A basic JUnit test class is just a Java class that contains one or more test methods. Each test method checks whether a small piece of code behaves the way you expect.
Here is a simple JUnit 5 example:
import org.junit.jupiter.api.Test;
import static org.junit.jupiter.api.Assertions.*;
class CalculatorTest {
@Test
void shouldAddTwoNumbers() {
int result = 2 + 3;
assertEquals(5, result);
}
}
1. The Test Class
class CalculatorTest {
// test methods go here
}
A JUnit test class is usually named after the class being tested, followed by Test.
For example:
| Class Being Tested | Test Class |
|---|---|
Calculator |
CalculatorTest |
UserService |
UserServiceTest |
OrderRepository |
OrderRepositoryTest |
The test class does not need a main() method. JUnit runs the tests for you.
2. The @Test Annotation
@Test
void shouldAddTwoNumbers() {
// test code
}
The @Test annotation tells JUnit:
This method is a test method. Run it as part of the test suite.
In JUnit 5, the annotation comes from:
import org.junit.jupiter.api.Test;
3. The Test Method
A test method usually:
- Creates some input or test data.
- Runs the code being tested.
- Checks the result.
Example:
@Test
void shouldAddTwoNumbers() {
int result = 2 + 3;
assertEquals(5, result);
}
The method name should describe the expected behavior. Common naming styles include:
void shouldAddTwoNumbers()
void returnsTrueWhenPasswordIsValid()
void throwsExceptionWhenEmailIsMissing()
4. Assertions
Assertions are checks that decide whether the test passes or fails.
Common JUnit 5 assertions include:
assertEquals(expected, actual);
assertTrue(condition);
assertFalse(condition);
assertNotNull(value);
assertNull(value);
assertThrows(Exception.class, () -> {
// code expected to throw exception
});
Example:
@Test
void shouldCheckUserName() {
String name = "Alice";
assertNotNull(name);
assertEquals("Alice", name);
assertTrue(name.startsWith("A"));
}
If all assertions pass, the test passes. If any assertion fails, the test fails.
Assertions are usually imported like this:
import static org.junit.jupiter.api.Assertions.*;
5. Basic Arrange-Act-Assert Pattern
Many test methods follow this structure:
@Test
void shouldCalculateTotalPrice() {
// Arrange
int price = 100;
int quantity = 3;
// Act
int total = price * quantity;
// Assert
assertEquals(300, total);
}
Arrange
Prepare the data or objects needed for the test.
int price = 100;
int quantity = 3;
Act
Run the code you want to test.
int total = price * quantity;
Assert
Check that the result is correct.
assertEquals(300, total);
6. A Complete Basic JUnit 5 Test Class
import org.junit.jupiter.api.Test;
import static org.junit.jupiter.api.Assertions.*;
class StringUtilsTest {
@Test
void shouldConvertTextToUpperCase() {
// Arrange
String text = "hello";
// Act
String result = text.toUpperCase();
// Assert
assertEquals("HELLO", result);
}
@Test
void shouldCheckIfTextContainsWord() {
// Arrange
String text = "Learning JUnit is useful";
// Act
boolean containsJUnit = text.contains("JUnit");
// Assert
assertTrue(containsJUnit);
}
}
7. Optional Setup Method
If several tests need the same object or data, you can use @BeforeEach.
import org.junit.jupiter.api.BeforeEach;
import org.junit.jupiter.api.Test;
import static org.junit.jupiter.api.Assertions.*;
class CalculatorTest {
private int baseNumber;
@BeforeEach
void setUp() {
baseNumber = 10;
}
@Test
void shouldAddNumber() {
int result = baseNumber + 5;
assertEquals(15, result);
}
@Test
void shouldMultiplyNumber() {
int result = baseNumber * 2;
assertEquals(20, result);
}
}
@BeforeEach runs before every test method.
8. JUnit 4 vs. JUnit 5 Structure
Older JUnit 3 or JUnit 4 tests may look different.
JUnit 5 style
import org.junit.jupiter.api.Test;
import static org.junit.jupiter.api.Assertions.*;
class AppTest {
@Test
void shouldWork() {
assertTrue(true);
}
}
Older JUnit 3 style
import junit.framework.TestCase;
public class AppTest extends TestCase {
public void testApp() {
assertTrue(true);
}
}
In modern Java projects, you will usually prefer JUnit 5 unless you are maintaining older code.
9. Typical Folder Location
In a Maven or Gradle Java project, test classes usually go under:
src/test/java
Application code usually goes under:
src/main/java
Example:
src
├── main
│ └── java
│ └── org.kodejava
│ └── Calculator.java
└── test
└── java
└── org.kodejava
└── CalculatorTest.java
Summary
A basic JUnit test class usually has:
- A class name ending in
Test. - One or more methods annotated with
@Test. - Assertions such as
assertEquals()orassertTrue(). - A clear structure: Arrange, Act, Assert.
- Optional setup methods such as
@BeforeEach.
In short:
import org.junit.jupiter.api.Test;
import static org.junit.jupiter.api.Assertions.*;
class ExampleTest {
@Test
void shouldDoSomething() {
// Arrange
String value = "JUnit";
// Act
boolean result = value.contains("Unit");
// Assert
assertTrue(result);
}
}