Java12 Collectors.teeing 你真的須要瞭解一下

前言

在 Java 12 裏面有個很是好用但在官方 JEP 沒有公佈的功能，由於它只是 Collector 中的一個小改動，它的做用是 merge 兩個 collector 的結果，這句話顯得很抽象，老規矩，咱們先來看個圖:

管道改造常常會用這個小東西，一般咱們叫它「三通」，它的主要做用就是將 downstream1 和 downstream2 的流入合併，而後從 merger 流出

有了這個形象的說明咱們就進入正題吧

Collectors.teeing

上面提到的小功能就是 Collectors.teeing API, 先來看一下 JDK 關於該 API 的說明，看着以爲難受的直接忽略，繼續向下看例子就行了:

/** * 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);
}
複製代碼

API 描述重的一句話很是關鍵:

Every element passed to the resulting collector is processed by both downstream collectors 結合「三通圖」來講明就是，集合中每個要被傳入 merger 的元素都會通過 downstream1 和 downstream2 的加工處理

其中 merger 類型是 BiFunction，也就是說接收兩個參數，並輸出一個值，請看它的 apply 方法

@FunctionalInterface
public 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);
}
複製代碼

至於能夠如何處理，咱們來看一些例子吧

例子

爲了更好的說明 teeing 的使用，列舉了四個例子，看過這四個例子再回看上面的 API 說明，相信你會柳暗花明了

計數和累加

先來看一個經典的問題，給定的數字集合，須要映射整數流中的元素數量和它們的和

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 +
				'}';
	}
}
複製代碼

經過 Collectors.teeing 處理

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());
複製代碼

• downstream1 經過 Collectors 的靜態方法 counting 進行集合計數
• downstream2 經過 Collectors 的靜態方法 summingInt 進行集合元素值的累加
• merger 經過 CountSum 構造器收集結果

運行結果:

CountSum{count=7, sum=46}

咱們經過 teeing 一次性獲得咱們想要的結果，繼續向下看其餘例子:

最大值與最小值

經過給定的集合， 一次性計算出集合的最大值與最小值，一樣新建一個類 MinMax，並建立構造器用於 merger 收集結果

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 +
				'}';
	}
}
複製代碼

經過 teeing API 計算結果:

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());
複製代碼

• downstream1 經過 Collectors 的靜態方法 minBy，經過 Comparator 比較器按照天然排序找到最小值
• downstream2 經過 Collectors 的靜態方法 maxBy，經過 Comparator 比較器按照天然排序找到最大值
• merger 經過 MinMax 構造器收集結果，只不過爲了應對 NPE，將 BiFunction 的兩個入參通過 Optional 處理

運行結果:

MinMax{min=1, max=12}

爲了驗證一下 Optional，咱們將集合中添加一個 null 元素，並修改一下排序規則來看一下排序結果:

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))));
複製代碼

• downstream1 處理規則是將 null 放在排序的最前面
• downstream2 處理規則是將 null 放在排序的最後面
• merger 處理時，都會執行 optional.orElse 方法，分別輸出最小值與最大值

運行結果:

MinMax{min=-2147483648, max=2147483647}

瓜的總重和單個重量

接下來舉一個更貼合實際的操做對象的例子

// 定義瓜的類型和重量
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;
	}
}

// 總重和單個重量列表
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 +
				'}';
	}
}
複製代碼

經過 teeing API 計算總重量和單個列表重量

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());
複製代碼

• downstream1 經過 Collectors 的靜態方法 summingInt 作重量累加
• downstream2 經過 Collectors 的靜態方法 mapping 提取出瓜的重量，並經過流的終結操做 toList() 獲取結果
• merger 經過 WeightsAndTotal 構造器獲取結果

運行結果:

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

繼續一個更貼合實際的例子吧:

預定人員列表和預定人數

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 +
				" }";
	}
}
複製代碼

經過 teeing API 處理

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);
複製代碼

• downstream1 經過 filtering 方法過濾出肯定參加的人，並 mapping 出他們的姓名，最終放到 toList 集合中
• downstream2 經過 summingInt 方法計數累加
• merger 經過 EventParticipation 構造器收集結果

其中咱們定義了 var result 來收集結果，並無指定類型，這個語法糖也加速了咱們編程的效率

運行結果:

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

總結

其實 teeing API 就是靈活應用 Collectors 裏面定義的靜態方法，將集合元素經過 downstream1 和 downstream2 進行處理，最終經過 merger 收集起來，當項目中有同時獲取兩個收集結果時，是時候應用咱們的 teeing API 了

靈魂追問

1. Collectors 裏面的靜態方法你應用的熟練嗎？
2. 項目中大家在用 JDK 的版本是多少？
3. Lambda 的使用熟練嗎？
4. 你的燈還亮着嗎？

1. Java併發系列文章，持續更新中
2. Maven 依賴傳遞性透徹理解
3. 讀取Excel還用POI?試試這個開源框架
4. 基礎面試，爲何面試官總喜歡問Java String

以讀偵探小說思惟輕鬆趣味學習 Java 技術棧相關知識，本着將複雜問題簡單化，抽象問題具體化和圖形化原則逐步分解技術問題，技術持續更新，請持續關注......