如何使用RxFire检查是否已在Firestore中创建文档

import com.google.firebase.firestore.DocumentReference;
import com.google.firebase.firestore.FirebaseFirestore;
import com.google.firebase.firestore.DocumentSnapshot;
import io.reactivex.rxjava3.core.Single;

public class FirestoreExample {
    public static void main(String[] args) {
        FirebaseFirestore db = FirebaseFirestore.getInstance();
        DocumentReference docRef = db.collection("users").document("user1");

        Single<DocumentSnapshot> documentSnapshotSingle = docRef.get().toSingle();

        documentSnapshotSingle.subscribe(documentSnapshot -> {
            if (documentSnapshot.exists()) {
                System.out.println("Document exists!");
            } else {
                System.out.println("Document does not exist!");
            }
        }, throwable -> {
            System.err.println("Error: " + throwable.getMessage());
        });
    }
}