JDK源碼

java.util.stream.Collectors#collectingAndThen方法的作用是將Collector的結果在執行一個額外的finisher轉換操作，其源碼如下：

/**
 * 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：輸入元素的類型
• A：下游Collector的中間堆積類型
• R：下游Collector的結果類型
• RR：結果Collector的結果類型

參數：此方法接受下面列出的兩個參數

• downstream: Collector的一個實例，可以使用任何Collector
• finisher: 類型是Function，該函數將應用于下游Collector的最終結果

返回值：返回一個執行下游Collector動作的Collector，然后在finisher函數的幫助下執行附加的轉換步驟。

范例：

創建不可變集合

// 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));

對象去重：

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));

參考文章：

• Java Collectors collectingAndThen()用法及代碼示例