Intro Java8 groupingBy Collector

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Intro Java8 groupingBy Collector

這篇介紹Intro Java8 groupingBy Collector。

Example Code Setup

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To demonstrate the usage of groupingBy(), let's define a BlogPost class (we will use a stream of BlogPost objects):

class BlogPost {
    String title;
    String author;
    BlogPostType type;
    int likes;
}

Next, the BlogPostType:

enum BlogPostType {
    NEWS,
    REVIEW,
    GUIDE
}

Then the List of BlogPost objects:

List<BlogPost> posts = Arrays.asList( ... );

Let's also define a Tuple class that will be used to group posts by the combination of their type and author attributes:

class Tuple {
    BlogPostType type;
    String author;
}

Example : Simple Grouping by a Single Column

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Map<BlogPostType, List<BlogPost>> postsPerType = posts.stream().collect(groupingBy(BlogPost::getType));

Example : groupingBy with a Complex Map Key Type

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Map<Tuple, List<BlogPost>> postsPerTypeAndAuthor = posts.stream().collect(groupingBy(post -> new Tuple(post.getType(), post.getAuthor())));