// Option 1: Initialize a Firestore client with a specific `projectId` and
//           authorization credential.
FirestoreOptions firestoreOptions =
    FirestoreOptions.getDefaultInstance().toBuilder()
        .setProjectId(projectId)
        .setCredentials(GoogleCredentials.getApplicationDefault())
        .build();
Firestore db = firestoreOptions.getService();

// Option 2: Initialize a Firestore client with default values inferred from
//           your environment.
Firestore db = FirestoreOptions.getDefaultInstance().getService();