你想要的是 比较器＃比较 ：

What you want is Comparator#comparing:

userMap.values().stream()
    .sorted(Comparator.comparing(User::getName, UserNameComparator.INSTANCE))
    .collect(Collectors.toList());

对于问题的第二部分，您只需使用

For the second part of your question, you would just use

Comparator.comparing(
    u->u.getProfile().getUsername(), 
    UserNameComparator.INSTANCE
)