MutationQueue.java

class MemoryMutationQueue {
  private final List<MutationBatch> queue;

  @Override
  public boolean isEmpty() {
    return queue.isEmpty();
  }
}

class SQLiteMutationQueue {
  private final SQLitePersistence db;

  @Override
  public boolean isEmpty() {
    return db.query("SELECT batch_id FROM mutations WHERE uid = ? LIMIT 1")
	 .binding(uid)
	 .isEmpty();
  }
}