收集器分组按java 8

考虑这两个类

学科类别：

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,...    
}

观察者类：

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

并考虑一下观察员名单

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));

到目前为止，我尝试并成功做到的是将观察者按主题分组，如下所示

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

但是我如何使用上述相同的逻辑按主题类别对观察者进行分组（Collectors, groupingBy, method reference,...)?

例如，我如何获得订阅了类别等于的主题的观察者sport使用这样的东西

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

Thanks.

Use

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

或定义getSubjectCategory() in Observer类及使用方法参考：

Collectors.groupingBy(Observer::getSubjectCategory)