Java8 Stream 之 toMap

场景1：将一个包含person的list转成key是手机号，value是该person对象的map
场景2：将一个包含person的list转成key是性别，value是属于该性别的所有person姓名的map

// 假装使用了lombok
// Person实体
@Data
class Person {
    private String name;
    private String phone;
    private String gender;
}
public static void main() {
    // 存储person的list
    private List<Person> personList = new ArrayList<Person>();
    personList.add(new Person("zhangsan", "18912345667", "Male"));  
    personList.add(new Person("lisi", "16712345667", "Male"));
    personList.add(new Person("susan", "13112345667", "Female")); 
    personList.add(new Person("marry", "15612345667","Female")); 

    Map<String, Person> map1 = new HashMap<String, Person>();
    Map<String, String> map2 = new HashMap<String, Person>();

    // 场景1：java8之前使用循环
    for (Person person : personList) {
        // 因为手机号的唯一性，所以没有考虑key冲突的情况。
        map1.put(person.getPhone(), person);
    }
    // 场景1：使用java8的流
   map1 = personList.stream().collect(Collectors.toMap(Person::getPhone, Function.identity()));

    // 场景2：java8之前使用循环
    for (Person person : personList) {
        String key = person.getGender();
        String value = person.getName();
        if (map2.containsKey(key)) {
            value = map2.get(key) + "," + value;
        }
        map2.put(key, value);
    }
    // 场景2：使用java8的流，但是有多个value对应1个key的情况
    // toMap的第三个参数是merge规则
     map2 = personList.stream().collect(Collectors.toMap(Person::getPhone, Function.identity()), (oldV, newV) -> oldV + "," + newV);
}