Java Lambda Stream Distinct（）在任意键上？[副本]

Question

下面是它的名称

BigDecimal totalShare = orders.stream().map(c -> Pairing.keyLeft(c.getCompany().getId(), c.getShare())).distinct().map(Pairing::getRightItem).reduce(BigDecimal.ZERO, (x,y) -> x.add(y));

这是配对类

    public final class Pairing<X,Y>  {
           private final X item1;
           private final Y item2;
           private final KeySetup keySetup;

           private static enum KeySetup {LEFT,RIGHT,BOTH};

           private Pairing(X item1, Y item2, KeySetup keySetup) {
                  this.item1 = item1;
                  this.item2 = item2;
                  this.keySetup = keySetup;
           }
           public X getLeftItem() { 
                  return item1;
           }
           public Y getRightItem() { 
                  return item2;
           }

           public static <X,Y> Pairing<X,Y> keyLeft(X item1, Y item2) { 
                  return new Pairing<X,Y>(item1, item2, KeySetup.LEFT);
           }

           public static <X,Y> Pairing<X,Y> keyRight(X item1, Y item2) { 
                  return new Pairing<X,Y>(item1, item2, KeySetup.RIGHT);
           }
           public static <X,Y> Pairing<X,Y> keyBoth(X item1, Y item2) { 
                  return new Pairing<X,Y>(item1, item2, KeySetup.BOTH);
           }
           public static <X,Y> Pairing<X,Y> forItems(X item1, Y item2) { 
                  return keyBoth(item1, item2);
           }

           @Override
           public int hashCode() {
                  final int prime = 31;
                  int result = 1;
                  if (keySetup.equals(KeySetup.LEFT) || keySetup.equals(KeySetup.BOTH)) {
                  result = prime * result + ((item1 == null) ? 0 : item1.hashCode());
                  }
                  if (keySetup.equals(KeySetup.RIGHT) || keySetup.equals(KeySetup.BOTH)) {
                  result = prime * result + ((item2 == null) ? 0 : item2.hashCode());
                  }
                  return result;
           }

           @Override
           public boolean equals(Object obj) {
                  if (this == obj)
                         return true;
                  if (obj == null)
                         return false;
                  if (getClass() != obj.getClass())
                         return false;
                  Pairing<?,?> other = (Pairing<?,?>) obj;
                  if (keySetup.equals(KeySetup.LEFT) || keySetup.equals(KeySetup.BOTH)) {
                         if (item1 == null) {
                               if (other.item1 != null)
                                      return false;
                         } else if (!item1.equals(other.item1))
                               return false;
                  }
                  if (keySetup.equals(KeySetup.RIGHT) || keySetup.equals(KeySetup.BOTH)) {
                         if (item2 == null) {
                               if (other.item2 != null)
                                      return false;
                         } else if (!item2.equals(other.item2))
                               return false;
                  }
                  return true;
           }

    }

更新：

测试了斯图尔特下面的功能，它似乎工作很好。下面的操作区分每个字符串的第一个字母。我想要弄清楚的是，ConcurrentHashMap如何只为整个流维护一个实例

public class DistinctByKey {

    public static <T> Predicate<T> distinctByKey(Function<? super T,Object> keyExtractor) {
        Map<Object,Boolean> seen = new ConcurrentHashMap<>();
        return t -> seen.putIfAbsent(keyExtractor.apply(t), Boolean.TRUE) == null;
    }

    public static void main(String[] args) { 

        final ImmutableList<String> arpts = ImmutableList.of("ABQ","ALB","CHI","CUN","PHX","PUJ","BWI");

        arpts.stream().filter(distinctByKey(f -> f.substring(0,1))).forEach(s -> System.out.println(s));
    }

输出为...

ABQ
CHI
PHX
BWI

Answer 1

Distinct操作是一个有状态的流水线操作；在本例中，它是一个有状态的过滤器。自己创建这些有点不方便，因为没有内置的东西，但是一个小的helper类应该可以做到这一点：

/**
 * Stateful filter. T is type of stream element, K is type of extracted key.
 */
static class DistinctByKey<T,K> {
    Map<K,Boolean> seen = new ConcurrentHashMap<>();
    Function<T,K> keyExtractor;
    public DistinctByKey(Function<T,K> ke) {
        this.keyExtractor = ke;
    }
    public boolean filter(T t) {
        return seen.putIfAbsent(keyExtractor.apply(t), Boolean.TRUE) == null;
    }
}

我不知道您的领域类，但我认为，有了这个helper类，您可以像下面这样做：

BigDecimal totalShare = orders.stream()
    .filter(new DistinctByKey<Order,CompanyId>(o -> o.getCompany().getId())::filter)
    .map(Order::getShare)
    .reduce(BigDecimal.ZERO, BigDecimal::add);

不幸的是，类型推断在表达式中不能得到足够的深入，所以我不得不显式地为DistinctByKey类指定类型参数。

可以去掉k类型参数，因为除了存储在映射中之外，它实际上并不用于其他任何用途。因此object就足够了。

/**
 * Stateful filter. T is type of stream element.
 */
static class DistinctByKey<T> {
    Map<Object,Boolean> seen = new ConcurrentHashMap<>();
    Function<T,Object> keyExtractor;
    public DistinctByKey(Function<T,Object> ke) {
        this.keyExtractor = ke;
    }
    public boolean filter(T t) {
        return seen.putIfAbsent(keyExtractor.apply(t), Boolean.TRUE) == null;
    }
}

BigDecimal totalShare = orders.stream()
    .filter(new DistinctByKey<Order>(o -> o.getCompany().getId())::filter)
    .map(Order::getShare)
    .reduce(BigDecimal.ZERO, BigDecimal::add);

这稍微简化了事情，但我仍然必须为构造函数指定类型参数。尝试使用diamond或静态工厂方法似乎并不能改善事情。我认为困难在于，当构造函数或静态方法调用在方法引用的实例表达式中时，编译器无法推断泛型类型参数。哦好吧。

（可能会简化它的另一个变体是使DistinctByKey 实现谓词 并将方法重命名为Eval。这将消除使用方法引用的需要，并可能改进类型推断。但是，它不可能像下面的解决方案那么好。）

public static <T> Predicate<T> distinctByKey(Function<? super T,Object> keyExtractor) {
    Map<Object,Boolean> seen = new ConcurrentHashMap<>();
    return t -> seen.putIfAbsent(keyExtractor.apply(t), Boolean.TRUE) == null;
}

BigDecimal totalShare = orders.stream()
    .filter(distinctByKey(o -> o.getCompany().getId()))
    .map(Order::getShare)
    .reduce(BigDecimal.ZERO, BigDecimal::add);