Keep thread alive in order to receive callback

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Java

asynchronous

I've used callbacks before, but only when my client code was not responsible for keeping the program/threads running (for instance in an Android app). All I want to do now, is to make a really simple Firestore Java client that listens for realtime database updates. My code is super simple and gives you the idea what I want to achieve but I don't know how.

What I have tried:

public class Main {

    public static void main(String[] args) {
        DatabaseClient databaseClient = new DatabaseClient();
        databaseClient.listenForUpdates(() -> {
            System.out.println("Alright, new update, gotcha");
        });
    }
}

public class DatabaseClient {

    private final Firestore firestore = FirestoreClient.getFirestore();

    void listenForUpdates(OnUpdateListener listener) {
        firestore.document("MyDocument").addSnapshotListener((value, error) -> {
            //Ignore 'value' and 'error' in this example
            listener.onUpdate();
        });
    }
}

public interface OnUpdateListener {

    void onUpdate();
}

As expected, it now executes run() once and then terminates. So, how to I keep the program running so that it can receive a callback when the database is updated? I guess I could add an infinite loop, put the thread to sleep, and check for updates, but I'd like to learn how make this work using callbacks.