JDK12 Collectors.teeing 功能真香

前言

在 Java 12 里面有个非常好用但在官方 JEP 没有公布的功能,因为它只是 Collector 中的一个小改动,它的作用是 merge 两个 collector 的结果,这句话显得很抽象,老规矩,我们先来看个图(这真是一个不和谐的图????):

管道改造经常会用这个小东西,通常我们叫它「三通」,它的主要作用就是将 downstream1 和 downstream2 的流入合并,然后从 merger 流出

有了这个形象的说明我们就进入正题吧「文中代码举例比较多,更好的阅读体验点击文末——阅读原文」

Collectors.teeing

上面提到的小功能就是 Collectors.teeing API, 先来看一下 JDK 关于该 API 的说明,看着觉得难受的直接忽略,继续向下看例子就好了:

/** * Returns a {@code Collector} that is a composite of two downstream collectors. * Every element passed to the resulting collector is processed by both downstream * collectors, then their results are merged using the specified merge function * into the final result. * * <p>The resulting collector functions do the following: * * <ul> * <li>supplier: creates a result container that contains result containers * obtained by calling each collector's supplier * <li>accumulator: calls each collector's accumulator with its result container * and the input element * <li>combiner: calls each collector's combiner with two result containers * <li>finisher: calls each collector's finisher with its result container, * then calls the supplied merger and returns its result. * </ul> * * <p>The resulting collector is {@link Collector.Characteristics#UNORDERED} if both downstream * collectors are unordered and {@link Collector.Characteristics#CONCURRENT} if both downstream * collectors are concurrent. * * @param <T>         the type of the input elements * @param <R1>        the result type of the first collector * @param <R2>        the result type of the second collector * @param <R>         the final result type * @param downstream1 the first downstream collector * @param downstream2 the second downstream collector * @param merger      the function which merges two results into the single one * @return a {@code Collector} which aggregates the results of two supplied collectors. * @since 12 */public static <T, R1, R2, R>Collector<T, ?, R> teeing(Collector<? super T, ?, R1> downstream1,                          Collector<? super T, ?, R2> downstream2,                          BiFunction<? super R1, ? super R2, R> merger) {    return teeing0(downstream1, downstream2, merger);}

API 描述重的一句话非常关键:

Every element passed to the resulting collector is processed by both downstream collectors结合「三通图」来说明就是,集合中每一个要被传入 merger 的元素都会经过 downstream1 和 downstream2 的加工处理

其中 merger 类型是 BiFunction,也就是说接收两个参数,并输出一个值,请看它的 apply 方法

@FunctionalInterfacepublic interface BiFunction<T, U, R> {
/** * Applies this function to the given arguments. * * @param t the first function argument * @param u the second function argument * @return the function result */ R apply(T t, U u);}

至于可以如何处理,我们来看一些例子吧

例子

为了更好的说明 teeing 的使用,列举了四个例子,看过这四个例子再回看上面的 API 说明,相信你会柳暗花明了

计数和累加

先来看一个经典的问题,给定的数字集合,需要映射整数流中的元素数量和它们的和

class CountSum {    private final Long count;    private final Integer sum;    public CountSum(Long count, Integer sum) {        this.count = count;        this.sum = sum;    }
@Override public String toString() { return "CountSum{" + "count=" + count + ", sum=" + sum + '}'; }}

通过 Collectors.teeing 处理

CountSum countsum = Stream.of(2, 11, 1, 5, 7, 8, 12)        .collect(Collectors.teeing(                counting(),                summingInt(e -> e),                CountSum::new));
System.out.println(countsum.toString());

  • downstream1 通过 Collectors 的静态方法 counting 进行集合计数

  • downstream2 通过 Collectors 的静态方法 summingInt 进行集合元素值的累加

  • merger 通过 CountSum 构造器收集结果

运行结果:

CountSum{count=7, sum=46}

我们通过 teeing 一次性得到我们想要的结果,继续向下看其他例子:

最大值与最小值

通过给定的集合, 一次性计算出集合的最大值与最小值,同样新建一个类 MinMax,并创建构造器用于 merger 收集结果

class MinMax {    private final Integer min;    private final Integer max;    public MinMax(Integer min, Integer max) {        this.min = min;        this.max = max;    }
@Override public String toString() { return "MinMax{" + "min=" + min + ", max=" + max + '}'; }}

通过 teeing API 计算结果:

MinMax minmax = Stream.of(2, 11, 1, 5, 7, 8, 12)        .collect(Collectors.teeing(                minBy(Comparator.naturalOrder()),                maxBy(Comparator.naturalOrder()),                (Optional<Integer> a, Optional<Integer> b) -> new MinMax(a.orElse(Integer.MIN_VALUE), b.orElse(Integer.MAX_VALUE))));
System.out.println(minmax.toString());

  • downstream1 通过 Collectors 的静态方法 minBy,通过 Comparator 比较器按照自然排序找到最小值

  • downstream2 通过 Collectors 的静态方法 maxBy,通过 Comparator 比较器按照自然排序找到最大值

  • merger 通过 MinMax 构造器收集结果,只不过为了应对 NPE,将 BiFunction 的两个入参经过 Optional 处理

运行结果:

MinMax{min=1, max=12}

为了验证一下 Optional,我们将集合中添加一个 null 元素,并修改一下排序规则来看一下排序结果:

MinMax minmax = Stream.of(null, 2, 11, 1, 5, 7, 8, 12)                .collect(Collectors.teeing(                        minBy(Comparator.nullsFirst(Comparator.naturalOrder())),                        maxBy(Comparator.nullsLast(Comparator.naturalOrder())),                        (Optional<Integer> a, Optional<Integer> b) -> new MinMax(a.orElse(Integer.MIN_VALUE), b.orElse(Integer.MAX_VALUE))));

  • downstream1 处理规则是将 null 放在排序的最前面

  • downstream2 处理规则是将 null 放在排序的最后面

  • merger 处理时,都会执行 optional.orElse 方法,分别输出最小值与最大值

运行结果:

MinMax{min=-2147483648, max=2147483647}

瓜的总重和单个重量

接下来举一个更贴合实际的操作对象的例子

// 定义瓜的类型和重量class Melon {    private final String type;    private final int weight;    public Melon(String type, int weight) {        this.type = type;        this.weight = weight;    }
public String getType() { return type; }
public int getWeight() { return weight; }}
// 总重和单个重量列表class WeightsAndTotal { private final int totalWeight; private final List<Integer> weights; public WeightsAndTotal(int totalWeight, List<Integer> weights) { this.totalWeight = totalWeight; this.weights = weights; }
@Override public String toString() { return "WeightsAndTotal{" + "totalWeight=" + totalWeight + ", weights=" + weights + '}'; }}

通过 teeing API 计算总重量和单个列表重量

List<Melon> melons = Arrays.asList(new Melon("Crenshaw", 1200),    new Melon("Gac", 3000), new Melon("Hemi", 2600),    new Melon("Hemi", 1600), new Melon("Gac", 1200),    new Melon("Apollo", 2600), new Melon("Horned", 1700),    new Melon("Gac", 3000), new Melon("Hemi", 2600));

WeightsAndTotal weightsAndTotal = melons.stream() .collect(Collectors.teeing( summingInt(Melon::getWeight), mapping(m -> m.getWeight(), toList()), WeightsAndTotal::new));
System.out.println(weightsAndTotal.toString());

  • downstream1 通过 Collectors 的静态方法 summingInt 做重量累加

  • downstream2 通过 Collectors 的静态方法 mapping 提取出瓜的重量,并通过流的终结操作 toList() 获取结果

  • merger 通过 WeightsAndTotal 构造器获取结果

运行结果:

WeightsAndTotal{totalWeight=19500, weights=[1200, 3000, 2600, 1600, 1200, 2600, 1700, 3000, 2600]}

继续一个更贴合实际的例子吧:

预约人员列表和预约人数

class Guest {    private String name;    private boolean participating;    private Integer participantsNumber;
public Guest(String name, boolean participating, Integer participantsNumber) { this.name = name; this.participating = participating; this.participantsNumber = participantsNumber; } public boolean isParticipating() { return participating; }
public Integer getParticipantsNumber() { return participantsNumber; }
public String getName() { return name; }}
class EventParticipation { private List<String> guestNameList; private Integer totalNumberOfParticipants;
public EventParticipation(List<String> guestNameList, Integer totalNumberOfParticipants) { this.guestNameList = guestNameList; this.totalNumberOfParticipants = totalNumberOfParticipants; }
@Override public String toString() { return "EventParticipation { " + "guests = " + guestNameList + ", total number of participants = " + totalNumberOfParticipants + " }"; }}

通过 teeing API 处理

var result = Stream.of(                new Guest("Marco", true, 3),                new Guest("David", false, 2),                new Guest("Roger",true, 6))                .collect(Collectors.teeing(                        Collectors.filtering(Guest::isParticipating, Collectors.mapping(Guest::getName, Collectors.toList())),                        Collectors.summingInt(Guest::getParticipantsNumber),                        EventParticipation::new                ));System.out.println(result);

  • downstream1 通过 filtering 方法过滤出确定参加的人,并 mapping 出他们的姓名,最终放到 toList 集合中

  • downstream2 通过 summingInt 方法计数累加

  • merger 通过 EventParticipation 构造器收集结果

其中我们定义了 var result 来收集结果,并没有指定类型,这个语法糖也加速了我们编程的效率

运行结果:

EventParticipation { guests = [Marco, Roger], total number of participants = 11 }

总结

其实 teeing API 就是灵活应用 Collectors 里面定义的静态方法,将集合元素通过 downstream1 和 downstream2 进行处理,最终通过 merger 收集起来,当项目中有同时获取两个收集结果时,是时候应用我们的 teeing API 了

灵魂追问

  1. Collectors 里面的静态方法你应用的熟练吗?

  2. 项目中你们在用 JDK 的版本是多少?

  3. Lambda 的使用熟练吗?

  4. 你的灯还亮着吗?


posted @ 2019-11-22 23:01  码农小胖哥  阅读(161)  评论(0编辑  收藏  举报