What Is Unit Testing?

Unit testing means checking if small pieces of your code—called "units"—work as expected. In Java, a unit is usually a method or a class. You write special code, known as a unit test, to make sure your logic is correct and reliable.

Why Is Unit Testing Important?

• Catches mistakes early, before your program goes live;
• Makes it easier to change and improve your code without breaking things;
• Helps you understand what your code should do and how it behaves;
• Saves time and effort when fixing bugs or adding new features.

How JUnit Jupiter Helps

JUnit Jupiter is the latest version of the popular JUnit testing framework for Java. It lets you write and run automated tests easily. With JUnit Jupiter, you can:

• Write clear, readable test methods using simple annotations like @Test;
• Run all your tests automatically with one command;
• Get detailed feedback about which tests passed or failed;
• Use advanced features like checking for exceptions or grouping related tests.

JUnit Jupiter makes unit testing a natural part of your Java development process, helping you build better, more reliable software.

Key Concepts of JUnit Jupiter

JUnit Jupiter is the modern testing framework included with JUnit 5. It helps you write and organize unit tests for your Java code. Here are the main concepts you need to know:

Test Methods

• Mark a method as a test by adding the @Test annotation;
• Each test method should check a small, specific piece of functionality;
• Test methods must be public and return void.

Example:

@Test
void testSum() {
    int result = 2 + 2;
    Assertions.assertEquals(4, result);
}

Assertions

Assertions check if your code produces the expected result. If an assertion fails, the test fails.

• Use Assertions.assertEquals(expected, actual) to check equality;
• Use Assertions.assertTrue(condition) to check if something is true;
• Use Assertions.assertThrows() to check if an exception is thrown.

Example:

Assertions.assertEquals(5, add(2, 3));
Assertions.assertTrue(isValidUser("admin"));

Test Lifecycle Annotations

JUnit Jupiter provides annotations to set up and clean up resources before and after each test:

• @BeforeEach: runs before every test method; use it to set up common test data;
• @AfterEach: runs after every test method; use it to clean up or reset changes.

Example:

@BeforeEach
void setUp() {
    userList.clear();
}

@AfterEach
void tearDown() {
    database.disconnect();
}

By using these annotations and assertions, you can write clear, repeatable tests that help ensure your code works as expected.