This example effectively illustrates the principles of Dependency Injection

Dependency Injection (DI) is a software design pattern that promotes loose coupling between classes by passing dependencies (objects that a class needs to function) into the class rather than having the class create them itself. This makes code more modular, testable, and maintainable.

Here's a simplified example of how DI works: 

1. Interface for the Dependency:

Java

interface Engine {
    void start();
}

2. Concrete Implementations of the Dependency:

Java

class WorkingEngine implements Engine {
    @Override
    public void start() {
        System.out.println("Engine started successfully.");
    }
}

class BrokenEngine implements Engine {
    @Override
    public void start() {
        throw new RuntimeException("Engine is broken.");
    }
}

3. The Class Using the Dependency:

Java

class Car {
    private final Engine engine;

    public Car(Engine engine) {
        this.engine = engine;
    }

    public void start() {
        engine.start();
    }
}

4. Test Cases:

Java

import org.junit.jupiter.api.Test;

public class CarTest {
    @Test
    public void carStartsWithWorkingEngine() {
        Engine workingEngine = new WorkingEngine();
        Car car = new Car(workingEngine);
        car.start();
        // Assert that the car started successfully
    }

    @Test
    public void carThrowsExceptionWithBrokenEngine() {
        Engine brokenEngine = new BrokenEngine();
        Car car = new Car(brokenEngine);
        assertThrows(RuntimeException.class, () -> car.start());
    }
}

Explanation:

• Interface: The Engine interface defines the contract for the dependency, allowing for different implementations.
• Concrete Implementations: The WorkingEngine and BrokenEngine classes implement the Engine interface, providing different behaviors.
• Dependency Injection: The Car class takes an Engine instance as a constructor parameter, injecting the dependency.
• Test Cases: The CarTest the class demonstrates how to test the Car class with different Engine implementations, ensuring its correctness under various conditions.

Key Points:

• Loose Coupling: The Car class is decoupled from specific Engine implementations, promoting flexibility and maintainability.
• Testability: The ability to inject different Engine instances makes testing easier and more thorough.
• Readability: The code is well-structured and easy to understand, with a clear separation of concerns.
• Error Handling: The BrokenEngine class demonstrates how to handle potential exceptions in the dependency.

This example effectively illustrates the principles of Dependency Injection and its benefits in building robust and testable Java applications.