Java unit tests and JUnit – tips and tricks for writing tests

Testing is a critical part of software development because it allows us to verify that different parts of our code work correctly and meet the requirements. Unit testing is one of the most important testing methods that deals with testing individual pieces of code, such as methods and classes, separately from the rest of the application.

In Java, unit testing is a common practice, but writing efficient and reliable unit tests requires some knowledge and skills. In this article, we’ll take a look at some useful tips and tricks to help you write quality unit tests.

Use the JUnit library

JUnit is the most popular testing library for the Java language that provides an environment for writing and running unit tests. It allows us to define test cases using annotations and provides a number of methods for verifying the expected results. For the beginning, you need to integrate JUnit into your project as a dependency.

Separate tests from the main source code

You should keep the test classes separate from the main source code to avoid the possibility of running them in a production environment. The best place for tests will be the relative path src/main/test, and this is also the directory where build tools such as Maven will look for implemented tests.

Name tests with descriptive names

When writing unit tests, it is important to name the test methods in an unambiguous and concise manner. Test methods should start with the word “test” and should clearly describe what the test is trying to verify. This helps in quickly identifying problems if any of the tests fail.

Start with simple tests

When you start writing tests, focus on simple cases that cover the basic functions of your classes. This way, you can make sure that the basic components of your application are working properly, and then you can gradually expand your test scenarios.

Test boundary values.

Boundary values are a common source of errors in code. When writing tests, make sure you include boundary values that can affect the behavior of your code. This includes uninitialized object values (null), negative values, maximum and minimum values, and also invalid inputs. Testing these thresholds can reveal potential bugs in the code.

Isolate tests from external dependencies

Unit tests should be independent and should not depend on external sources such as databases, files or web services. To achieve this, you can use mocking or stubbing to simulate the behaviour of these dependencies. This ensures that test failures are not caused by external dependency issues.

Use assert methods

The JUnit library provides various assert methods , which allow you to verify that the expected output of your test is correct. The most commonly used ones are assertEquals(), assertTrue(), assertFalse(), etc. The use of proper assert methods is crucial for successful test validation.

Write parameterized tests

JUnit supports parameterized tests that allow you to run the same tests with different input values. This way you can simplify the testing of different scenarios and minimize duplicates in the test code.

Run tests regularly

Don’t forget to run all unit tests regularly. It is often the case that changes in one part of the code can affect another part of the application. Running regular tests allows you to find issues before they reach the production environment.

Test exception raising

Improper exception handling can cause your program to crash. Therefore, test whether your methods correctly evaluate the expected exceptions and can correctly react to these exceptions.

In JUnit, you can use the @Test annotation along with the expected parameter to verify that the method will raise an exception. For example: @Test(expected = IllegalArgumentException.class)

Analyse code coverage

Test coverage is a metric that shows the percentage of code covered by tests. There are many tools (e.g. JaCoCo, Cobertura) that allow you to analyze code coverage by tests. These tools will show you what percentage of your code is covered by tests, allowing you to identify parts of your code that are not tested.

Keep your tests up to date

When you make changes to the code, don’t forget to update the corresponding tests afterwards. Always keep your test scenarios up-to-date and make sure new features are properly tested.

With these tips and tricks, you should be ready to start writing effective Unit tests in Java. Remember that well-written tests improve the quality of your code and can save you a lot of time later on analyzing bugs in your code.

About the author

Jozef Wagner

Java Developer Senior

Viac ako 10 rokov programujem v Jave, momentálne pracujem v msg life Slovakia ako Java programátor senior a pomáham zákazníkom implementovať ich požiadavky do poistného softvéru Life Factory. Vo voľnom čase si rád oddýchnem v lese, prípadne si zahrám nejakú dobrú počítačovú hru.

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