JUnit Interview Questions for Freshers

What is JUnit?

JUnit is a popular open-source testing framework for Java. It is specifically designed to assist developers in writing and running automated tests for their Java applications. JUnit provides a set of annotations, assertions, and test runners that simplify the process of creating and executing tests.

JUnit follows the principles of unit testing, which involves testing individual units of code, such as methods or classes, in isolation. By writing unit tests, developers can ensure that each unit of their code behaves as expected and produces the correct output for a given set of inputs.

JUnit tests are typically written as methods within test classes. These methods are annotated with various JUnit annotations to define the test cases and the expected results. The framework provides a set of assertion methods that allow developers to validate the actual output of the code against the expected output.

JUnit test cases can be run individually or as a suite of tests. JUnit provides test runners, which are special classes that execute the tests and generate reports indicating the success or failure of each test. The reports help developers identify issues and debug their code.

JUnit has become a standard choice for unit testing in Java development due to its simplicity, flexibility, and widespread adoption. It integrates well with popular development tools and frameworks, making it easier for developers to incorporate testing into their software development workflow.

What are the features of JUnit?

JUnit offers several key features that make it a powerful testing framework for Java applications:

1. Annotations: JUnit provides a set of annotations that allow developers to define and control the behavior of test methods. These annotations include `@Test`, `@Before`, `@After`, `@BeforeClass`, `@AfterClass`, and more. Annotations make it easy to mark test methods, setup and teardown methods, and test class-level initialization and cleanup.

2. Assertions: JUnit provides a range of assertion methods that enable developers to verify the expected outcomes of their tests. These assertions include `assertEquals`, `assertTrue`, `assertFalse`, `assertNull`, `assertNotNull`, and many others. Assertions help compare actual and expected values, making it straightforward to determine if a test has passed or failed.

3. Test Runners: JUnit supports various test runners that execute test cases and generate reports. The primary test runner is `JUnitCore`, which can be used to run JUnit tests programmatically. Additionally, there are IDE-specific runners, build tool integrations, and third-party runners that extend JUnit’s functionality.

4. Parameterized Tests: JUnit allows parameterized tests, where a single test method can be executed with multiple sets of input values. This feature is useful when testing the same functionality with different data or scenarios. Parameterized tests help reduce code duplication and provide a concise way to test various inputs.

5. Test Suites: JUnit enables the creation of test suites, which allow developers to group related test classes and run them together. Test suites provide a convenient way to organize and execute multiple tests as a single entity.

6. Exception Testing: JUnit provides support for testing expected exceptions. With the `@Test` annotation, developers can specify the expected exception type that should be thrown during the execution of a test method. This allows for precise verification of exception handling within the code.

7. Test Fixtures: JUnit supports the use of test fixtures, which are methods annotated with `@Before` and `@After`. Test fixtures help set up the necessary test environment before running each test method and perform cleanup operations afterward. They ensure that each test runs in a controlled and isolated environment.

8. Test Coverage: JUnit integrates with code coverage tools, such as JaCoCo or Emma, to measure the extent to which the code is exercised by the tests. Code coverage reports provide insights into the effectiveness of the test suite and help identify areas of code that are not adequately tested.

These features, among others, make JUnit a comprehensive and flexible testing framework that facilitates the creation, execution, and analysis of tests for Java applications.

What are some of the important annotations provided by JUnit?

Here are some important annotations provided by JUnit along with examples:


import org.junit.Test;

public class MyTestClass {
public void testAddition() {
int result = 2 + 2;
assertEquals(4, result);

The @Test annotation marks the testAddition() method as a test method. JUnit will execute this method as part of the test suite.

@Before and @After:

import org.junit.Before;
import org.junit.After;

public class MyTestClass {
public void setup() {
// Perform setup actions before each test method

public void teardown() {
// Perform cleanup actions after each test method

// Test methods go here


The @Before annotation marks the setup() method to be executed before each test method. The @After annotation marks the teardown() method to be executed after each test method. These methods are used for setup and cleanup tasks.

@BeforeClass and @AfterClass:

import org.junit.BeforeClass;
import org.junit.AfterClass;

public class MyTestClass {
public static void setupClass() {
// Perform setup actions once before all test methods

public static void teardownClass() {
// Perform cleanup actions once after all test methods

// Test methods go here


The @BeforeClass annotation marks the setupClass() method to be executed once before all test methods in the test class. The @AfterClass annotation marks the teardownClass() method to be executed once after all test methods in the test class. These methods are useful for setup and cleanup tasks shared among multiple tests.


import org.junit.Ignore;
import org.junit.Test;

public class MyTestClass {
@Ignore("Temporarily ignored")
public void testMethod() {
// Test logic

The @Ignore annotation marks the test method to be ignored or skipped during test execution. It is useful for temporarily disabling tests that are failing or require further investigation. The optional message provides a reason for skipping the test.

These are just a few examples of important annotations provided by JUnit. Each annotation serves a specific purpose and allows developers to control various aspects of their tests, such as test execution, setup, teardown, and skipping tests.


import org.junit.runner.RunWith;
import org.junit.runners.Parameterized;

public class MyParameterizedTestClass {
// Parameterized test logic


The @RunWith annotation specifies a custom test runner class. In this example, the Parameterized runner is used to run parameterized tests. It allows running the same test method with different sets of input values.


import org.junit.Test;
import org.junit.runner.RunWith;
import org.junit.runners.Parameterized;
import java.util.Arrays;
import java.util.Collection;

public class MyParameterizedTestClass {
public int input;

public static Collection<Object[]> data() {
return Arrays.asList(new Object[][] {
{ 1 }, { 2 }, { 3 }

public void testMethod() {
// Test logic using the parameterized input value


The @Parameterized annotation is used in conjunction with the @RunWith(Parameterized.class) annotation to create parameterized tests. It allows defining a data source and specifying the parameters used for each test run. Each set of parameters generates an instance of the test class, and the test method is executed with those parameter values.


import org.junit.Rule;
import org.junit.Test;
import org.junit.rules.TemporaryFolder;

public class MyTestClass {
public TemporaryFolder tempFolder = new TemporaryFolder();

public void testMethod() {
// Test logic using the temporary folder rule

The @Rule annotation allows the use of custom test rules. In this example, the TemporaryFolder rule is used to create a temporary folder that is automatically cleaned up after the test execution. Rules provide additional functionality and can be used to augment the behavior of tests.

These annotations, along with the ones mentioned earlier, contribute to the flexibility and control provided by JUnit in writing and executing tests. Each annotation caters to specific testing scenarios and helps in managing the test lifecycle, data sources, test runners, and custom rules.

What is Unit Testing?

Unit testing is a software testing technique in which individual units of code are tested to verify that they function correctly in isolation. A unit refers to the smallest testable component of an application, such as a method, function, or class.

The primary goal of unit testing is to ensure that each unit of code behaves as expected and produces the correct output for a given set of inputs. By testing units in isolation, developers can identify and fix bugs early in the development process, improving the overall quality and reliability of the software.

Unit tests are typically written by the developers themselves and are executed frequently during the development cycle. They focus on testing the smallest possible pieces of functionality and rely on stubs or mock objects to simulate dependencies or interactions with other units. This isolation allows developers to pinpoint the source of any failures or unexpected behaviors.

Unit tests are usually automated, meaning they can be run automatically and repeatedly without requiring manual intervention. This automation enables developers to quickly detect regressions, ensure that new changes don’t break existing functionality, and provide a safety net for refactoring code.

What are the benefits of unit testing?

1. Early bug detection: Unit tests catch issues early in the development process, making it easier and cheaper to fix them.
2. Improved code quality: Writing unit tests often leads to more modular and well-structured code, as it forces developers to think about the design and functionality of their units.
3. Faster debugging: When a unit test fails, it provides a clear indication of which unit is causing the problem, facilitating faster debugging and resolution.
4. Refactoring support: Unit tests provide confidence when refactoring or modifying code, ensuring that existing functionality remains intact.
5. Documentation and specifications: Unit tests can serve as living documentation, illustrating how a unit is intended to be used and showcasing its expected behavior.

Overall, unit testing is a critical practice in software development that helps ensure the reliability, maintainability, and quality of code.

Is it mandatory to write test cases for every logic?

While it is not mandatory to write test cases for every logic in your code, it is generally considered a good practice to have test coverage for critical and complex parts of your codebase. Writing comprehensive tests helps ensure the correctness and reliability of your software, especially as the complexity of the logic increases.

Here are a few considerations to keep in mind when deciding which parts of your code to test:

1. Critical Functionality: Test cases are especially important for critical functionality that is vital to the correct operation of your application. These may include core algorithms, important business logic, or security-related components. Ensuring that these critical areas of your code are thoroughly tested helps mitigate risks and prevents potential failures.

2. Edge Cases and Boundaries: Test cases should cover edge cases and boundaries where the behavior of your code might differ. These are scenarios where input values are at the extremes or on the edge of what is expected. Testing such cases helps uncover potential issues and ensures that your code handles these situations correctly.

3. Error Handling and Exception Paths: It is crucial to write test cases that cover error handling and exception paths within your code. By simulating and validating error conditions, you can verify that your code responds appropriately and gracefully handles unexpected situations.

4. Code Coverage and Risk Assessment: Analyzing code coverage metrics can help identify areas of your code that are not adequately tested. Tools such as code coverage reports can highlight sections of code that lack test coverage, allowing you to prioritize your testing efforts based on the risk associated with each untested area.

5. Refactoring and Maintenance: When refactoring or modifying existing code, having a comprehensive suite of tests ensures that you can confidently make changes without introducing unintended side effects. Tests act as a safety net, alerting you to any regressions that may occur due to code modifications.

6. Time and Resource Constraints: While it is ideal to have complete test coverage, the reality is that time and resource constraints may limit the extent of testing. In such cases, focus on critical and high-risk areas of your code, and aim to achieve a balance between the time spent on writing tests and the potential benefits they provide.

Remember that the decision of which logic to test ultimately depends on factors such as project requirements, complexity, time constraints, and risk assessment. Strive to achieve a balance between test coverage and the practicality of testing given the project’s constraints.

What will happen if the return type of the JUnit method is String?

The test case execution will fail because the JUnit methods are designed to return void.

What is the importance of @Test annotation?

@Test annotation is used for marking the method as a test method.

What is the difference between thenReturn and doReturn?

In the context of Mockito, `thenReturn` and `doReturn` are two methods used to stub method invocations on mocked objects. They are used to define the behavior of mocked methods when they are called during testing. However, there is a subtle difference between the two.

1. `thenReturn`:
The `thenReturn` method is typically used to stub a method and specify the return value of the method when it is invoked. It is generally used for methods that have a non-void return type. Here’s an example:

// Mocking the dependency
SomeDependency dependency = Mockito.mock(SomeDependency.class);

// Stubbing a method using thenReturn
Mockito.when(dependency.someMethod()).thenReturn("Mocked value");

// Testing the behavior
String result = dependency.someMethod();
assertEquals("Mocked value", result);


In this example, when the `someMethod()` of the `dependency` object is called, it will always return the specified value of `”Mocked value”`.

2. `doReturn`:
The `doReturn` method is used in scenarios where you want to stub a method with a void return type or when the method throws an exception. Since void methods do not have a return value, you cannot use `thenReturn`. Instead, you use `doReturn` to specify the behavior. Here’s an example:

// Mocking the dependency
SomeDependency dependency = Mockito.mock(SomeDependency.class);

// Stubbing a void method using doReturn
Mockito.doReturn("Mocked value").when(dependency).someVoidMethod();

// Testing the behavior


In this example, when the `someVoidMethod()` of the `dependency` object is called, it will not actually execute the method but will instead return the specified value of `”Mocked value”`.

To summarize, the key difference between `thenReturn` and `doReturn` lies in the ability to stub methods with a void return type or when you want to avoid the actual execution of the method. Use `thenReturn` for non-void methods, and use `doReturn` for void methods or when you want to avoid the actual execution of the method.