从单个列表并行输入映射和列表

Question

List<Model> models = ....

List<String> anotherlist = models.parallelStream().map(Model::getName).collect(Collectors.toList());

Map<String, Model> modelsMap = new ConcurrentHashMap<String, Model>();
for (final Model model : models) {
   modelsMap.put(model.getId(), model);
}

Answer 1

看来你想要的只是：

ConcurrentMap<String, Model> modelsMap = 
        models.parallelStream()
              .collect(toConcurrentMap(Model::getId, identity()));

这将通过使用Collectors.ToConcurrentMap收集模型列表来创建ConcurrentMap。键映射器是返回模型名称的函数，值映射器是标识函数。

注意，这将抛出一个异常是重复ID的情况。

Pair<Map<String, Model>, List<String>> pair = 
        models.parallelStream()
              .collect(pairing(
                    toConcurrentMap(Model::getId, identity()),
                    mapping(Model::getName, toList()),
                    Pair::new)
              );

static <T, A1, A2, R1, R2, R> Collector<T, ?, R> pairing(Collector<T, A1, R1> c1, 
        Collector<T, A2, R2> c2, BiFunction<R1, R2, R> finisher) {
    EnumSet<Characteristics> c = EnumSet.noneOf(Characteristics.class);
    c.addAll(c1.characteristics());
    c.retainAll(c2.characteristics());
    c.remove(Characteristics.IDENTITY_FINISH);
    return Collector.of(() -> new Object[] {c1.supplier().get(), c2.supplier().get()},
            (acc, v) -> {
                c1.accumulator().accept((A1)acc[0], v);
                c2.accumulator().accept((A2)acc[1], v);
            },
            (acc1, acc2) -> {
                acc1[0] = c1.combiner().apply((A1)acc1[0], (A1)acc2[0]);
                acc1[1] = c2.combiner().apply((A2)acc1[1], (A2)acc2[1]);
                return acc1;
            },
            acc -> {
                R1 r1 = c1.finisher().apply((A1)acc[0]);
                R2 r2 = c2.finisher().apply((A2)acc[1]);
                return finisher.apply(r1, r2);
            }, c.toArray(new Characteristics[c.size()]));
}

public class Pair<T, U> {
    private final T first;
    private final U second;

    public Pair(T first, U second) {
        this.first = first;
        this.second = second;
    }

    public T getFirst() {
        return first;
    }
    public U getSecond() {
        return second;
    }
}