java - 末尾带有 null 的集合比较器

Question

我有一些User对象，可以有name和email，我希望它们按名称排序，然后按电子邮件排序，但首先显示名称不为空的用户。我正在尝试这样:

 Collections.sort(usersList, new Comparator<User>() {
            @Override
            public int compare(User obj1, User obj2) {
                int res = 0;

               //to make sure null names at the end

                if (obj1.getFullName() == null && obj2.getFullName()  == null)
                    res = 0;

                if (obj1.getFullName()  == null)
                    res = 1;

                if (obj2.getFullName() == null)
                    res = -1;

                //logic for name sorting then email sorting

                if (obj1.getFullName() != null  && obj2.getFullName() != null)
                    res = obj1.getFullName().compareTo(obj2.getFullName());

                if (res == 0)
                    res = obj1.getEmail().compareTo(obj2.getEmail());

                return res;
            }
        });

因此，我首先得到姓名，但按电子邮件而不是姓名排序。

Answer 1

这可以通过 Java-8 的 Comparator 轻松完成实用程序:

List<User> users = Arrays.asList(new User("pete", "pete@email"), new User(null, null), new User("alex", null), new User(null, "email"));

Collections.sort(users, Comparator.comparing(User::getName, Comparator.nullsLast(Comparator.naturalOrder()))
                       .thenComparing(Comparator.comparing(User::getEmail, Comparator.nullsLast(Comparator.naturalOrder()))));

System.out.println(users);

输出:

[
    User{name='alex', email='null'}, 
    User{name='pete', email='pete@email'}, 
    User{name='null', email='email'}, 
    User{name='null', email='null'}
]