Question

我创建了一个收集器，该收集器可以将流简化为具有某些客户可以购买的商品的键并将客户名称作为值的地图，我的实现在顺序流中进行了深入研究但是当我尝试使用parallel时根本无法使用，结果集总是包含一个客户名称。

List<Customer> customerList = this.mall.getCustomerList();

Supplier<Object> supplier = ConcurrentHashMap<String,Set<String>>::new;

BiConsumer<Object, Customer> accumulator = ((o, customer) -> customer.getWantToBuy().stream().map(Item::getName).forEach(
            item -> ((ConcurrentHashMap<String,Set<String>>)o)
                    .merge(item,new HashSet<String>(Collections.singleton(customer.getName())),
                            (s,s2) -> {
                                HashSet<String> res = new HashSet<>(s);
                                res.addAll(s2);
                                return res;
                            })
    ));

BinaryOperator<Object> combiner = (o,o2) -> {
        ConcurrentHashMap<String,Set<String>> res = new ConcurrentHashMap<>((ConcurrentHashMap<String,Set<String>>)o);
        res.putAll((ConcurrentHashMap<String,Set<String>>)o2);
        return res;
    };

Function<Object, Map<String, Set<String>>> finisher = (o) -> new HashMap<>((ConcurrentHashMap<String,Set<String>>)o);

Collector<Customer, ?, Map<String, Set<String>>> toItemAsKey =
        new CollectorImpl<>(supplier, accumulator, combiner, finisher, EnumSet.of(
            Collector.Characteristics.CONCURRENT,
            Collector.Characteristics.IDENTITY_FINISH));

Map<String, Set<String>> itemMap = customerList.stream().parallel().collect(toItemAsKey);

我的accumulator实现或另一个Function中肯定存在问题，但我无法解决！谁能建议我该怎么办？

Answer 1

您的组合器未正确实现。
您将覆盖所有具有相同密钥的条目。您要为现有键添加值。

BinaryOperator<ConcurrentHashMap<String,Set<String>>> combiner = (o,o2) -> {
        ConcurrentHashMap<String,Set<String>> res = new ConcurrentHashMap<>(o);
        o2.forEach((key, set) -> set.forEach(string -> res.computeIfAbsent(key, k -> new HashSet<>())
                                                          .add(string)));
        return res;
    };

累加器不能在并行流中正常工作

1 个答案: