output: #fixed code
                                  @Override
      public void receiveEmptyACK(){
          if(!emptyAckNotificationTimes.add(System.currentTimeMillis())){
              log.error("Could not add notification time for empty ACK.");
          }
      }
input: #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