#vulnerable code 
                                @Override
    public void receiveEmptyACK(){
        if(receiveEnabled && !emptyAckNotificationTimes.add(System.currentTimeMillis())){
            log.error("Could not add notification time for empty ACK.");
        }
    } 
                            #location 3 
                            #vulnerability type THREAD_SAFETY_VIOLATION
#fixed code
                                @Override
    public void receiveEmptyACK(){
        if(!emptyAckNotificationTimes.add(System.currentTimeMillis())){
            log.error("Could not add notification time for empty ACK.");
        }
    }