List<属性值类型> 属性List = 对象List.stream().map(对象::get方法()).collect(Collectors.toList());

List<Integer> idList = list.stream().map(User::getId).collect(Collectors.toList());

//或者

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

public class HelloWorld {

    public static void main(String[] args) {

        List<User> list = new ArrayList<>();

        for(int i=1;i<=10;i++) {

            User u = new User(i, "用户-" + i);

            list.add(u);

        }

        //取出id列表

        //List<Integer> idList = list.stream().map(User::getId).collect(Collectors.toList());

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

        System.out.println("id列表：" + idList);

    }

    private static class User{

        int id;

        String name;

        public User(int id, String name) {

            this.id = id;

            this.name = name;

        }

        public int getId() {

            return id;

        }

        public void setId(int id) {

            this.id = id;

        }

        public String getName() {

            return name;

        }

        public void setName(String name) {

            this.name = name;

        }

        @Override

        public String toString() {

            return "User{" +

                    "id=" + id +

                    ", name='" + name + '\'' +

                    '}';

        }

    }

}