java8之lamda groupingby多层 嵌套[通俗易懂]

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@Test
public void r(){
    List<Person> javaProgrammers = new ArrayList<Person>() {
        {
            add(new Person("Elsdon", "1", "Java programmer", "male", 43, 2000));
            add(new Person("Elsdon", "1", "Java programmer", "female", 23, 1500));
            add(new Person("Elsdon", "Jaycob", "Java programmer", "male", 33, 1800));
            add(new Person("Elsdon", "Jaycob", "Java programmer", "female", 32, 1600));
            add(new Person("Elsdon", "Jaycob", "Java programmer", "male", 22, 1200));
            add(new Person("Maude", "2", "Java programmer", "female", 27, 1900));
            add(new Person("Maude", "2", "Java programmer", "male", 30, 2300));
            add(new Person("Maude", "Jaimie", "Java programmer", "female", 35, 1700));
            add(new Person("Maude", "Jaimie", "Java programmer", "male", 33, 2000));
            add(new Person("Maude", "Jaimie", "Java programmer", "female", 34, 1300));
        }
    };

    List<Person> phpProgrammers = new ArrayList<Person>() {
        {
            add(new Person("Jarrod", "Pace", "PHP programmer", "male", 34, 1550));
            add(new Person("Clarette", "Cicely", "PHP programmer", "female", 23, 1200));
            add(new Person("Victor", "Channing", "PHP programmer", "male", 32, 1600));
            add(new Person("Tori", "Sheryl", "PHP programmer", "female", 21, 1000));
            add(new Person("Osborne", "Shad", "PHP programmer", "male", 32, 1100));
            add(new Person("Rosalind", "Layla", "PHP programmer", "female", 25, 1300));
            add(new Person("Fraser", "Hewie", "PHP programmer", "male", 36, 1100));
            add(new Person("Quinn", "Tamara", "PHP programmer", "female", 21, 1000));
            add(new Person("Alvin", "Lance", "PHP programmer", "male", 38, 1600));
            add(new Person("Evonne", "Shari", "PHP programmer", "female", 40, 1800));
        }
    };

 Map<String, Map<String, List<Person>>> z = javaProgrammers.stream().collect(Collectors.groupingBy(t > t.getFirstName(), Collectors.groupingBy(r -> r.getLastName())));
        z.forEach((k, v) -> {
            v.forEach((k1, v1) -> {
                v1.forEach(v2 -> System.out.println(v2.getFirstName() + v2.getLastName()));
            });
        });
    }

补充下源码的理解

首先groupBy有三个方法，前两个方法是起到一个参数补充作用（有具体知道这个的术语的吗）

第一种是我们常用的方法只需要传递一个Function用于获取key值。
public static <T, K> Collector<T, ?, Map<K, List<T>>>
groupingBy(Function<? super T, ? extends K> classifier) {
    return groupingBy(classifier, toList());
}

这个就是嵌套写法。需要传递一个新的Collector进去。理解起来就是接收Collector类型作为参数之一，同时返回一个Collector类型，这就是嵌套调用的基础。

public static <T, K, A, D>
Collector<T, ?, Map<K, D>> groupingBy(Function<? super T, ? extends K> classifier,
                                      Collector<? super T, A, D> downstream) {
    return groupingBy(classifier, HashMap::new, downstream);
}

真正的实现是第三个方法。 downstreamAccumulator.accept(container, t);从这里我们知道，先执行的外层的 Accumulator方法，

  public static <T, K, D, A, M extends Map<K, D>>
    Collector<T, ?, M> groupingBy(Function<? super T, ? extends K> classifier,
                                  Supplier<M> mapFactory,
                                  Collector<? super T, A, D> downstream) {
        Supplier<A> downstreamSupplier = downstream.supplier();
        BiConsumer<A, ? super T> downstreamAccumulator = downstream.accumulator();
        BiConsumer<Map<K, A>, T> accumulator = (m, t) -> {
            K key = Objects.requireNonNull(classifier.apply(t), "element cannot be mapped to a null key");
            A container = m.computeIfAbsent(key, k -> downstreamSupplier.get());
            downstreamAccumulator.accept(container, t);
        };
        BinaryOperator<Map<K, A>> merger = Collectors.<K, A, Map<K, A>>mapMerger(downstream.combiner());
        @SuppressWarnings("unchecked")
        Supplier<Map<K, A>> mangledFactory = (Supplier<Map<K, A>>) mapFactory;

        if (downstream.characteristics().contains(Collector.Characteristics.IDENTITY_FINISH)) {
            return new CollectorImpl<>(mangledFactory, accumulator, merger, CH_ID);
        }
        else {
            @SuppressWarnings("unchecked")
            Function<A, A> downstreamFinisher = (Function<A, A>) downstream.finisher();
            Function<Map<K, A>, M> finisher = intermediate -> {
                intermediate.replaceAll((k, v) -> downstreamFinisher.apply(v));
                @SuppressWarnings("unchecked")
                M castResult = (M) intermediate;
                return castResult;
            };
            return new CollectorImpl<>(mangledFactory, accumulator, merger, finisher, CH_NOID);
        }
    }