Already familiar with Java but need to brush up on JUnit before your next interview? This collection of JUnit interview questions and answers is designed for experienced developers who want a quick yet thorough refresher. We cover everything from the evolution of JUnit 4 to JUnit 5, including key differences in annotations, lifecycle methods, and test execution.
You’ll revisit best practices for writing maintainable test cases, structuring test suites, and integrating JUnit with build tools like Maven or Gradle. Whether you’re transitioning to a new tech role or aiming for a senior position, these insights will help you articulate your testing strategy with confidence. Think of this as your technical tune-up—sharp, efficient, and tailored for professionals who already speak code fluently and want to make a career in Java.
Answer: Yes, you can test use the main() method for unit testing. It provides a key advantage that you can white-box test a class which means you can test the internals or private methods. You can’t do that with the unit tests as primarily the test framework only tests an interface.
Answer: No, we don’t need to write a separate test class for every class that needs to be tested. If a small test group shares a common test fixture, you may move those tests to a new test class. If you have two groups of tests that need to be executed separately, then it is prudent to place them in a separate test class.
Answer: The test runner holds a strong reference to all test instances for a test execution duration. It means that no tests may be garbage collected for a very long test run with many Test instances unless the entire test runs. Thus, we can use the tearDown() method to collect the garbage beforehand and explicitly set an object to null.
Answer: A Mock object is used for unit testing. If you have any object whose methods you need to test, and those methods depend on another object, you create a mock of the dependency instead of an actual or real instance of that dependency. It enables you to test your object in isolation.
Answer: Cactus means a simple test framework used for unit testing the server-side Java code. It intends to lower the cost of writing the tests for a server-side code. It uses JUnit and extends it. Cactus implements the in-container strategy, which means that all tests are executed inside a container.
Answer: WebUnit refers to a Java-based testing framework that helps to test web applications. It comes with a unified & simple testing interface & wraps around the existing testing frameworks to allow quick testing of web applications.
Answer: JWebUnit offers a high-level Java API to test the web applications with a set of assertions. It includes navigation through form entry, submission, and links. It also includes validation of table contents and other features of business web applications. An easy navigation method that comes with a ready-to-use assertion facilitates rapid test creation than only using JUnit or HtmlUnit.
Answer: XMLUnit performs an XML validation with the Validator class. You can create an instance of it by using the for-language factory method when passing in a schema to use in validation. The schema is passed as a URI leading to its location, XMLUnit abstracts a schema location, it supports in the language class as constants.
Answer: XMLUnit is used as an XMLTestCase, a single JUnit extension class, and a set of supporting classes. Supporting classes assertions are made about the following:
- The distinction between two XML pieces like Diff & DetailedDiff classes.
- The validation of an XML piece through a validator class.
- The result of transforming an XML piece by using XSLT through a Transform class.
- An evaluation of an XPath expression from the XML via classes that implements an XPath engine interface.
- Individual nodes in an XML piece exposed by DOM Traversal through a Node Test class.
Answer: @Before annotation: syntax: @Before public void myMethod() This method should always be executed before each test. Such methods are usually deployed for initialization before performing an actual test in the test environment. @BeforeClass annotation: syntax: @BeforeClass public static void myMethod() This method should also be executed before all the tests. It runs only once. The method should be declared static and used for database connectivity tasks before any execution.
Answer: @After public void runAfterEachTest() This method is executed after every test, used to clean the test and temporary data used by the test. @AfterClass public static void runAfterAllTheTest() This method is executed when all the test execution is complete and gets executed only one time. It is usually used for closing a database connection. This method should be declared as static.
Answer: JUnitCore class is an inbuilt class in the JUnit package; it is based on the Façade design pattern; this class is only used to run definite test classes.
Answer: JUnit provides a special test handling or test suite using the @Rule annotation. You can easily redefine a test’s behavior by using the @Rule annotation. JUnit API provides various built-in rules that a tester can use, or even you can write your own rule.
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| Junit | TestNG |
|---|---|
| In JUnit, the naming convention for annotation is a bit complex and complicated. For example, “Before,” “After,” & “Expected.” | In TestNG, it is way easier to understand the annotations such as the “BeforMethod,” “AfterMethod,” & “ExpectedException.” |
| In JUnit, for a method declaration, you need to follow a particular style like using “@AfterClass” and “@BeforeClass.” | In TestNG, there are no such restrictions like you need to declare methods in a particular format. |
| In the JUnit method, the name constraint is present. | The name constraint is not present in the TestNG method, and you can easily determine any test method names. |
| JUnit framework does not have a “Dependency Test” or “Parameterized Test” features. | TestNG uses “dependOnMethods” for implementing dependency testing. |
| In JUnit, the grouping of test cases is not available. | In TestNG, the grouping of test cases is available. |
| JUnit does not support parallel execution on the Selenium test cases. | In TestNG, parallel execution of the Selenium test cases is possible. |
| It cannot rerun the failed test cases. | It can rerun the failed test cases. |
Answer: Exceptional conditions during a program execution need special processing methods. The exception handling process in the program computation can be of many types. Below are some exception handling methods in the JUnit, such as:
- Try catch idiom
- JUnit rule
- @Test annotation
- Catch exception library
- Customs annotation
Answer: The developers often use JUnit to implement unit tests in Java. It was originally designed for unit testing, which is more of a coding process than a testing process. However, many QA engineers and testers use JUnit for unit testing. JUnit is used for the following reasons:
- It automates testing and prompts quicker tests.
- JUnit tests can be compiled with a build to execute regression testing at the unit level.
- It enables test code reuse.
- JUnit tests behave like a document for the unit tests when there is a transfer.
Answer: When your code is not ready, it will fail if executed; in that case, you will use @Ignore annotation.
- It will not execute the test method annotated with @Ignore;
- Also, it will not execute any test methods of the test class if annotated with @Ignore.
Answer: Cyclomatic complexity of code is a quantitative measure of the no. of linearly independent paths. It refers to a software metric used to indicate a program’s complexity. It is computed using a program’s Control Flow Graph. The nodes in a graph represent a program’s smallest group of commands and a directed edge in which it connects the two nodes, i.e., if the second command might immediately follow the first command.
Answer: In case of assertions, if an Assert condition is not met, the test case execution will get aborted. The remaining tests are skipped, and the test cases are marked as failed. These assertions are used as checkpoints for validating or testing business-critical transactions. In the case of Verify, tests will continue to run until the last test is executed even if assert conditions are not met. Soft Asserts or Verify will report the errors at the end of a test. Tests will not get aborted if any condition is not met. Testers need to invoke an assertAll() method to view the results.
Answer: In the Software development cycle, we can use a different number of testing methods such as:
- Black Box Testing- In this method, while testing a software tester, you cannot see the internal structure of a program and source code. While performing the test, a tester just focuses on the inputs and the expected outputs without knowing how an application works and these inputs are processed. This testing method aims to validate a software’s functionality to ensure that the software works correctly as per the requirements and meets the user’s demands. The black box testing method can be applied to most software testing levels, including unit testing, integration testing, user acceptance testing, and system testing.
- White Box Testing– In contrast to black-box testing, white-box testing is a software testing method, which the tester can see an internal system code. This testing method is also known as clear box testing, Open box testing, or glass box testing. Unlike black-box testing, the white box testing method requires testers to know how to implement software and how it works. Thus, software developers are responsible for this process. This method applies to all the testing levels, but it is mainly used in unit and integration testing. The main purpose of white-box testing is to enhance security with the flow of inputs and outputs through an application and improve the design and usability.
- Agile Testing– Agile Testing refers to a popular software testing practice that follows the principles of agile software development. It means testing software to quickly finding any issues or defects within the agile context and get feedback for faster and better project development.
- Grey Box Testing- Grey box testing is also a software testing method. It is a combination of black-box testing and white-box testing. In this method, the tester has partial knowledge of internal structures and based on his knowledge; he designs the test cases but tests at the black-box level. This method is mainly applicable to integration testing.
- Ad-Hoc Testing– Ad hoc testing is also known as random or monkey testing. It is a type of testing method which is performed without documentation and planning. The tests are conducted randomly and informally without any formal expected results. It is the least formal method as the tester improvises the steps and arbitrarily executes them. Although defects found from this method are difficult to reproduce as there are no written test cases, this method is crucial as it can help find important defects quickly that cannot be found elsewhere using formal methods.