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1. Overview

Unit Testing is a crucial step in software design and implementation.

Not only does it improve the efficiency and effectiveness of the code, but it also makes the code more robust and reduces the regressions in future development and maintenance.

In this tutorial, we'll discuss a few best practices for unit testing in Java.

2. What Is Unit Testing?

Unit Testing is a methodology of testing source code for its fitment of use in production.

We start out writing unit tests by creating various test cases to verify the behaviors of an individual unit of source code.

Then, the complete test suite executes to catch the regressions, either in the implementation phase or while building packages for various stages of deployments like staging and production.

Let's take a look at a simple scenario. To start with, let's create the Circle class and implement the calculateArea method in it:

public class Circle {

    public static double calculateArea(double radius) {
        return Math.PI * radius * radius;

Then, we'll create unit tests for the Circle class to make sure the calculateArea method works as expected.

Let's create the CalculatorTest class in the src/main/test directory:

public class CircleTest {

    public void testCalculateArea() {

In this case, we're using JUnit's @Test annotation along with build tools like Maven or Gradle to run the test.

3. Best Practices

3.1. Source Code

It's a good idea to keep the test classes separate from the main source code. So, they are developed, executed, and maintained separately from the production code.

Also, it avoids any possibility of running test code in the production environment.

We can follow the steps of the build tools like Maven and Gradle that look for src/main/test directory for test implementations.

3.2. Package Naming Convention

We should create a similar package structure in the src/main/test directory for test classes. Thus, improving the readability and maintainability of the test code.

In simple words, the package of the test class should match the package of the source class whose unit of source code it'll test.

For instance, if our Circle class exists in the com.baeldung.math package, the CircleTest class should also exist in the com.baeldung.math package under the src/main/test directory structure.

3.3. Test Case Naming Convention

The test names should be insightful, and users should understand the behavior and expectation of the test by just glancing name itself.

For example, as we discussed, the name of our unit test was testCalculateArea which vaguely gives any meaningful information about the test scenario and expectation.

Therefore, we should name a test with the action and expectation such as testCalculateAreaWithGeneralDoubleValueRadiusThatReturnsAreaInDouble, testCalculateAreaWithLargeDoubleValueRadiusThatReturnsAreaAsInfinity.

However, we can still improve the names for better readability. Often it's helpful to name the test cases in given_when_then to elaborate on the purpose of a unit test. For instance:

public class CircleTest {


    public void givenRadius_whenCalculateArea_thenReturnArea() {

    public void givenDoubleMaxValueAsRadius_whenCalculateArea_thenReturnAreaAsInfinity() {

Not only the name of a unit test, but we should also describe code blocks in the Given, When, and Then format. In addition, it helps to differentiate the test into three parts: input, action, and output.

First, the code block corresponding to the given section creates the test objects, mocks the data, and arranges input.

Next, the code block for the when section represents a specific action or test scenario. Likewise, the then section points out the output of the code, which is verified against the expected result using assertions.

3.4. Expected vs. Actual

A test case should have an assertion between expected and actual values. To corroborate the idea of the expected vs. actual values, we can look at the definition of the assertEquals method of the JUnit's Assert class:

public static void assertEquals(Object expected, Object actual)

Let's use the assertion in one of our already discussed test cases:

public void givenRadius_whenCalculateArea_thenReturnArea() {
    double actualArea = Circle.calculateArea(1d);
    double expectedArea = 3.141592653589793;
    Assert.assertEquals(expectedArea, actualArea); 

It's suggested to prefix the variable names with the actual and expected keyword to improve the readability of the test code.

3.5. Prefer Simple Test Case

In the previous test case, we can see that the expected value was hard-coded. This is done to avoid rewriting or reusing actual code implementation in the test case to get the expected value.

It's not encouraged to calculate the area of the circle to match against the return value of the calculateArea method:

public void givenRadius_whenCalculateArea_thenReturnArea() {
    double actualArea = Circle.calculateArea(2d);
    double expectedArea = 3.141592653589793 * 2 * 2;
    Assert.assertEquals(expectedArea, actualArea); 

In this assertion, we're calculating both expected and actual values using similar logic, resulting in similar results forever. Hence, our test case won't have any value added to the unit testing of code.

Therefore, we should create a simple test case that asserts hard-coded expected value against the actual one.

Although it's sometimes required to write the logic in the test case, we shouldn't overdo it. Also, as commonly seen, we should never implement production logic in a test case to pass the assertions.

3.6. Appropriate Assertions

Always use proper assertions to verify the expected vs. actual results. We should use various methods available in the Assert class of JUnit or similar frameworks like AssertJ.

For instance, we've already used the Assert.assertEquals method for value assertion. Similarly, we can use assertNotEquals to check if the expected and actual values are not equal.

Other methods like assertNotNull, assertTrue, and assertNotSame are beneficial in distinct assertions.

3.7. Specific Unit Tests

Instead of adding multiple assertions to the same unit test, we should create separate test cases.

Of course, it's sometimes tempting to verify multiple scenarios in the same test, but it's a good idea to keep them separate. Then, in the case of test failures, it'll be easier to determine which specific scenario failed and, likewise, simpler to fix the code.

Therefore, always write a unit test to test a single specific scenario.

A unit test won't get overly complicated to understand. Moreover, it'll be easier to debug and maintain unit tests later.

3.8. Test Production Scenarios

Unit testing is more rewarding when we write tests considering real scenarios in mind.

Principally, it helps to make unit tests more relatable. Also, it proves essential in understanding the behavior of the code in certain production cases.

3.9. Mock External Services

Although unit tests concentrate on specific and smaller pieces of code, there is a chance that the code is dependent on external services for some logic.

Therefore, we should mock the external services and merely test the logic and execution of our code for varying scenarios.

We can use various frameworks like Mockito, EasyMock, and JMockit for mocking external services.

3.10. Avoid Code Redundancy

Create more and more helper functions to generate the commonly-used objects and mock the data or external services for similar unit tests.

As with other recommendations, this enhances the readability and maintainability of the test code.

3.11. Annotations

Often, testing frameworks provide annotations for various purposes like performing setup, executing code before, and tearing down after running a test.

Various annotations like JUnit's @Before, @BeforeClass, and @After and from other test frameworks like TestNG are at our disposal.

We should leverage annotations to prepare the system for tests by creating data, arranging objects, and dropping all of it after every test to keep test cases isolated from each other.

3.12. 80% Test Coverage

More test coverage for the source code is always beneficial. However, it's not the only goal to achieve. We should make a well-informed decision and choose a better trade-off that works for our implementation, deadlines, and the team.

As a thumb rule, we should try to cover 80% of the code by unit tests.

Additionally, we can use tools like JaCoCo and Cobertura along with Maven or Gradle to generate code coverage reports.

3.13. TDD Approach

Test-Driven Development (TDD) is the methodology where we create test cases before and in ongoing implementation. The approach couples with the process of designing and implementing the source code.

The benefit includes testable production code from the start, robust implementation with easy refactorings, and fewer regressions.

3.14. Automation

We can improve the reliability of the code by automating the execution of the entire test suite while creating new builds.

Primarily, this helps to avoid unfortunate regressions in various release environments. It also ensures rapid feedback before a broken code is released.

Therefore, unit test execution should be part of CI-CD pipelines and alert the stakeholders in case of malfunctions.

4. Conclusion

In this article, we explored some best practices of Unit Testing in Java. Following best practices can help in many aspects of software development.

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