终止或中断Java 8流循环

  • Z时代
  • 2024-01-10
  • 分类：问答

我有一个Java 8流循环，其内容如下：

    void matchSellOrder(Market market, Order sellOrder) {
        System.out.println("selling " + market.pair() + " : " + sellOrder);
        market.buyOrders()
                .stream()
                .filter(buyOrder -> buyOrder.price >= sellOrder.price)
                .sorted(BY_ASCENDING_PRICE)
                .forEach((buyOrder) -> {
                    double tradeVolume = Math.min(buyOrder.quantity, sellOrder.quantity);
                    double price = buyOrder.price;
                    buyOrder.quantity -= tradeVolume;
                    sellOrder.quantity -= tradeVolume;
                    Trade trade = new Trade.Builder(market, price, tradeVolume, Trade.Type.SELL).build();
                    CommonUtil.convertToJSON(trade);
                    if (sellOrder.quantity == 0) {
                        System.out.println("order fulfilled");
                        // break loop there
                    }
                });
    }

满足某些条件时如何跳出循环？什么是关闭流的正确方法呢？

我滥用流技术，假设它是一个循环，不是为此而设计的。这是我使用下面提供的答案最终得到的代码：

        List<Order> applicableSortedBuyOrders = market.buyOrders()
                .stream()
                .filter(buyOrder -> buyOrder.price >= sellOrder.price)
                .sorted(BY_ASCENDING_PRICE)
                .collect(toList());
        for(Order buyOrder : applicableSortedBuyOrders){
            double tradeVolume = Math.min(buyOrder.quantity, sellOrder.quantity);
            double price = buyOrder.price;
            buyOrder.quantity -= tradeVolume;
            sellOrder.quantity -= tradeVolume;
            Trade trade = new Trade.Builder(market, price, tradeVolume, Trade.Type.SELL).build();
            CommonUtil.printAsJSON(trade);
            if (sellOrder.quantity == 0) {
                System.out.println("order fulfilled");
                break;
            }
        }

回答：

Stream.forEach不是循环，也并非设计为使用终止break。如果流是并行流，则lambda主体可以同时在不同的线程上执行（这不容易打破，并且很容易产生错误的结果）。

最好使用带有while循环的迭代器：

Iterator<BuyOrderType> iter = market.buyOrders() // replace BuyOrderType with correct type here
            .stream()
            .filter(buyOrder -> buyOrder.price >= sellOrder.price)
            .sorted(BY_ASCENDING_PRICE).iterator();
while (iter.hasNext()) {
    BuyOrderType buyOrder = iter.next()  // replace BuyOrderType with correct type here
    double tradeVolume = Math.min(buyOrder.quantity, sellOrder.quantity);
    double price = buyOrder.price;
    buyOrder.quantity -= tradeVolume;
    sellOrder.quantity -= tradeVolume;
    Trade trade = new Trade.Builder(market, price, tradeVolume, Trade.Type.SELL).build();
    CommonUtil.convertToJSON(trade);
    if (sellOrder.quantity == 0) {
        System.out.println("order fulfilled");
        break;
    }
}

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