Asynchronous read for a list of key

Hii I want to apply Asynch GET method . I called GET function in loop as my input is List of key and I have to return List of values to respective keys. As i am storing output of each iteration in a synchronized List.But as my database is on more then one instances due to this i think my list will not folllow synchronization. Can someone suggest me how to implement this to make list synchronized even more then one server

Multiple get requests from different AerospikeClient instances can be placed on the same event loop. This does not guarantee the receive order, but the listener callback will be thread safe without having to use locks.