience using this testing framework for Python applications. One project where I applied PyTest was a web scraping application that extracted data from multiple sources. Writing unit tests for each module of the application helped us identify issues quickly and significantly reduced debugging time. Additionally, we were able to reuse the tests as we added more features to the application, saving us time and effort in the long run.
  3. Selenium WebDriver: I have experience using this popular testing tool to automate browser tests. One project where I used Selenium WebDriver was a web application for a travel agency. By automating critical user flows, such as booking a flight or hotel, we were able to dramatically reduce the amount of time and effort required for manual testing. This allowed us to release updates more frequently and with greater confidence that the application was working as expected.

Overall, my experience with unit testing frameworks and tools has taught me the importance of testing early and often, and has helped me deliver high-quality, reliable software to clients and end-users.

3. How do you determine what to test with unit tests?

When it comes to determining what to test with unit tests, I always start by looking at the requirements and specifications for the code I will be testing. By taking an organized approach and understanding the requirements at a fundamental level, it’s easier to ensure that I’m creating useful and effective tests that cover all the necessary functionality.

  1. In addition to the requirements and specifications, I also take into account any known areas of the code that are prone to errors or are considered high-risk. By prioritizing these areas, I can make sure that my tests are focusing on the most critical parts of the codebase.

  2. Another factor to consider is the complexity of the code. Higher complexity requires more testing to ensure that all possible scenarios are covered. By using automation tools, I can generate additional tests that may not have been obvious without a deeper understanding of the codebase.

  3. Finally, I also rely on feedback from my team members or stakeholders to determine which portions of the codebase are most important for testing. By getting their input, I can make sure that all key functionality is covered and that our tests are aligned with overall project goals.

Ultimately, my goal with unit testing is to create comprehensive tests that will minimize the risk of defects and ensure the highest quality code possible. By being proactive and thoughtful in my approach to testing, I can help ensure that our development team is delivering solutions that meet all requirements and exceed stakeholder expectations.

4. How do you ensure that your unit tests remain relevant as the code grows and changes?

One of the biggest challenges in unit testing is ensuring that the tests remain relevant as the codebase grows and changes. To overcome this challenge, I follow these strategies:

  1. Continuous Integration: I ensure that my unit tests are always run as part of our continuous integration (CI) process. This helps us catch any regressions early and ensures that our tests remain relevant each time new code is committed.
  2. Refactoring and maintenance: I regularly review and maintain my unit tests as part of our codebase's regular refactoring processes. This includes making sure that the tests remain accurate and appropriately cover the latest changes in the system.
  3. Metric-driven approach: I use a metric-driven approach to track the effectiveness of my unit tests. For example, I use code coverage tools that help me identify tests that fail to cover certain parts of the codebase. This approach ensures that my tests remain relevant and drive quality improvements.
  4. Test-Driven Development (TDD): I advocate for TDD and apply it to our codebase, which helps ensure that new code is only written when a relevant test case exists. This approach ensures that our unit tests remain relevant as the system grows and helps to identify regression issues early on.

Overall, by following these strategies, I've seen a significant increase in code quality and a reduction in regression issues. For instance, over the last year, by increasing our unit test coverage by 25%, we reduced the number of regressions by 30%. These results demonstrate the effectiveness of my approach in ensuring that our unit tests remain relevant and updated as our codebase evolves.

5. Can you describe the relationship between unit tests and other testing types?

Unit testing is the first line of defense in the software testing process. It checks the functionality of individual software components, also known as units, and ensures they behave as expected. The main objective of unit testing is to identify the errors early in the development cycle when they are easier and less expensive to fix.

Other types of testing, such as integration testing and system testing, are built on top of unit testing. Integration testing checks the interaction between different units or components. It ensures that units integrated together are still working as expected after being combined. Once the units are integrated and their interactions are tested, system testing is executed to evaluate the entire system’s compliance and performance.

According to a study by IBM, early detection of defects through unit testing saves up to 80% of the time and cost that could be spent finding and fixing the same issue at a later stage in the development cycle. Therefore, relying on unit tests should be an essential part of any development project. By confirming that each unit functions correctly, developers can proceed to the later stages of the development cycle with confidence, knowing that each integrated unit has been tested and is working correctly.

In conclusion, unit testing is a crucial part of software development that acts as the foundation for integration and system testing. Early detection through unit testing helps keep projects on track and saves time and resources in the long run.

6. What are some common mistakes you've seen in unit testing?

One of the most common mistakes I have seen in unit testing is not testing all possible scenarios. Some developers may assume that testing their code in a few scenarios is enough, but this can lead to bugs going undetected.

  1. For example, I once worked on a project where we did not thoroughly test a specific edge case. This resulted in the application crashing when a certain user action was taken.
  2. In another project, we did not properly test how our code interacts with third-party APIs. This resulted in unexpected errors that were difficult to debug.

Additionally, I have seen developers make the mistake of not updating their unit tests when making changes to the code. In one project, we made a change to a function but forgot to update the corresponding unit tests. As a result, the tests continued to pass, even though the function was no longer working correctly.

To avoid these mistakes, it's important to thoroughly test all possible scenarios and update tests when making changes to the code. This can catch bugs early on and save time and resources in the long run.

7. What is your approach to debugging failed unit tests?

When a unit test fails, my approach to debugging is to start by reading the failure message and the code location where the error occurred. I then go through the code and check if there are any variables or parameters that are not correctly defined or values that have been hard-coded. I try to find out if there is any logic error or if there are any issues with the sequence of operations taking place.

  1. If it’s an issue with a particular line of code, I put a breakpoint and execute the portion of the code step by step to identify where the error is occurring. Once I locate the issue, I fix it before running the unit tests again to ensure they pass.
  2. If there is a failure pattern happening across multiple test cases, I review the testing data and use the scientific method to explore possible causes of the error. For instance, I deal with a small set of testing data to identify patterns, diagnose errors, and fix the issue once identified.

Additionally, I might try rerunning the tests in debugging mode and reading line by line of code to identify the source of the problem. I dive deeper into logs, exceptions, and other relevant information that might have been generated during the testing process. Lastly, If I cannot identify the root cause even after trying all the other approaches, I collaborate with other experts in that particular field and brainstorm possible solutions to the issue at hand.

Through this approach, I have been able to identify and fix multiple bugs in my previous projects. For instance, about 80% of my unit tests passed without issues in my most recent project. In this way, I am confident that I can make a significant contribution to your organization’s remote team while ensuring efficiency and effectiveness in debugging and fixing failed unit tests.

8. How do you know when you have enough unit test coverage?

Sample Answer:

  1. Code Coverage Metrics
  2. Data-driven tools can help calculate coverage statistics, such as measuring the percentage of lines of code that are executed by unit tests. For instance, if your code coverage is at 80%, you can assume that 80% of your code is adequately unit-tested.

  3. Bug Detection Ratio
  4. An additional method is to compare the number of bugs discovered by unit tests against the total bugs found in the system. If the majority of bugs are found by unit tests, this is a signal that there is still less unit test coverage.

  5. Risk Assessment
  6. The degree of test coverage required may rely on the magnitude of the project, the risk of errors, and other variables. For instance, test coverage of 100% may be critical for medical devices or military systems, while other projects might need a lesser degree of coverage.

Overall, the appropriate degree of test coverage is subjective to the specific task, but a comprehensive evaluation of code coverage, bug detection ratio, and risk evaluation can contribute to making an informed decision.

9. How do you prioritize which unit tests to create?

As a software developer, I prioritize unit tests based on their importance and how often they impact the software. By doing so, I ensure that critical functionality is thoroughly tested, and changes to the application do not cause regression errors. Here are some factors that I tend to consider while prioritizing unit tests:

  1. Business requirements: I start by reviewing the business requirements and mapping them to the application's modules. This enables me to understand which modules are more critical and which ones don't require extensive attention. For example, if the application has a payment module, thorough testing is necessary to ensure that transactions are performed securely and accurately.
  2. Code complexity: I analyze the application's code complexity and its dependencies. I usually start with testing simple, functional components like utility functions and gradually move towards testing bigger, more complex modules. This helps me identify issues earlier in the development cycle.
  3. Risk assessment: I assess how likely the code is to break and how severe that would be. If code is mission-critical, it should have comprehensive testing to prevent costly production errors that affect the business revenue. On the other hand, modules that are unlikely to fail can have fewer tests, and I can use my judgment to decide on what is necessary.
  4. Code coverage: Achieving high code coverage is indicative of better quality code. As a developer, I aim for high test coverage, making sure that all code paths are tested. This can help identify edge cases that can cause errors, leading to better overall software quality. I track code coverage metrics and aim for 90%+ code coverage rates.

By considering these elements and analysing the software, I am able to prioritise the unit tests that are essential to have in place. This approach results in a well-tested application that is more stable and less prone to errors. It also lowers the rate of client complaints and increases user trust in the software.

10. Can you provide an example of a complex unit testing scenario you've encountered and how you approached it?

During my time at XYZ Inc, I was tasked with testing a complex financial calculation engine. One of the scenarios revolved around the calculation of compound interest on a loan. This required testing all possible inputs and combinations of inputs, including principal amount, interest rate, compounding frequency, and term of the loan.

Since this was a critical component of the application, I decided to approach the testing process with a structured methodology. I created a series of test cases to cover all possible scenarios, including inputs at the boundaries, edge cases, and invalid inputs. I also used a variety of testing tools and techniques, including mocking and stubbing, to verify and validate the results.

After running the tests, I discovered a few issues with the calculation engine. Some of the edge cases were not being handled correctly, leading to incorrect results in some cases. I worked with the development team to triage and fix the issues, and then re-ran the tests to ensure everything was working correctly.

  1. The results of the testing were impressive. We found and fixed all the issues, ensuring that the financial engine was calculating compound interest correctly in all scenarios.
  2. I learned a lot from this experience, including the importance of a structured testing methodology, attention to detail, and the value of working closely with the development team.
  3. Overall, this was a challenging but rewarding experience, and I'm confident that my approach to unit testing would be a valuable asset to any team.


Congratulations on learning about unit testing interview questions and answers for 2023! The next step in your remote qa engineer job search journey is to write an impressive cover letter that highlights your skills and experiences. You can check out our guide on writing a cover letter to help you stand out to potential employers. Don't forget to tailor your cover letter to the company and job you're applying for! Another next step is to create an impressive CV showcasing your qualifications as a qa engineer. Check out our guide on writing a resume for qa engineers to help you create a standout resume that accurately represents your experience and skills. Remember, your resume should be tailored to each job you apply for, so make sure you adjust it accordingly. If you're looking for remote qa engineer jobs, be sure to use our website to search for open positions. Our job board is constantly updated with the latest and greatest remote job opportunities, so you can be sure you're applying to top-quality positions. Good luck with your job search!

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