Published on Fri Jul 25 2025 10:30:00 GMT+0000 (Coordinated Universal Time) by Purusothaman Ramanujam

Unit Testing in Java with JUnit 5: A Beginner’s Guide

Introduction

Testing is a crucial part of software development. Unit tests help ensure your code works as expected and make it easier to maintain and refactor. JUnit 5 is the latest version of the most popular testing framework for Java, bringing many improvements and new features.

In this post, you’ll learn how to write and run your first unit tests using JUnit 5.

What is JUnit 5?

JUnit 5 is a modern, flexible, and powerful testing framework for Java. It consists of three main modules:

JUnit Platform: Launches testing frameworks on the JVM
JUnit Jupiter: New programming model and extension model for writing tests
JUnit Vintage: Supports running JUnit 3 and 4 tests

Adding JUnit 5 to Your Project

Maven Dependency

<dependency>
    <groupId>org.junit.jupiter</groupId>
    <artifactId>junit-jupiter</artifactId>
    <version>5.10.0</version>
    <scope>test</scope>
</dependency>

Gradle Dependency

testImplementation 'org.junit.jupiter:junit-jupiter:5.10.0'

Writing Your First Test

Let’s write a simple class and test it.

public class Calculator {
    public int add(int a, int b) {
        return a + b;
    }
    public int subtract(int a, int b) {
        return a - b;
    }
}

Now, let’s write tests for this class:

import org.junit.jupiter.api.Test;
import static org.junit.jupiter.api.Assertions.*;

class CalculatorTest {
    @Test
    void testAdd() {
        Calculator calc = new Calculator();
        assertEquals(5, calc.add(2, 3));
    }

    @Test
    void testSubtract() {
        Calculator calc = new Calculator();
        assertEquals(1, calc.subtract(3, 2));
    }
}

@Test marks a method as a test case.
assertEquals(expected, actual) checks if the result matches the expectation.

Running Tests

Most IDEs (IntelliJ IDEA, Eclipse, VS Code) let you right-click and run tests. You can also run them from the command line:

Maven: mvn test
Gradle: ./gradlew test

More JUnit 5 Features

Assertions

JUnit 5 provides many assertions:

assertTrue(condition);
assertFalse(condition);
assertNull(value);
assertNotNull(value);
assertThrows(Exception.class, () -> { ... });

Parameterized Tests

Run the same test with different inputs:

import org.junit.jupiter.params.ParameterizedTest;
import org.junit.jupiter.params.provider.ValueSource;

class ParameterizedExample {
    @ParameterizedTest
    @ValueSource(strings = {"racecar", "radar", "level"})
    void testPalindrome(String word) {
        assertTrue(isPalindrome(word));
    }
    boolean isPalindrome(String str) {
        return new StringBuilder(str).reverse().toString().equals(str);
    }
}

Test Lifecycle Methods

Set up or clean up before/after tests:

import org.junit.jupiter.api.BeforeEach;
import org.junit.jupiter.api.AfterEach;

class LifecycleExample {
    @BeforeEach
    void setUp() {
        // Runs before each test
    }
    @AfterEach
    void tearDown() {
        // Runs after each test
    }
}

Best Practices

Test Small Units: Test one method or behavior at a time.
Name Tests Clearly: Use descriptive method names.
Keep Tests Independent: Each test should run on its own.
Use Setup Methods: Use @BeforeEach for common setup code.
Test Edge Cases: Don’t just test the happy path.

Conclusion

JUnit 5 makes unit testing in Java easy and powerful. Start by writing simple tests, then explore advanced features like parameterized tests and lifecycle methods. Good tests help you write better, more reliable code.