Reduction of List of Objects based on priority of String

Question

Lets assume I have a list List<Foo> mylist = parseFromCSV();

Where

parseFromCSV()

returns a list of object of Foo class. Foo class has her private attributes:

private mailId;
private status;

... getters and setters...

A Possibly pair of objects in

mylist: [{MailId=100, Status=BOUNCED, [email protected]},{MailId=100, Status=OPENED, [email protected]}, {MailId=200, Status=CLICKED, EMAIL = [email protected]}, {MailId=100, Status=HARDBOUNCED, EMAIL = [email protected]}]

I can indeed have duplicates in my list because the objects listed represents the CSV lines parsed into Java Objects. Each MailId has multiple different possible status of feedback.

What I am interested in, is to retrieve only the "important" status in my set of possible statuses for one MailId and persist into my database.

OPENED > CLICKED > HARDBOUNCED > BOUNCED (priority of the statuses).

In my example: for MailId= 100 I will only persist the OPENED status into my status column in database.

What will be a good way of achieving this? Should I create a new List (Set?) containing each pair filtered of {MailId, Status(Prioritized)}, ... {MailIdN, StatusN(Prioritized)...} ? Can we add priority on String using Enumerations or making a new comparator for my objects of class Foo?

The persisting operations into mysql database using hibernate should look like this:

persistStatusIntoTable(String MailId, String Status(Taken only status based on priority))

Thanks to all of your help !

I'd suggest putting the status as its own separate class which has a private int priority (or something like that), using a custom Comparator<String> would also work, but may be quite hard to make if you add more statusses. (In some weird scenario.) Then create a new List and add all elements from myList to it that pass your status priority condition. — Wietlol
– Wietlol, Commented Mar 22, 2017 at 23:40
I think it will be a bad idea to make it into a separate class since the status represent a field of my Entity class FeedBackReceived . I think I should go on making a custom Comparator<String> like you said. The possible statuses I can have are all set to those I mention. The comparator will be generic and applied for other existing Lists with different type of Objects that include also statuses. I have List<foo>, List<Bar>, List<FooBar> representing different email Accounts — algorithmic
– algorithmic, Commented Mar 22, 2017 at 23:58
Thanks to both for your answers and time. @Dolev what do you mean with tables? database tables? — algorithmic
– algorithmic, Commented Mar 23, 2017 at 0:04

Markus Benko · Accepted Answer · 2017-03-23 07:46:59Z

If you don't want to use 3rd party libraries and don't want to use an enum class:

// initialization which you will have to do once somewhere in your code
List<String> ordering = Arrays.asList("OPENED", "CLICKED", "HARDBOUNCED", "BOUNCED");
Map<String, Integer> priorityMap = IntStream.range(0, ordering.size()).boxed()
        .collect(Collectors.toMap(ordering::get, i -> i));

// example input
List<Foo> myList = Arrays.asList(
        new Foo("100", "BOUNCED"), new Foo("100", "OPENED"),
        new Foo("101", "BOUNCED"), new Foo("101", "HARDBOUNCED"), new Foo("101", "BOUNCED"));

// determine foo objects that should be persisted 
Collection<Foo> toBePersisted = myList.stream()
        .filter(t -> priorityMap.containsKey(t.getStatus()))
        .collect(Collectors.toMap(Foo::getMailId, t -> t,
                BinaryOperator.minBy(Comparator.comparing(t -> priorityMap.get(t.getStatus())))))
        .values();

What this code does is:

set up a map which maps status to priority (lower int = higher priority)
filter Foo instances that have an invalid status
group Foo instances by mailId
keep Foo instance with minimum priority integer (= highest priority) per group
get values of resulting map

Eugene · Accepted Answer · 2017-03-23 06:22:30Z

I assume a couple of things here (that could be easily changed). First Status is an enum (but you could do the same for a String for example).

Then I'm using google guava Ordering to simplify building the Comparator (you could do this without it).

 Comparator<Status> c = Ordering.explicit(
            ImmutableList.of(Status.OPENED, Status.CLICKED, Status.HARDBOUNCED, Status.BOUNCED));

    List<Foo> list = Stream.of(new Foo(Status.BOUNCED, 57), new Foo(Status.OPENED, 57),
            new Foo(Status.CLICKED, 58), new Foo(Status.BOUNCED, 57),
            new Foo(Status.HARDBOUNCED, 58))
            .collect(Collectors.groupingBy(Foo::getId,
                    Collectors.mapping(Foo::getStatus, Collectors.toCollection(() -> new PriorityQueue<>(c)))))
            .entrySet()
            .stream()
            .map(e -> new Foo(e.getValue().remove(), e.getKey()))
            .collect(Collectors.toList());

    System.out.println(list); // [id = 57 status = OPENED, id = 58 status = CLICKED]

The idea is to create a Comparator that defines the order of your Statuses; then use that comparator to populate a PriorityQueue (for some id); ultimately I am only interested in the first one - the most important as you call it (and that is the first according to the Comparator).

And here is the version that uses a String versus an enum for Status:

 Comparator<String> c = Ordering.explicit(
            ImmutableList.of("OPENED", "CLICKED", "HARDBOUNCED", "BOUNCED"));

    List<Foo> list = Stream.of(new Foo("BOUNCED", 57), new Foo("OPENED", 57),
            new Foo("CLICKED", 58), new Foo("BOUNCED", 57),
            new Foo("HARDBOUNCED", 58))
            .collect(Collectors.groupingBy(Foo::getId,
                    Collectors.mapping(Foo::getStatus, Collectors.toCollection(() -> new PriorityQueue<>(c)))))
            .entrySet()
            .stream()
            .map(e -> new Foo(e.getValue().remove(), e.getKey()))
            .collect(Collectors.toList());

Lot of thanks your solution is for no doubt correct and solve my problem however I checkmarked the above answer since it does not require any 3rd library and in my case I don't want to use on. I am sure it will help somebody else in the future! But thank you again !

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