JUnit is an excellent solution for developers working with the Java programming language. It can be used to implement unit testing as well as UI testing. It is one of the most sought after competencies for organizations looking to improve their code quality and programming speed. This article would discuss the standard JUnit interview questions to help you prepare and make it to that shortlist! 

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Top 15 JUnit Interview Questions & Answers

1. What is JUnit?

JUnit is a framework used for conducting unit testing of Java code. It is considered the foundation of developer-side testing on the JVM engine. Therefore, JUnit is an integral part of the programming field and test-driven development. 

2. What do you understand by testing and unit testing?

Testing is the process of checking whether the application fulfills the requirements and achieves the desired functionalities. Unit testing refers to assessing an individual functionality or a unit of the application.  

3. Differentiate between manual and automated testing.

Manual testing is executed without test scripts and requires dedicated human effort for the various steps involved. On the other hand, automated testing can be done without human assistance using technology tools and software programs. Test automating is cheaper and less time-consuming than manual testing. Also, manual testing is less reliable as it cannot be programmed. 

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4. Do you have to write a test case for every logic?

A test case is a code that is written to establish the logic of a program. In JUnit, it is unnecessary to write a test case for every logic but only for those that can be reasonably broken. 

A unit test case would comprise a collection of input data and expected output. The org.junit package contains several classes and interfaces to help you in unit testing, such as Assert, Test, Before, After, etc. 

5. Mention some examples of tools with which you can integrate JUnit?

The JUnit is a user-friendly framework for Java development. It can be easily extended and integrated with Eclipse IDE, Apache Ant, and Maven. 

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6. List some useful JUnit extensions and explain their purpose. 

There are four main JUnit extensions, namely Cactus, JWebUnit, XMLUnit, and MockObject. We have described their uses in detail below. 

• Cactus: It is a framework for testing server-side Java code that implements tests inside a container. The cactus ecosystem has two components, namely the Cactus Framework and Cactus Integration Modules. The framework is the engine that provides the API for writing tests, while the integration modules take care of the front-end with plugins’ help.
• JWebUnit: This java-based framework provides a unified testing interface by combining the capabilities of HtmlUnit and Selenium. JWebUnit makes it a breeze to navigate web applications via links, table content validation, form entry and submission, and other features. In addition to the high-level Java API, it allows rapid test creation with its ready-to-use assertions. 

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• XMLUnit: It offers a single extension class called XMLTestCase. XMLUnit also provides supporting classes that allow assertions about the validity of an XML piece (Validator class), differences between two pieces of XML (Diff and DetailedDiff), the outcome of transforming XML (Transformer class), among others.
• MockObject: When it is impossible or impracticable to include real objects in a unit test, mock objects can prove to be a boon. You can simulate the behavior of complex objects and incorporate them with the following coding steps:

1. Create instances of mock objects
2. Set state and expectations
3. Invoke domain code using mock objects as parameters
4. Verify consistency

7. How do you test the ‘protected’ and ‘private’ methods?

In the protected method, test class and target class are declared in the same package. However, in the private method, there is no direct way of testing. Either you have to change your method to ‘protected’ or do the testing manually. 

8. What happens in JUnit when the return type is “string”?

In this situation, the compilation would pass, but the execution will fail. This would occur because all JUnit test methods are designed to return “void.” 

9. Define (i) Fixtures (ii) Test Suit (iii) Test Runner (iv) JUnit Classes

(i) Fixtures are used as the baseline for executing tests, establishing the repeatability of the results. A test fixture comprises a fixed state of a set of objects. Methods include:

• setUp(): It runs before a test invocation.
• tearDown(): It runs after a test method.

(ii) Test Runner: It executes the test cases.

(iii) JUnit Classes: They contain methods to be used in writing and testing JUnits. 

(iv) Test Suite is a bundle or collection of unit test cases to be run together. You can run a suite test in JUnit with either @Suite or the @RunWith annotation. 

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10. Mention a few frequently used JUnit annotations.

Annotations are simply meta-tags that you can add to your code. The @Test annotation marks a particular method as a test method. Some other useful annotations include @Before, @BeforeClass, @After, and @AfterClass, and @Ignores. To implement a JUnit test, you would need to know the importance of these annotations. So, let us get acquainted with them one by one. 

• Test: This annotation tells JUnit that it can run the public void method as a test case. It is basically a replacement for org.junit.TestCase
• Before: If you want to execute any statements (for example, preconditions) before a particular test case, you use the @Before. This annotation allows you to run similar objects before test methods.
• Before Class: You can use the @BeforeClass annotation in JUnit to run statements before all test cases.
• After: It causes some statements to run after each test case, such as to delete temporary files, reset variables, etc.
• After Class: The @AfterClass annotation lets you execute statements after all the test cases. For instance, the release of all resources after the execution is done.
• Ignore: It is used to ignore certain statements during the execution. For example, disabling test cases.

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11. Explain the different types of JUnit classes. 

Assert, TestCase, and TestResult are some of the vital JUnit classes. Assert provides methods to the test cases. TestCase contains a test case, defining the fixture to execute multiple tests. TestResult includes methods to collect the results. 

12. When do you write a unit test?

In a typical development cycle, unit tests are written before the code. This practice enables developers to maintain the quality of their code. 

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13. Why should you refrain from using System.out.printIn() for debugging?

If you use System.out.printIn() to debug your code, it would benefit in the long run. Every time the program is run, it would result in manual scanning of the entire output to ensure that the code is operating correctly. So, it would require relatively less time to code JUnit methods and perform testing on class files.

14. Where does the test garbage in JUnit go?

The test runner holds the references for the duration of the test. In the case of an extended test having many test instances, the garbage may not be collected until the end of the test run. We can use the tearDown() to collect the garbage before the test run completes. In this method, we explicitly set an object to null. 

15. How do you install Junit?

The first step is to download JUnit 5, the latest version of JUnit ( it would be a file named junit.zip). We would be required to unzip the distribution file to a directory, %JUNIT_HOME%. Then, we would add JUnit to the classpath. 

Next, we would test the installation. This would involve running sample tests (located not in the junit.jar file, but the installation directory) distributed with Junit. Lastly, we would confirm that all the tests pass with an “OK” message. If they don’t, we would go back and verify whether junit.jar is in the classpath.  

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Conclusion

With the above JUnit interview questions, you can brush up your concepts in preparation for developer jobs. Positions like Test Automation Engineer and Software Engineer also include JUnit in their desired skillset. The recruiters want to gauge your technical knowledge and communication ability. So, the content of your answer and how you present it would make all the difference. 

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