java stream Collectors.groupingBy使用

待处理数据

@Data
public class Book {

    /**
     * 书名
     */
    private String name;
    /**
     * 品类
     */
    private Integer category;

    /**
     * 级别：入门，初级，。。。
     */
    private Integer level;
    /**
     * 作者
     */
    private String author;
    /**
     * 价格
     */
    private Double price;
}

groupingBy使用

    @Test
    public void groupingBy(){
        // 每个category对应的book详情
        Map<Integer, List<Book>> category2Books = books.stream().collect(Collectors.groupingBy(Book::getCategory));
        System.out.println(category2Books);
        // 每个category book数量
        Map<Integer, Long> collect = books.stream().collect(Collectors.groupingBy(Book::getCategory, Collectors.counting()));
        System.out.println(collect);
        // 每个category book价格总和
        Map<Integer, Double> collect1 =
            books.stream().collect(Collectors.groupingBy(Book::getCategory, Collectors.summingDouble(Book::getPrice)));
        System.out.println(collect1);
        // 按category排序，最贵book
        TreeMap<Integer, Optional<Book>> collect2 = books.stream().collect(Collectors.groupingBy(Book::getCategory, TreeMap::new,
            Collectors.maxBy(Comparator.comparingDouble(Book::getPrice))));
        System.out.println(collect2);
    }