How to use JUnit

Using JUnit means writing unit tests for your Java code using the JUnit framework. Whether you're using JUnit 5 (modern) or JUnit 4 (legacy), the basics are similar: annotate methods with @Test, write assertions, and run them using an IDE or build tool.

✅ 1. Set Up JUnit

🧱 If You're Using Maven (JUnit 5):

Add to pom.xml:

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

Then run tests using:

mvn test

🧱 If You're Using Gradle (JUnit 5):

dependencies {
    testImplementation 'org.junit.jupiter:junit-jupiter:5.10.0'
}

test {
    useJUnitPlatform()
}

✅ 2. Write a Simple JUnit Test (JUnit 5)

📦 Class Under Test:

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

🧪 JUnit Test:

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

class CalculatorTest {

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

✅ Tip: Always import static org.junit.jupiter.api.Assertions.* for clean assertions.

✅ 3. Common Annotations

💡 Example:

@BeforeEach
void setUp() {
    // init logic
}

@AfterEach
void tearDown() {
    // cleanup logic
}

✅ 4. Assertions You’ll Use Often

assertEquals(expected, actual);
assertTrue(condition);
assertFalse(condition);
assertThrows(Exception.class, () -> { ... });
assertNotNull(object);
assertNull(object);

✅ 5. Run Tests

• 🖥️ In IntelliJ/Eclipse: Right-click the test file or method > Run
• 🔧 With Maven/Gradle: mvn test or ./gradlew test
• 🧪 CLI (Console Launcher): Use if you're not using a build tool or IDE

✅ 6. Folder Structure (Maven/Gradle Convention)

src/
├── main/
│   └── java/       → your application code
└── test/
    └── java/       → your test code

📌 Summary