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Collecting groupBy java 8

Consider these two classes

Thematic class:

class Subject{
    long id;
    String category;
    String description;
    public Subject(long id,String category,String description){
        this.id=id;
        this.category=category;
        this.description=description;
    }
    // getters, setters, toString,...    
}

Observer class:

class Observer{
    long id;
    Subject subject;
    public Observer(long id,Subject subject){
        this.id=id;
        this.subject=subject;
    }
    // getters, setters, toString,...
}

And consider this list of observers

Subject s1 = new Subject(1, "sport", "subject 1");
Subject s2 = new Subject(2, "music", "subject 2");
Subject s3 = new Subject(3, "sport", "subject 3");

List<Observer> observers = Arrays.asList(new Observer(1, s1),
        new Observer(2, s1), new Observer(3, s2), new Observer(4, s3));

What I have tried so far and has managed to do this is to group observers by subject by following

Map<Subject, List<Observer>> observersGroupedBySubject = observers.stream().collect(
                Collectors.groupingBy(Observer::getSubject));

But how can I be grouped by subject category of observers, using the same logic above ( Collectors

, groupingBy

, method reference

...)?

For example, how can I get observers who have subscribed to a topic whose category is equal sport

using something like this

Map<String, List<Observer>> observersGroupedBySubjectCategory = ...;
observersGroupedBySubjectCategory.get("sport");

Thank.

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3 answers


Using

Collectors.groupingBy(o -> o.getSubject().getCategory())


or define getSubjectCategory()

in a class Observer

and use a method reference:

Collectors.groupingBy(Observer::getSubjectCategory)
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But how can I group observers by subject category using the same over logic (Collectors, groupingBy, reference method, ...)?

As I explained here (second part), you just need to provide a function that will accept a subject category for a given observer. So in this case it would be:



groupingBy(o -> o.getSubject().getCategory())

This will give you Map<String, List<Observer>>

.

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Use a filter.

Map<Subject, List<Observer>> observersGroupedBySubject = observers
                .stream()
                .filter((u) -> u.getSubject().getCategory()
                        .equalsIgnoreCase("sport"))
                .collect(Collectors.groupingBy(Observer::getSubject));
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