Java

Unit Testing (JUnit)

Verify your code works. Writing Unit Tests with JUnit 5.

By TechCoder TeamLast updated: 2026-06-02
In a Nutshell

Verify your code works. Writing Unit Tests with JUnit 5. This hands-on tutorial focuses on practical implementation of unit testing (junit) concepts.

Unit Testing (JUnit)

Unit testing checks if a small piece of code (a unit) behaves as expected. JUnit is the standard framework for this.

Why Test?

  • Catch bugs early.
  • Safe refactoring (change code without fear).
  • Documentation (tests show how code is used).

Annotations

  • @Test: Marks a method as a test case.
  • @BeforeEach: Runs before each test.
  • @AfterEach: Runs after each test.
  • @BeforeAll / @AfterAll: Runs once for the class.

Assertions

We assert that expected results match actual results.

  • assertEquals(expected, actual)
  • assertTrue(condition)
  • assertThrows(Exception.class, () -> ...)

Mocking

In unit tests, we don't want to hit the real database. We use libraries like Mockito to fake (mock) dependencies.

Interactive Code

Write a test case!

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Quiz

Quiz

Question 1 of 3

Which annotation marks a test method?

@Check
@Test
@Run

Next Steps

How do we run these tests and manage thousands of libraries? Build Tools.