The Same Pattern, a Different Dependency

The pattern travels a long way from a database. Here it is behind an entirely different kind of dependency — a mail client sending ride confirmations.

java

interface MailClient {
    void send(String to, String body);
}

class SmtpMailClient implements MailClient {
    public void send(String to, String body) { /* talks to a real SMTP server */ }
}

class StubMailClient implements MailClient {
    public void send(String to, String body) {
        System.out.println("stub sent to " + to + ": " + body);
    }
}

class NotificationService {
    private final MailClient mailClient;

    NotificationService(MailClient mailClient) {
        this.mailClient = mailClient;
    }

    void notifyRideConfirmed(String rider) {
        mailClient.send(rider, "Your ride is confirmed");
    }
}

text

stub sent to alex@example.com: Your ride is confirmed

run in a container, javac 21

A constructor parameter instead of a new, in both cases — one swaps a database, the other swaps a mail server, and the fix reads the same way each time. That repetition is the actual lesson: this is not a database trick. It is a shape, and it is the shape every later topic in this subject assumes you can already see.