code: public OkCoinFuturesTradeHistoryResult[] getFuturesTradesHistory(
        String symbol, long since, String date) throws IOException {
  
      OkCoinFuturesTradeHistoryResult[] futuresHistory =
          okCoin.getFuturesTradeHistory(apikey, since, symbol, date, signatureCreator());
      return (futuresHistory);
    }
docs: 获取OKEX合约交易历史（非个人）
  
  @param symbol
  @param since
  @param date
  @return
  @throws IOException