先定义一个用户类

class User {
    Integer id;
    String name;

    public Integer getId() {
        return id;
    }
    public void setId(Integer id) {
        this.id = id;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
}

提取所有的用户ID

List<Integer> userIds = users.stream().map(u -> u.getId()).collect(Collectors.toList());

提取所有的用户ID并去重

List<Integer> userIds = users.stream().map(u -> u.getId()).distinct().collect(Collectors.toList());

处理为HashMap -> key为用户ID，value为该User

Map<Integer, User> userMap = users.stream().collect(Collectors.toMap(User :: getId, u -> u));

处理为HashMap -> key为用户ID，value为该User，如果User重复，则key会重复：

Exception in thread "main" java.lang.IllegalStateException: Duplicate key 123456

需要指定在key重复时选择保留旧值还是新值

Map<Integer, User> userMap = users.stream().collect(Collectors.toMap(User :: getId, u -> u, (oldV, newV) -> newV));

按照id的大小进行排序

List<User> users = new ArrayList<User>();
users.sort((u1, u2) -> u1.getId().compareTo(u2.getId()));