Writing Your First JUnit Test

In this article on Writing Your First JUnit Test, I will cover the basic annotations like @Test, @Before, and @After. Basically, it is a step-by-step guide to writing a simple JUnit test case.

JUnit is a powerful and widely used testing framework for Java applications. It allows you to automate the testing of your code, ensuring that it behaves as expected. In this guide, we’ll walk you through the process of writing your very first JUnit test. By the end of this tutorial, you’ll have a solid foundation for creating and running tests in your Java projects.

Prerequisites

Before we dive into writing JUnit tests, make sure you have the following prerequisites in place.

Java Development Environment. To begin with, you should have Java installed on your system. Meanwhile, you can download and install the latest version of Java from the official Oracle website.
Integrated Development Environment (IDE). In fact, you can use any Java IDE of your choice. Popular options include Eclipse, IntelliJ IDEA, and Visual Studio Code with appropriate Java extensions.
JUnit Library. Also ensure that you have the JUnit library added to your project. Most modern IDEs make it easy to add JUnit as a dependency through build tools like Maven or Gradle.

Creating a Test Class

Open your Java IDE. At first, launch your chosen Java IDE and open your project.
Create a new Java class. After that, right-click on your project or package, navigate to “New,” and select “Class.” Name your class something like MyMathTest. This will be your test class for testing a hypothetical MyMath class.
Add JUnit annotations. In order to turn this class into a test class, you need to add the @Test annotation from the JUnit library to the methods you want to run as tests. The following code shows an example.

import org.junit.jupiter.api.Test;

public class MyMathTest {

    @Test
    public void testAddition() {
        // Your test code for addition goes here
    }
}

Writing a Test Method

Now that you have a test class with a test method, let’s write some test code for the MyMath class. In this example, we’ll test the add method of MyMath.

Create an instance of the class under test. In your test method, create an instance of the class you want to test. For example:

MyMath math = new MyMath();

Invoke the method to be tested. Then, call the method you want to test and store the result in a variable.

int result = math.add(2, 3);

Assertions. Use JUnit’s assertion methods to verify that the actual output matches your expected output. The most common assertion method is assertEquals:

import static org.junit.jupiter.api.Assertions.assertEquals;

@Test
public void testAddition() {
    MyMath math = new MyMath();
    int result = math.add(2, 3);
    assertEquals(5, result);
}

Running the Test

Now that you’ve written your first JUnit test, it’s time to run it.

Right-click on the test class. In your IDE, right-click on the MyMathTest class and select “Run As” -> “JUnit Test.”
View the results. The IDE will display the results of the test execution in the JUnit tab. If all goes well, you should see a green bar indicating that your test passed.

Congratulations! You’ve just written and executed your first JUnit test. You now have the foundation to write more tests for your Java projects, helping you catch and fix bugs early in the development process.

Next: Parameterized Tests in JUnit

JUnit Tutorial

Prerequisites

Creating a Test Class

Writing a Test Method

Running the Test

Further Reading

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Prerequisites

Creating a Test Class

Writing a Test Method

Running the Test

Further Reading

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