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()?

Answer 1

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 )