Guava way of sorting List according to another list?
Solution 1
The fully "functional" way, using Guava, would combine Ordering#explicit()
with Ordering#onResultOf()
public class UserService {
@Inject private UserDao userDao;
public List<User> getUsersWithIds(List<Integer> userIds) {
List<User> users = userDao.loadUsersWithIds(userIds);
Ordering<User> orderById = Ordering.explicit(userIds).onResultOf(UserFunctions.getId());
return orderById.immutableSortedCopy(users);
}
}
You could declare an anonymous function inline, but I like to declare my functions as static factory methods in a separate class, for a cleaner code (the verbosity of Java's function declarations is hidden in the utility class):
/**
* Static factory methods to create {@link Function}s for {@link User}s.
*/
public final class UserFunctions {
private UserFunctions() { /* prevents instantiation */ }
/**
* @return a {@link Function} that returns an {@link User}'s id.
*/
public static Function<User, Integer> getId() {
return GetIdFunction.INSTANCE;
}
// enum singleton pattern
private enum GetIdFunction implements Function<User, Integer> {
INSTANCE;
public Integer apply(User user) {
return user.getId();
}
}
}
As mentioned by @Arne in a comment, this can be simplified in Java 8, using a method reference instead of the UserFunctions
class:
public class UserService {
@Inject private UserDao userDao;
public List<User> getUsersWithIds(List<Integer> userIds) {
List<User> users = userDao.loadUsersWithIds(userIds);
Ordering<User> orderById = Ordering.explicit(userIds).onResultOf(User::getId);
return orderById.immutableSortedCopy(immutableSortedCopy(users));
}
}
Solution 2
I don't think Guava has anything specific to do this. But it's just a matter of writing this comparator:
Collections.sort(userList, new Comparator<User>() {
@Override
public int compare(User u1, User u2) {
int i1 = idList.indexOf(u1.getId());
int i2 = idList.indexOf(u2.getId());
return Ints.compare(i1, i2);
}
}
Now that I think about it, it can also be implemented this way:
final Ordering<Integer> idOrdering = Ordering.explicit(idList);
Collections.sort(userList, new Comparator<User>() {
@Override
public int compare(User u1, User u2) {
return idOrdering.compare(u1.getId(), u2.getId());
}
}
which is probably more efficient.
Solution 3
Others have already answered your question using Guava. Here is a Functional Java answer.
Please note that you'll have to use immutable data structures from the library in order to avail of all the goodness.
F<User, Integer> indexInIdList = new F<User, Integer>() {
public Integer f(User u) {
return idList.elementIndex(Equal.intEqual, u.getId()).toNull();
}
};
userList.sort(Ord.intOrd.comap(indexInIdList));
Cemo
Updated on July 27, 2022Comments
-
Cemo almost 2 years
I have
List<Integer>
consisting Ids of my Users. And after a database query, I am retrievingList<User>
. I would like to order this list according to first Id list.List<User>
may not include some of the Ids. What is the Guava way for sorting this list?