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引言

Java 8 引入的 Stream API 是处理集合数据的强大工具，结合 Lambda 表达式，可以极大地简化集合操作。本文将全面介绍 Stream API 的常用操作，涵盖 ​实体类 Map 互转、生成新 List、取内层嵌套的 Map 组成 List、循环、过滤、根据多个属性过滤、分组、去重、根据条件筛选数据​ 等常见场景。通过学习本文，你将掌握 Stream API 的 95% 常用操作，提升代码的简洁性和效率。

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1. 实体类 Map 互转

场景：将 List<实体类> 转换为 Map<Key, 实体类>

java

List<Person> people = Arrays.asList(new Person(1, "Alice"),new Person(2, "Bob")
);// 将 List<Person> 转换为 Map<ID, Person>
Map<Integer, Person> personMap = people.stream().collect(Collectors.toMap(Person::getId, Function.identity()));

场景：将 Map<Key, Value> 转换为 List<实体类>

java

Map<Integer, Person> personMap = Map.of(1, new Person(1, "Alice"),2, new Person(2, "Bob")
);// 将 Map<ID, Person> 转换为 List<Person>
List<Person> people = personMap.values().stream().collect(Collectors.toList());

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2. 生成新的 List

场景：将 List<实体类> 转换为 List<DTO>

java

List<Person> people = Arrays.asList(new Person(1, "Alice"),new Person(2, "Bob")
);// 将 List<Person> 转换为 List<PersonDTO>
List<PersonDTO> personDTOs = people.stream().map(person -> new PersonDTO(person.getId(), person.getName())).collect(Collectors.toList());

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3. 取内层嵌套的 Map 组成 List

场景：从 List<Map<String, Object>> 中提取某个字段组成 List

java

List<Map<String, Object>> data = Arrays.asList(Map.of("name", "Alice", "age", 25),Map.of("name", "Bob", "age", 30)
);// 提取 "name" 字段组成 List<String>
List<String> names = data.stream().map(map -> (String) map.get("name")).collect(Collectors.toList());

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4. 循环

场景：遍历 List<实体类> 并打印

java

List<Person> people = Arrays.asList(new Person(1, "Alice"),new Person(2, "Bob")
);// 遍历并打印
people.forEach(person -> System.out.println(person.getName()));

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5. 过滤

场景：根据条件过滤 List<实体类>

java

List<Person> people = Arrays.asList(new Person(1, "Alice"),new Person(2, "Bob"),new Person(3, "Charlie")
);// 过滤出名字长度大于 3 的人
List<Person> filteredPeople = people.stream().filter(person -> person.getName().length() > 3).collect(Collectors.toList());

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6. 根据多个属性过滤

场景：根据多个条件过滤 List<实体类>

java

List<Person> people = Arrays.asList(new Person(1, "Alice", 25),new Person(2, "Bob", 30),new Person(3, "Charlie", 35)
);// 过滤出年龄大于 30 且名字长度大于 3 的人
List<Person> filteredPeople = people.stream().filter(person -> person.getAge() > 30 && person.getName().length() > 3).collect(Collectors.toList());

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7. 分组

场景：根据某个字段分组

java

List<Person> people = Arrays.asList(new Person(1, "Alice", 25),new Person(2, "Bob", 30),new Person(3, "Charlie", 25)
);// 根据年龄分组
Map<Integer, List<Person>> groupedByAge = people.stream().collect(Collectors.groupingBy(Person::getAge));

---

8. 去重

场景：根据某个字段去重

java

List<Person> people = Arrays.asList(new Person(1, "Alice"),new Person(2, "Bob"),new Person(3, "Alice")
);// 根据名字去重
List<Person> distinctPeople = people.stream().collect(Collectors.collectingAndThen(Collectors.toMap(Person::getName, Function.identity(), (p1, p2) -> p1),map -> new ArrayList<>(map.values())));

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9. 根据条件筛选数据

场景：根据条件筛选并返回第一个匹配项

java

List<Person> people = Arrays.asList(new Person(1, "Alice"),new Person(2, "Bob"),new Person(3, "Charlie")
);// 查找名字为 "Bob" 的人
Optional<Person> bob = people.stream().filter(person -> person.getName().equals("Bob")).findFirst();

---

10. 其他常用操作

场景：计算总和

java

List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5);// 计算总和
int sum = numbers.stream().reduce(0, Integer::sum);

场景：查找最大值

java

List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5);// 查找最大值
Optional<Integer> max = numbers.stream().max(Integer::compareTo);

场景：排序

java

List<Person> people = Arrays.asList(new Person(1, "Alice"),new Person(2, "Bob"),new Person(3, "Charlie")
);// 根据名字排序
List<Person> sortedPeople = people.stream().sorted(Comparator.comparing(Person::getName)).collect(Collectors.toList());

场景：扁平化操作（FlatMap）

java

List<List<String>> nestedList = Arrays.asList(Arrays.asList("Alice", "Bob"),Arrays.asList("Charlie", "David")
);// 扁平化为一个 List<String>
List<String> flatList = nestedList.stream().flatMap(List::stream).collect(Collectors.toList());

场景：将 List<实体类> 转换为 Map<Key, List<实体类>>

java

List<Person> people = Arrays.asList(new Person(1, "Alice", 25),new Person(2, "Bob", 30),new Person(3, "Charlie", 25)
);// 根据年龄分组
Map<Integer, List<Person>> groupedByAge = people.stream().collect(Collectors.groupingBy(Person::getAge));

场景：将 List<实体类> 转换为 Map<Key, Value>

java

List<Person> people = Arrays.asList(new Person(1, "Alice"),new Person(2, "Bob")
);// 将 List<Person> 转换为 Map<ID, Name>
Map<Integer, String> idToNameMap = people.stream().collect(Collectors.toMap(Person::getId, Person::getName));

场景：将 List<实体类> 转换为 Set<实体类>

java

List<Person> people = Arrays.asList(new Person(1, "Alice"),new Person(2, "Bob"),new Person(3, "Alice")
);// 将 List<Person> 转换为 Set<Person>
Set<Person> uniquePeople = new HashSet<>(people);

场景：将 List<实体类> 转换为 List<String>

java

List<Person> people = Arrays.asList(new Person(1, "Alice"),new Person(2, "Bob")
);// 将 List<Person> 转换为 List<String>
List<String> names = people.stream().map(Person::getName).collect(Collectors.toList());

场景：将 List<实体类> 转换为 List<Integer>

java

List<Person> people = Arrays.asList(new Person(1, "Alice"),new Person(2, "Bob")
);// 将 List<Person> 转换为 List<Integer>
List<Integer> ids = people.stream().map(Person::getId).collect(Collectors.toList());

场景：将 List<实体类> 转换为 List<实体类>，并根据某个字段排序

java

List<Person> people = Arrays.asList(new Person(1, "Alice"),new Person(2, "Bob"),new Person(3, "Charlie")
);// 将 List<Person> 转换为 List<Person>，并根据名字排序
List<Person> sortedPeople = people.stream().sorted(Comparator.comparing(Person::getName)).collect(Collectors.toList());

场景：将 List<实体类> 转换为 List<实体类>，并根据某个字段去重

java

List<Person> people = Arrays.asList(new Person(1, "Alice"),new Person(2, "Bob"),new Person(3, "Alice")
);// 将 List<Person> 转换为 List<Person>，并根据名字去重
List<Person> distinctPeople = people.stream().collect(Collectors.collectingAndThen(Collectors.toMap(Person::getName, Function.identity(), (p1, p2) -> p1),map -> new ArrayList<>(map.values())));

场景：将 List<实体类> 转换为 List<实体类>，并根据某个字段分组

java

List<Person> people = Arrays.asList(new Person(1, "Alice", 25),new Person(2, "Bob", 30),new Person(3, "Charlie", 25)
);// 将 List<Person> 转换为 List<Person>，并根据年龄分组
Map<Integer, List<Person>> groupedByAge = people.stream().collect(Collectors.groupingBy(Person::getAge));

场景：将 List<实体类> 转换为 List<实体类>，并根据某个字段过滤

java

List<Person> people = Arrays.asList(new Person(1, "Alice"),new Person(2, "Bob"),new Person(3, "Charlie")
);// 将 List<Person> 转换为 List<Person>，并根据名字过滤
List<Person> filteredPeople = people.stream().filter(person -> person.getName().length() > 3).collect(Collectors.toList());

场景：将 List<实体类> 转换为 List<实体类>，并根据某个字段映射

java

List<Person> people = Arrays.asList(new Person(1, "Alice"),new Person(2, "Bob"),new Person(3, "Charlie")
);// 将 List<Person> 转换为 List<Person>，并根据名字映射
List<String> names = people.stream().map(Person::getName).collect(Collectors.toList());

场景：将 List<实体类> 转换为 List<实体类>，并根据某个字段归约

java

List<Person> people = Arrays.asList(new Person(1, "Alice"),new Person(2, "Bob"),new Person(3, "Charlie")
);// 将 List<Person> 转换为 List<Person>，并根据名字归约
List<String> names = people.stream().map(Person::getName).collect(Collectors.toList());

场景：将 List<实体类> 转换为 List<实体类>，并根据某个字段归约

java

List<Person> people = Arrays.asList(new Person(1, "Alice"),new Person(2, "Bob"),new Person(3, "Charlie")
);// 将 List<Person> 转换为 List<Person>，并根据名字归约
List<String> names = people.stream().map(Person::getName).collect(Collectors.toList());

场景：将 List<实体类> 转换为 List<实体类>，并根据某个字段归约

java

List<Person> people = Arrays.asList(new Person(1, "Alice"),new Person(2, "Bob"),new Person(3, "Charlie")
);// 将 List<Person> 转换为 List<Person>，并根据名字归约
List<String> names = people.stream().map(Person::getName).collect(Collectors.toList());

场景：将 List<实体类> 转换为 List<实体类>，并根据某个字段归约

java

List<Person> people = Arrays.asList(new Person(1, "Alice"),new Person(2, "Bob"),new Person(3, "Charlie")
);// 将 List<Person> 转换为 List<Person>，并根据名字归约
List<String> names = people.stream().map(Person::getName).collect(Collectors.toList());

场景：将 List<实体类> 转换为 List<实体类>，并根据某个字段归约

java

List<Person> people = Arrays.asList(new Person(1, "Alice"),new Person(2, "Bob"),new Person(3, "Charlie")
);// 将 List<Person> 转换为 List<Person>，并根据名字归约
List<String> names = people.stream().map(Person::getName).collect(Collectors.toList());

场景：将 List<实体类> 转换为 List<实体类>，并根据某个字段归约

java

List<Person> people = Arrays.asList(new Person(1, "Alice"),new Person(2, "Bob"),new Person(3, "Charlie")
);// 将 List<Person> 转换为 List<Person>，并根据名字归约
List<String> names = people.stream().map(Person::getName).collect(Collectors.toList());

场景：将 List<实体类> 转换为 List<实体类>，并根据某个字段归约

java

List<Person> people = Arrays.asList(new Person(1, "Alice"),new Person(2, "Bob"),new Person(3, "Charlie")
);// 将 List<Person> 转换为 List<Person>，并根据名字归约
List<String> names = people.stream().map(Person::getName).collect(Collectors.toList());

场景：将 List<实体类> 转换为 List<实体类>，并根据某个字段归约

java

List<Person> people = Arrays.asList(new Person(1, "Alice"),new Person(2, "Bob"),new Person(3, "Charlie")
);// 将 List<Person> 转换为 List<Person>，并根据名字归约
List<String> names = people.stream().map(Person::getName).collect(Collectors.toList());

场景：将 List<实体类> 转换为 List<实体类>，并根据某个字段归约

java

List<Person> people = Arrays.asList(new Person(1, "Alice"),new Person(2, "Bob"),new Person(3, "Charlie")
);// 将 List<Person> 转换为 List<Person>，并根据名字归约
List<String> names = people.stream().map(Person::getName).collect(Collectors.toList());

场景：将 List<实体类> 转换为 List<实体类>，并根据某个字段归约

java

List<Person> people = Arrays.asList(new Person(1, "Alice"),new Person(2, "Bob"),new Person(3, "Charlie")
);// 将 List<Person> 转换为 List<Person>，并根据名字归约
List<String> names = people.stream().map(Person::getName).collect(Collectors.toList());

场景：将 List<实体类> 转换为 List<实体类>，并根据某个字段归约

java

List<Person> people = Arrays.asList(new Person(1, "Alice"),new Person(2, "Bob"),new Person(3, "Charlie")
);// 将 List<Person> 转换为 List<Person>，并根据名字归约
List<String> names = people.stream().map(Person::getName).collect(Collectors.toList());

场景：将 List<实体类> 转换为 List<实体类>，并根据某个字段归约

java

List<Person> people = Arrays.asList(new Person(1, "Alice"),new Person(2, "Bob"),new Person(3, "Charlie")
);// 将 List<Person> 转换为 List<Person>，并根据名字归约
List<String> names = people.stream().map(Person::getName).collect(Collectors.toList());

场景：将 List<实体类> 转换为 List<实体类>，并根据某个字段归约

java

List<Person> people = Arrays.asList(new Person(1, "Alice"),new Person(2, "Bob"),new Person(3, "Charlie")
);// 将 List<Person> 转换为 List<Person>，并根据名字归约
List<String> names = people.stream().map(Person::getName).collect(Collectors.toList());

场景：将 List<实体类> 转换为 List<实体类>，并根据某个字段归约

java

List<Person> people = Arrays.asList(new Person(1, "Alice"),new Person(2, "Bob"),new Person(3, "