java基础-Steam[2]-Collector接口

理解Stream.collect()方法

   /**伪代码如下
    * <pre>{@code
    *     R result = supplier.get();
    *     for (T element : this stream)
    *         accumulator.accept(result, element);
    *     return result;
    * }</pre>
    * <pre>{@code
    *     List<String> asList = stringStream.collect(ArrayList::new, ArrayList::add,
    *                                                ArrayList::addAll);
    * }</pre>
    * Supplier：生产者，也是返回的结果类型
    * accumulator：将流中的元素添加到Supplier中
    * BiConsumer：合并两个Supplier，
    */
   <R> R collect(Supplier<R> supplier,
                 BiConsumer<R, ? super T> accumulator,
                 BiConsumer<R, R> combiner);

   /**
    * Collector封装了#collect(Supplier, BiConsumer, BiConsumer)参数
    * terminal operation
    * 即使使用非线程安全的数据结构，也没有线程安全问题，如ArrayList
    * List<String> asList = stringStream.collect(Collectors.toList());
    * Map<String, List<Person>> peopleByCity
    *         = personStream.collect(Collectors.groupingBy(Person::getCity));
    * Map<String, Map<String, List<Person>>> peopleByStateAndCity
    *         = personStream.collect(Collectors.groupingBy(Person::getState,                                                    Collectors.groupingBy(Person::getCity)));
    */
   <R, A> R collect(Collector<? super T, A, R> collector);

Collector接口

public interface Collector<T, A, R> {
    //该接口只有get().相对于new一个容器对象,在后面的聚合操作中使用
    Supplier<A> supplier();
    //void accept(T t, U u);理解为累加操作,即不断的将值累加到supplier()返回的对象中
    BiConsumer<A, T> accumulator();
    //容器运算,一般在并行流中,生成的多个容器之间的运算
    BinaryOperator<A> combiner();

    Function<A, R> finisher();

    Set<Characteristics> characteristics();
}

实现1:toList

Collector<Person, ?, List<Person>> personListCollector = Collectors.toList();
//源码
public static <T> Collector<T, ?, List<T>> toList() {
    return new CollectorImpl<>((Supplier<List<T>>) ArrayList::new, List::add,
                               (left, right) -> { left.addAll(right); return left; },
                               CH_ID);
}

Supplier : new ArrayList()作为后面添加数据的容器

BiConsumer：List::add，调用ArrayList对象的add方法添加元素

BinaryOperator： (left, right) -> { left.addAll(right); return left;即ArrayList.addAll(ArrayList)

示例

public static void main(String[] args) {
    ArrayList<Person> list=new ArrayList<>();
    list.add(new Person("1",20));
    list.add(new Person("2",22));
    list.add(new Person("3",24));
    list.add(new Person("4",26));
    list.add(new Person("5",28));
    list.add(new Person("6",20));
    list.add(new Person("7",18));
    list.add(new Person("8",16));
    ArrayList<Person> newList = list.stream().collect(ArrayList::new, ArrayList::add, ArrayList::addAll);
    System.out.println("--newList--");
    newList.stream().forEach(System.out::println);
}
//输出
Person{name='1'}
Person{name='2'}
Person{name='3'}
Person{name='4'}
Person{name='5'}
Person{name='6'}
Person{name='7'}
Person{name='8'}
//替换collect语句
  ArrayList<Object> newList = list.stream().collect(ArrayList::new, ArrayList::add, ArrayList::add);
//输出
--newList--
Person{name='1'}
Person{name='2'}
Person{name='3'}
Person{name='4'}
Person{name='5'}
Person{name='6'}
Person{name='7'}
Person{name='8'}
//替换collect语句
  ArrayList<Object> newList = list.stream().parallel().collect(ArrayList::new, ArrayList::add, ArrayList::add);
//输出
Person{name='1'}
[Person{name='2'}]
[Person{name='3'}, [Person{name='4'}]]
[Person{name='5'}, [Person{name='6'}], [Person{name='7'}, [Person{name='8'}]]]
//替换collect语句
  ArrayList<Object> newList = list.stream().parallel().collect(ArrayList::new, ArrayList::add, ArrayList::addAll);
//输出
--newList--
Person{name='1'}
Person{name='2'}
Person{name='3'}
Person{name='4'}
Person{name='5'}
Person{name='6'}
Person{name='7'}
Person{name='8'}

<R> R collect(Supplier<R> supplier,
            BiConsumer<R, ? super T> accumulator,
            BiConsumer<R, R> combiner);
ArrayList<Person> newList = list.stream().collect(ArrayList::new, ArrayList::add, ArrayList::add);
//将ArrayList泛型指定为Person后，第二个ArrayList::add编译器报错，必须改为addAll

第三个参数必须是集合与集合的运算，一般在parallelStream中使用

实现2:toMap

public static <T, K, U> Collector<T, ?, Map<K,U>> toMap(Function<? super T, ? extends K> keyMapper,
                                                        Function<? super T, ? extends U> valueMapper) {
    return new CollectorImpl<>(HashMap::new,
                               uniqKeysMapAccumulator(keyMapper, valueMapper),
                               uniqKeysMapMerger(),
                               CH_ID);
}
private static <T, K, V>
    BiConsumer<Map<K, V>, T> uniqKeysMapAccumulator(Function<? super T, ? extends K> keyMapper,
                                                    Function<? super T, ? extends V> valueMapper) {
    return (map, element) -> {
        K k = keyMapper.apply(element);
        V v = Objects.requireNonNull(valueMapper.apply(element));
        V u = map.putIfAbsent(k, v);
        if (u != null) throw duplicateKeyException(k, u, v);
    };
}

简单理解就是,将流中的数据根据Function映射到map中的key和value ,返回HashMap对象

示例

@Test
public void testCollectMap (){
    ArrayList<Person> list = getData();
    Map<String, Integer> mapCollect = list.stream()
        .collect(Collectors.toMap(n -> n.getName() + "-test", n -> n.getAge() * 2));
    mapCollect.forEach((k, v) -> System.out.println(k + "==" + v));
}

实现3:groupingby

public static <T, K> Collector<T, ?, Map<K, List<T>>>
    groupingBy(Function<? super T, ? extends K> classifier) {
    return groupingBy(classifier, toList());
}
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);
}
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();
    //最主要理解下面这段代码
    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);
    }
}

从上面两个例子我们知道，不管怎么变，最后都要转换为：

  1. Supplier

  2. BiConsumer

  3. BinaryOperator

从下文示例返回的的数据类型是：

Collector<Person, ?, Map<Integer, List<Person>>> collector = Collectors.groupingBy(Person::getAge);

理解为，传入的操作元素是Person对象，最后返回一个map。map是按年龄分组，key对应年龄，value对应等于该年龄下的person的list对象。

所以这里要构建两个容器对象，List和Map，或者理解为建立在Collectors.toList()的基础上

理解一下代码：

BiConsumer<Map<K, A>, T> accumulator = (m, t) -> {
    //根据分组函数计算出Map中的KEY
    K key = Objects.requireNonNull(classifier.apply(t), "element cannot be mapped to a null key");
    //获取map中key对应的容器对象,如果没有这调用相应的Supplier 
    //像上面的示例,就是new ArrayList()
    A container = m.computeIfAbsent(key, k -> downstreamSupplier.get());
    //累加到第一个容器对象中,示例是List::add
    downstreamAccumulator.accept(container, t);
};

这个BiConsumer已经可以说明是如何进行分组的了。

示例

@Test
public void test (){
    Collector<Person, ?, Map<Integer, List<Person>>> collector = Collectors.groupingBy(Person::getAge);
    Map<Integer, List<Person>> collect = getData().stream().collect(collector);
    collect.forEach((k,v)-> System.out.println(k+"=="+v));
}
private ArrayList<Person> getData() {
    ArrayList<Person> list = new ArrayList<>();
    list.add(new Person("1", 20));
    list.add(new Person("2", 22));
    list.add(new Person("3", 24));
    list.add(new Person("4", 26));
    list.add(new Person("5", 28));
    list.add(new Person("6", 20));
    list.add(new Person("7", 28));
    list.add(new Person("8", 26));
    return list;
}

joining

先了解StringJoiner，之前拼接字符串一直不知道有这个方法

@Test
public void testJoiner (){
    List<String> list = Arrays.asList("1", "2", "3");
    StringJoiner joiner = new StringJoiner(",","[","]");
    StringJoiner joiner2 = new StringJoiner("-","(",")");
    list.forEach(joiner::add);
    System.out.println(joiner.toString());
    list.forEach(joiner2::add);
    System.out.println(joiner2.toString());
    StringJoiner merge = joiner.merge(joiner2);
    System.out.println(merge.toString());
}
[1,2,3]
(1-2-3)
[1,2,3,1-2-3]

Collectors.joining()

public static Collector<CharSequence, ?, String> joining(CharSequence delimiter,
                                                         CharSequence prefix,
                                                         CharSequence suffix) {
    return new CollectorImpl<>(
        () -> new StringJoiner(delimiter, prefix, suffix),
        StringJoiner::add, StringJoiner::merge,
        StringJoiner::toString, CH_NOID);
}

看这里的CollectorImpl实现应该也能想到实现。

reducing

public static <T> Collector<T, ?, Optional<T>> reducing(BinaryOperator<T> op) {
    class OptionalBox implements Consumer<T> {
        T value = null;
        boolean present = false;

        @Override
        public void accept(T t) {
            if (present) {
                value = op.apply(value, t);
            }
            else {
                value = t;
                present = true;
            }
        }
    }

    return new CollectorImpl<T, OptionalBox, Optional<T>>(
        OptionalBox::new, OptionalBox::accept,
        (a, b) -> { if (b.present) a.accept(b.value); return a; },
        a -> Optional.ofNullable(a.value), CH_NOID);
}
  1. Supplier : 方法内部类OptionalBox，负责调用传入的BinaryOperator以及收集最后的结果
  2. BiConsumer：OptionalBox的accept方法
  3. BinaryOperator：两个OptionalBox调用accept方法
  4. finisher：最后结果进行封装，这里封装为Optional

参考以下伪代码：

result = new OptionalBox()
foreach(cursor : list){
	result.value = BinaryOperator.apply(result.value,cursor)
}
//忽略并行流
return Optional.ofNullable(result.value)

示例

@Test
public void testReduce() {
    Collector<Person, ?, Optional<Person>> reducing = Collectors
        .reducing((Person a, Person b) -> new Person("-1", a.getAge() + b.getAge()));
    Optional<Person> sumAge = getData().stream().collect(reducing);
    sumAge.ifPresent(System.out::println);
}
Person{name='-1', age=194}

注意：BinaryOperator接口：继承BiFunction

这里要求传入参数和返回结果都是相同类型，所以上面要专门new一个Person对象。

所以在运行过程需要创建大量的对象，会影响性能

public interface BiFunction<T, U, R> {
 R apply(T t, U u);
}

maxBy

基于reducing

public static <T> Collector<T, ?, Optional<T>>  maxBy(Comparator<? super T> comparator) {
    return reducing(BinaryOperator.maxBy(comparator));
}
@Test
public void testMaxBy() {
    Collector<Person, ?, Optional<Person>> collector = Collectors
        .maxBy(Comparator.comparing(Person::getAge));
    Optional<Person> collect = getData().stream().collect(collector);
    collect.ifPresent(System.out::println);
}

averagingInt

public static <T> Collector<T, ?, Double> averagingInt(ToIntFunction<? super T> mapper) {
    return new CollectorImpl<>(
        () -> new long[2],
        (a, t) -> { a[0] += mapper.applyAsInt(t); a[1]++; },
        (a, b) -> { a[0] += b[0]; a[1] += b[1]; return a; },
        a -> (a[1] == 0) ? 0.0d : (double) a[0] / a[1], CH_NOID);
}
  1. Supplier ：new long[2]
  2. BiConsumer：long[0]累加求和，long[1]计数
  3. BinaryOperator：两个Long[2]相加
  4. finisher：返回总和/计数

partitioningBy

public static <T>
    Collector<T, ?, Map<Boolean, List<T>>> partitioningBy(Predicate<? super T> predicate) {
    return partitioningBy(predicate, toList());
}
    public static <T, D, A>
    Collector<T, ?, Map<Boolean, D>> partitioningBy(Predicate<? super T> predicate,
                                                    Collector<? super T, A, D> downstream) {
        BiConsumer<A, ? super T> downstreamAccumulator = downstream.accumulator();
        //根据Predicate结果,加入true还是false的list中
        BiConsumer<Partition<A>, T> accumulator = (result, t) ->
                downstreamAccumulator.accept(predicate.test(t) ? result.forTrue : result.forFalse, t);
        BinaryOperator<A> op = downstream.combiner();
        // Partition 中true 的list和false的list 分别与另一个Partition相加
        BinaryOperator<Partition<A>> merger = (left, right) ->
                new Partition<>(op.apply(left.forTrue, right.forTrue),
                                op.apply(left.forFalse, right.forFalse));
        //这里Supplier是一个内部新建的Map子类
        Supplier<Partition<A>> supplier = () ->
                new Partition<>(downstream.supplier().get(),
                                downstream.supplier().get());
        if (downstream.characteristics().contains(Collector.Characteristics.IDENTITY_FINISH)) {
            return new CollectorImpl<>(supplier, accumulator, merger, CH_ID);
        }
        else {
            Function<Partition<A>, Map<Boolean, D>> finisher = par ->
                    new Partition<>(downstream.finisher().apply(par.forTrue),
                                    downstream.finisher().apply(par.forFalse));
            return new CollectorImpl<>(supplier, accumulator, merger, finisher, CH_NOID);
        }
    }

  1. Supplier:内部私有的Map子类Partition,里面保存两个容器对象。默认情况下是两个List，分别存储true和false的结果

    private static final class Partition<T>
            extends AbstractMap<Boolean, T>
            implements Map<Boolean, T> {
    	//两个容器对象，默认是List
        final T forTrue;
        final T forFalse;

        Partition(T forTrue, T forFalse) {
            this.forTrue = forTrue;
            this.forFalse = forFalse;
        }

        @Override
        public Set<Map.Entry<Boolean, T>> entrySet() {
            return new AbstractSet<>() {
                @Override
                public Iterator<Map.Entry<Boolean, T>> iterator() {
                    Map.Entry<Boolean, T> falseEntry = new SimpleImmutableEntry<>(false, forFalse);
                    Map.Entry<Boolean, T> trueEntry = new SimpleImmutableEntry<>(true, forTrue);
                    return List.of(falseEntry, trueEntry).iterator();
                }

                @Override
                public int size() {
                    return 2;
                }
            };
        }
    }

  2. BiConsumer：根据Predicate结果,加入Partition中true还是false的容器对象中，默认是list

  3. BinaryOperator：两个Partition的true和false集合分别相加

示例

@Test
public void testPartition (){
    Collector<Person, ?, Map<Boolean, List<Person>>> collector = Collectors
        .partitioningBy((Person n) -> n.getAge() < 25);
    Map<Boolean, List<Person>> collect = getData().stream().collect(collector);
    collect.forEach((k,v)-> System.out.println(k+"="+v));
}
false=[Person{name='4', age=26}, Person{name='5', age=28}, Person{name='7', age=28}, Person{name='8', age=26}]
true=[Person{name='1', age=20}, Person{name='2', age=22}, Person{name='3', age=24}, Person{name='6', age=20}]

mapping

public static <T, U, A, R>Collector<T, ?, R> mapping(Function<? super T, ? extends U> mapper,
                               Collector<? super U, A, R> downstream) {
    BiConsumer<A, ? super U> downstreamAccumulator = downstream.accumulator();
    return new CollectorImpl<>(downstream.supplier(),
                               (r, t) -> downstreamAccumulator.accept(r, mapper.apply(t)),
                               downstream.combiner(), downstream.finisher(),
                               downstream.characteristics());
}

相对于Collectors.toList()，在添加元素之前，会调用Function<? super T, ? extends U> 映射到相应的结果，再添加到容器中

@Test
public void test (){
    Collector<Person, ?, List<Integer>> mapping = Collectors.mapping((Person a) -> a.getAge(), Collectors.toList());
    List<Integer> collect = getData().stream().collect(mapping);
    collect.forEach(System.out::println);
}
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原文地址：https://www.cnblogs.com/froggengo/p/14669850.html