HoRain云--Java Stream 自定义 Collector：从 collectingAndThen 到并行收集

本文讲解 Java Stream Collector 原理包括内置收集器、自定义 Collector、combiner、并行收集和实战案例。正文1. Collector 接口java复制下载public interface CollectorT, A, R { SupplierA supplier(); BiConsumerA, T accumulator(); BinaryOperatorA combiner(); FunctionA, R finisher(); SetCharacteristics characteristics(); }T元素类型A累加器类型R结果类型2. 内置收集器java复制下载list.stream().collect(Collectors.toList()); list.stream().collect(Collectors.toSet()); list.stream().collect(Collectors.toMap(User::getId, User::getName)); list.stream().collect(Collectors.groupingBy(User::getCity)); list.stream().collect(Collectors.joining(, )); list.stream().collect(Collectors.counting());3. 自定义 Collectorjava复制下载public class ImmutableListCollectorT implements CollectorT, ListT, ListT { public SupplierListT supplier() { return ArrayList::new; } public BiConsumerListT, T accumulator() { return List::add; } public BinaryOperatorListT combiner() { return (a, b) - { a.addAll(b); return a; }; } public FunctionListT, ListT finisher() { return Collections::unmodifiableList; } public SetCharacteristics characteristics() { return Collections.emptySet(); } }使用java复制下载ListString result stream.collect(new ImmutableListCollector());4. Collector.of 简化java复制下载CollectorUser, ?, MapString, Integer collector Collector.of( HashMap::new, (map, user) - map.merge(user.getCity(), 1, Integer::sum), (m1, m2) - { m2.forEach((k, v) - m1.merge(k, v, Integer::sum)); return m1; } );5. CharacteristicsCONCURRENT并行时可多线程累加同一容器。UNORDERED不保证顺序。IDENTITY_FINISHfinisher 为恒等函数。6. collectingAndThenjava复制下载ListString unmodifiable list.stream() .collect(Collectors.collectingAndThen(Collectors.toList(), Collections::unmodifiableList));7. teeingJava 12同时收集两个结果java复制下载record Stats(long count, double sum) {} Stats stats list.stream().collect(Collectors.teeing( Collectors.counting(), Collectors.summingDouble(Item::getPrice), Stats::new ));8. 实战分组后排序java复制下载MapString, ListUser result users.stream() .collect(Collectors.groupingBy( User::getCity, Collectors.collectingAndThen( Collectors.toList(), list - list.stream() .sorted(Comparator.comparing(User::getAge)) .toList() ) ));9. 并行收集java复制下载MapString, Long counts list.parallelStream() .collect(Collectors.groupingByConcurrent(User::getCity, Collectors.counting()));并行时使用groupingByConcurrent提升性能。10. 常见坑自定义 Collector 未正确实现 combiner并行结果错误。使用有状态 lambda。toMap 键冲突未处理。并行流与线程安全容器混用。11. 总结自定义 Collector 让 Stream 收集更灵活。理解 supplier、accumulator、combiner、finisher 后可以实现复杂聚合逻辑。