Cookbooks
Java
Chapter 9. Testing and Debugging
Writing tests

Java:
Writing tests

How to:

Java developers primarily use two testing frameworks: JUnit and TestNG. Here, we’ll focus on JUnit, the more popular choice for writing tests due to its simplicity and widespread adoption.

JUnit Basics

To use JUnit in your Maven project, add the following dependency to your pom.xml:

<dependency>
    <groupId>org.junit.jupiter</groupId>
    <artifactId>junit-jupiter</artifactId>
    <version>5.9.0</version>
    <scope>test</scope>
</dependency>

A basic test in JUnit looks like this:

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

public class CalculatorTest {
    
    @Test
    public void testAdd() {
        Calculator calculator = new Calculator();
        assertEquals(5, calculator.add(2, 3), "2 + 3 should equal 5");
    }
}

Executing this test will either pass, indicating the add method works as expected, or fail, showing an error message.

Mocking with Mockito

In real-world scenarios, objects often depend on other objects. Mockito is a popular mocking framework that helps in creating mock objects for the purpose of testing.

Add Mockito to your Maven project:

<dependency>
    <groupId>org.mockito</groupId>
    <artifactId>mockito-core</artifactId>
    <version>4.5.1</version>
    <scope>test</scope>
</dependency>

A simple use case with Mockito:

import org.junit.jupiter.api.Test;
import static org.junit.jupiter.api.Assertions.assertEquals;
import static org.mockito.Mockito.*;

public class UserServiceTest {

    @Test
    public void testGetUsername() {
        // Create a mock UserRepository
        UserRepository mockRepository = mock(UserRepository.class);

        // Define behavior for mock object
        when(mockRepository.getUsername(1)).thenReturn("john_doe");

        UserService userService = new UserService(mockRepository);
        
        assertEquals("john_doe", userService.getUsername(1), "User ID 1 should be john_doe");
    }
}

This mock allows us to test UserService without needing an actual UserRepository, focusing the test on the logic within UserService itself.

Using a debugger