Use Java 8 Stream API to filter objects based on an ID and Date

Question

I have a Contact class, for which each instance has a unique contactId.

public class Contact {
    private Long contactId;

    ... other variables, getters, setters, etc ...
}

And a Log class that details an action performed by a Contact on a certain lastUpdated date.

public class Log {
    private Contact contact;
    private Date lastUpdated;
    private String action;

    ... other variables, getters, setters, etc ...
}

Now, in my code I have a List<Log> that can contain multiple Log instances for a single Contact. I would like to filter the list to include only one Log instance for each Contact, based on the lastUpdated variable in the Log object. The resulting list should contain the newest Log instance for each Contact.

I could do this by creating a Map<Contact, List<Log>>, then looping through and getting the Log instance with max lastUpdated variable for each Contact, but this seems like it could be done much simpler with the Java 8 Stream API.

How would one accomplish this using the Java 8 Stream API?

It probably can, but how about this: how would you do this *without* the Stream API? How would you want to go about doing something like this? — Makoto, Jul 27 '15 at 16:17
How is this Question different than [the one you posted an hour earlier](http://stackoverflow.com/q/31657036/642706)? Multiple good answers there showed you how to use steams to filter to get latest date. — Basil Bourque, Jul 27 '15 at 16:40
@BasilBourque, the question earlier was how to get the object with a max date from a list. This question is more complex. — Andrew Mairose, Jul 27 '15 at 16:43

score 7 · Accepted Answer · answered Jul 27 '15 at 16:22

7

You can chain several collectors to get what you want:

import static java.util.stream.Collectors.*;

List<Log> list = ...
Map<Contact, Log> logs = list.stream()
    .collect(groupingBy(Log::getContact,
        collectingAndThen(maxBy(Comparator.comparing(Log::getLastUpdated)), Optional::get)));

answered Jul 27 '15 at 16:22

Tagir Valeev

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This is close to what I was looking for, but I need the output as a `List`, not a `Map`. – Andrew Mairose Jul 27 '15 at 16:44
2

@AndrewMairose, use `new ArrayList<>(logs.values())` after. Or probably simply `logs.values()` if `Collection` is fine for you. – Tagir Valeev Jul 27 '15 at 17:00
Yup @TagirValeev that's what I ended up doing. I just tacked on a .values() at the end to do it all in one statement. – Andrew Mairose Jul 27 '15 at 18:58

Use Java 8 Stream API to filter objects based on an ID and Date

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