基于 JDK 1.8 的 Java Stream 常用操作大全，涵盖开发中最常用的所有场景（过滤、排序、分组、求和、聚合、去重、映射、合并、多条件排序等）。
所有代码都是 JDK1.8 兼容的。

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⭐ JDK 1.8 Java Stream 最全操作整理

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1. 创建 Stream

Stream<String> stream1 = list.stream();
Stream<String> stream2 = Stream.of("a", "b", "c");

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2. 遍历 forEach

list.stream().forEach(System.out::println);

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3. 过滤 filter

List<String> filtered = list.stream()
        .filter(s -> s.length() > 3)
        .collect(Collectors.toList());

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4. 排序 sorted

4.1 默认排序

List<String> sorted = list.stream()
        .sorted()
        .collect(Collectors.toList());

4.2 按对象字段排序

list.stream()
    .sorted(Comparator.comparing(User::getAge))
    .collect(Collectors.toList());

4.3 多字段排序（先 age，后 name）

list.stream()
    .sorted(
        Comparator.comparing(User::getAge)
                  .thenComparing(User::getName)
    )
    .collect(Collectors.toList());

4.4 倒序排序

list.stream()
    .sorted(Comparator.comparing(User::getAge).reversed())
    .collect(Collectors.toList());

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5. 转换 map

5.1 字段提取

List<String> names = list.stream()
        .map(User::getName)
        .collect(Collectors.toList());

5.2 对元素做运算

List<Integer> lengths = list.stream()
        .map(String::length)
        .collect(Collectors.toList());

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6. 去重 distinct

list.stream().distinct().collect(Collectors.toList());

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7. 去重（按对象某个字段）

list.stream()
    .filter(StreamDistinctByKey.distinctByKey(User::getId))
    .collect(Collectors.toList());

工具方法：

public static <T> Predicate<T> distinctByKey(Function<? super T, ?> keyExtractor) {
    Set<Object> seen = ConcurrentHashMap.newKeySet();
    return t -> seen.add(keyExtractor.apply(t));
}

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8. 转 Map（key-value）

8.1 简单对象转 Map

Map<Long, String> map = list.stream()
        .collect(Collectors.toMap(User::getId, User::getName));

8.2 key 冲突处理（后者覆盖前者）

.collect(Collectors.toMap(User::getId, User::getName, (a, b) -> b));

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9. 分组 groupingBy

9.1 按字段分组

Map<String, List<User>> map = list.stream()
        .collect(Collectors.groupingBy(User::getCity));

9.2 分组并统计数量

Map<String, Long> countMap = list.stream()
        .collect(Collectors.groupingBy(User::getCity, Collectors.counting()));

9.3 分组求和

Map<String, Integer> sumMap = list.stream()
        .collect(Collectors.groupingBy(User::getCity,
                Collectors.summingInt(User::getAge)));

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10. 聚合操作（sum / max / min）

10.1 求和

int sum = list.stream()
        .mapToInt(User::getAge)
        .sum();

10.2 最大值

User max = list.stream()
        .max(Comparator.comparing(User::getAge))
        .orElse(null);

10.3 最小值

User min = list.stream()
        .min(Comparator.comparing(User::getAge))
        .orElse(null);

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11. 匹配操作 anyMatch / allMatch / noneMatch

boolean any = list.stream().anyMatch(u -> u.getAge() > 18);
boolean all = list.stream().allMatch(u -> u.getAge() > 18);
boolean none = list.stream().noneMatch(u -> u.getAge() < 0);

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12. 截取 limit / skip

12.1 取前 10 条

list.stream().limit(10).collect(Collectors.toList());

12.2 跳过前 10 条

list.stream().skip(10).collect(Collectors.toList());

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13. flatMap（多层结构展开）

例如：List<List>

List<String> flat = list.stream()
        .flatMap(Collection::stream)
        .collect(Collectors.toList());

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14. 合并两个 Stream

Stream.concat(stream1, stream2)
        .collect(Collectors.toList());

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15. Optional 结合 Stream

Optional<User> first = list.stream()
        .filter(u -> u.getAge() > 20)
        .findFirst();

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16. 并行 Stream (parallel)

注意：多线程环境要小心！！！

list.parallelStream()
        .map(...)
        .collect(Collectors.toList());

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✔ 如你问过的例子：

对 ['000','001','010'] 排序

List<String> list = Arrays.asList("000","001","010");

List<String> sorted = list.stream()
        .sorted()
        .collect(Collectors.toList());

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