Dependency Injection

Dependency injection (DI) is a design technique in which an object receives the other objects it needs (its dependencies) from outside, usually through its constructor, instead of creating them itself. This keeps classes loosely coupled: a class depends on an interface, and the actual implementation is chosen elsewhere.

DI makes code easier to test, because real dependencies such as a database can be replaced with fakes, and easier to change. It is a form of inversion of control (IoC). Frameworks such as Spring (Java), ASP.NET Core and Angular create objects and inject their dependencies automatically.

package in.ihelpyou;

interface MessageSender {
	void send(String text);
}

class EmailSender implements MessageSender {
	public void send(String text) { System.out.println("Email: " + text); }
}

class OrderService {
	private final MessageSender sender;

	// The dependency is passed in, not created here
	OrderService(MessageSender sender) { this.sender = sender; }

	void placeOrder() { sender.send("Order placed"); }
}

public class Main {
	public static void main(String[] args) {
		new OrderService(new EmailSender()).placeOrder();
	}
}