Java 8 Collectors.collectingAndThen()
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，該函數將應用於下游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));