StockServer

class Server {
  Map<Stock, StockTx> map = new HashMap<Stock, StockTx>();
  
  public void publish() {
    Map<Stock, StockTx> mapBack = null;
    synchronized(this) {
      mapBack = map;
      map = new HashMap<Stock, StockTx>();
    }
    
    // following
  }
}

class StockTx {
  String transactionType;
  String price;
}