Sorting by property in Java 8 stream

Question

Oh, those tricky Java 8 streams with lambdas. They are very powerful, yet the intricacies take a bit to wrap one's header around it all.

Let's say I have a User type with a property User.getName(). Let's say I have a map of those users Map<String, User> associated with names (login usernames, for example). Let's further say I have an instance of a comparator UserNameComparator.INSTANCE to sort usernames (perhaps with fancy collators and such).

So how do I get a list of the users in the map, sorted by username? I can ignore the map keys and do this:

return userMap.values()
    .stream()
    .sorted((u1, u2) -> {
      return UserNameComparator.INSTANCE.compare(u1.getName(), u2.getName());
    })
    .collect(Collectors.toList());

But that line where I have to extract the name to use the UserNameComparator.INSTANCE seems like too much manual work. Is there any way I can simply supply User::getName as some mapping function, just for the sorting, and still get the User instances back in the collected list?

Bonus: What if the thing I wanted to sort on were two levels deep, such as User.getProfile().getUsername()?

Solution

What you want is Comparator#comparing:

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

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

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