将List分组转成Map是日常开发中常见的需求，我们就来总结一下常见的几种写法。

有如下代码：

@Dataclass Person {    private String uuid;    private String name;    private String gender;    private int age;    public Person(String name, String gender, int age) { this.uuid = UUID.randomUUID().toString(); this.name = name; this.gender = gender; this.age = age;    }}

和一个List集合：

List<Person> persons = new ArrayList<>();persons.add(new Person("张三", "男", 27));persons.add(new Person("李四", "男", 14));persons.add(new Person("王五", "女", 17));persons.add(new Person("赵六", "女", 34));

1. partitioningBy 分两组

partitioningBy要求传入一个Predicate，会按照满足条件和不满足条件分成两组，得到的结果是Map<Boolean, List>结构，比如我们按是否未成年分成两组：

Map<Boolean, List<Person>> personsByAge = persons.stream().collect(Collectors.partitioningBy(p -> p.getAge() > 18));System.out.println(JSON.toJSONString(personsByAge));

Output：

{false: [{"age": 14,"gender": "男","name": "李四","uuid": "9fc3be98-f676-42a4-9f02-ebdab328103a"}, {"age": 17,"gender": "女","name": "王五","uuid": "3621044d-25a1-4946-a765-57b074f63f26"}],true: [{"age": 27,"gender": "男","name": "张三","uuid": "3f87ec59-29a1-4137-b95b-ae755f0e06ca"}, {"age": 34,"gender": "女","name": "赵六","uuid": "04ed8e9f-545b-49f5-a28b-ce0cccd15663"}]}

2. groupingBy 分多组

比如按照性别进行分组，得到的是Map<String, List>结构：

Map<String, List<Person>> personByGender = persons.stream().collect(Collectors.groupingBy(Person::getGender));System.out.println(JSON.toJSONString(personByGender));

Output：

{"女": [{"age": 17,"gender": "女","name": "王五","uuid": "feb8ca82-789f-445e-9e85-c14aa1d70546"}, {"age": 34,"gender": "女","name": "赵六","uuid": "6402b5ec-03cd-45d1-aa6d-7134509ca670"}],"男": [{"age": 27,"gender": "男","name": "张三","uuid": "e2c5ec58-5767-4807-8470-56a016dbc5eb"}, {"age": 14,"gender": "男","name": "李四","uuid": "d10aad57-038b-4ff8-8b36-86045d657c5a"}]}

3. toMap 自定义

前面介绍的partitioningBy和groupingBy返回Map的value部分都是List结构的，有时我们需要value是对象的一个属性，比如我们想构造一个uuid到name的映射，以方便通过uuid快速获取人员的名字：

Map<String, String> uuidNameMap = persons.stream().collect(Collectors.toMap(Person::getUuid, Person::getName));System.out.println(JSON.toJSONString(uuidNameMap));

Output：

{"7a021022-fa62-4f57-bf33-873b8e030cc3": "王五","e0bad9e6-2c3c-417e-9d27-3b321312421a": "张三","895b0f95-b4fd-481e-ba6c-33f0b636e6cf": "李四","fcb6f403-8489-4853-98c5-6f41341165ba": "赵六"}

实际情况有可能同一个key会对应多个value，就有可能抛Duplicate key异常。这时可以传入第三个参数决定重复时如何选择，比如我们想构造的映射，但是考虑可能有重名的可能，就可以这么做：

Map<String, String> nameUuidMap = persons.stream().collect(Collectors.toMap(Person::getName, Person::getUuid, (p1, p2) -> p1));System.out.println(JSON.toJSONString(nameUuidMap));

这里(p1, p2) -> p1表示如果重复则取前者。