public String toString() {
    String s = "Bid            | Ask\n" +
               "Price   | Size | Price   | Size\n" +
               "-------------------------------\n";
                              
    for(int i = 0;i < ROWS;i++) {
      s += bids[i].get("price") + "\t" + bids[i].get("size") + "\t";
      s += asks[i].get("price") + "\t" + asks[i].get("size") + "\n";
    }
    
    s += "VWAP    | Size | VWAP    | Size\n" + 
         "-------------------------------\n" + 
         bidVWAP() + "\t" + bidSizeSum() + "\t" +
         askVWAP() + "\t" + askSizeSum() + "\t";
    
    return s;
  }