• Java 8 Collectors.collectingAndThen()


    java.util.stream.Collectors#collectingAndThen方法的作用是将Collector的结果在执行一个额外的finisher转换操作,其源码如下:

    /**
    * Adapts a {@code Collector} to perform an additional finishing
    * transformation. For example, one could adapt the {@link #toList()}
    * collector to always produce an immutable list with:
    * <pre>{@code
    * List<String> people
    * = people.stream().collect(collectingAndThen(toList(), Collections::unmodifiableList));
    * }</pre>
    *
    * @param <T> the type of the input elements
    * @param <A> intermediate accumulation type of the downstream collector
    * @param <R> result type of the downstream collector
    * @param <RR> result type of the resulting collector
    * @param downstream a collector
    * @param finisher a function to be applied to the final result of the downstream collector
    * @return a collector which performs the action of the downstream collector,
    * followed by an additional finishing step
    */
    public static<T, A, R, RR> Collector<T, A, RR> collectingAndThen(Collector<T, A, R> downstream, Function<R, RR> finisher) {
    Set<Collector.Characteristics> characteristics = downstream.characteristics();
    if (characteristics.contains(Collector.Characteristics.IDENTITY_FINISH)) {
    if (characteristics.size() == 1) {
    characteristics = Collectors.CH_NOID;
    } else {
    characteristics = EnumSet.copyOf(characteristics);
    characteristics.remove(Collector.Characteristics.IDENTITY_FINISH);
    characteristics = Collections.unmodifiableSet(characteristics);
    }
    }
    return new CollectorImpl<>(downstream.supplier(),
    downstream.accumulator(),
    downstream.combiner(),
    downstream.finisher().andThen(finisher),
    characteristics);
    }
    • T:输入元素的类型
    • A:下游Collector的中间堆积类型
    • R:下游Collector的结果类型
    • RR:结果Collector的结果类型

    参数:此方法接受下面列出的两个参数

    • downstream: Collector的一个实例,可以使用任何Collector
    • finisher: 类型是Function,该函数将应用于下游Collector的最终结果

    返回值:返回一个执行下游Collector动作的Collector,然后在finisher函数的帮助下执行附加的转换步骤。

    范例:

    创建不可变集合

    // Create an Immutable List
    List<String> lt = Stream.of("GEEKS", "For", "GEEKS").collect(Collectors.collectingAndThen(Collectors.toList(), Collections::<String>unmodifiableList));
    
    // Create an Immutable Set
    Set<String> st = Stream.of("GEEKS", "FOR", "GEEKS").collect(Collectors.collectingAndThen(Collectors.toSet(), Collections::<String>unmodifiableSet));
    
    // Create an Immutable Map
    Map<String, String> mp = Stream.of(new String[][] {{ "1", "Geeks" }, { "2", "For" }, { "3", "Geeks" }})
              .collect(Collectors.collectingAndThen(Collectors.toMap(p -> p[0], p -> p[1]), Collections::<String, String>unmodifiableMap));

     对象去重:

    List<Xxx> distinctList = rowList.stream()
            .collect(Collectors.collectingAndThen(
                    Collectors.toCollection(() -> new TreeSet<>(Comparator.comparing(obj -> String.join("-", obj.getA(), obj.getB(), obj.getC())))),
                    ArrayList::new
            ));
    Map<Integer, BigDecimal> mongthValuePair = map.computeIfAbsent(year, ArrayList::new)
            .stream().collect(Collectors.toMap(A::getMonth, A::getValue));

    ^_^

    参考文章:

    Java Collectors collectingAndThen()用法及代码示例

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  • 原文地址:https://www.cnblogs.com/gotodsp/p/15120273.html
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