java 8流收集最大对象并按属性区分

Question

一些列表在这里

List<Book> list = new ArrayList<>();
{
   list.add(new Book("Core Java", 200));
   list.add(new Book("Core Java", 500));
   list.add(new Book("Core Java", 800));
   list.add(new Book("Learning Freemarker", 150));          
   list.add(new Book("Learning Freemarker", 1350));   
   list.add(new Book("Learning Freemarker", 1250));   
   list.add(new Book("Spring MVC", 300));
   list.add(new Book("Spring MVC", 600)); 
   list.add(new Book("Spring MVC", 1600));
}

我想显示这样的书单

Core Java", 800
Learning Freemarker", 1350
Spring MVC", 1600

每1个元素

list .stream().distinct()
     .sorted(Comparator.comparing(Book::bookname)
     .thenComparing(Book::getPrice)).collect(Collectors.toList());

此代码仅排序。

Answer 1

首先，您可以按Book名称进行分组并将它们收集到Map<String, List<Book>>，然后map.values()从每种类型中收集最高价格的书

List<Book> books = list.stream()
                       .collect(Collectors.groupingBy(Book::getName))
                       .values()
                       .stream()
                       .map(book -> Collections.max(book, Comparator.comparingInt(Book::getCost)))
                       .collect(Collectors.toList());

Collectors.toMap与收集和查找最大元素相比，@Holger 使用的另一种解决方案将更有效

List<Book> books = list.stream()
            .collect(Collectors.collectingAndThen(
                    Collectors.toMap(Book::getName, Function.identity(),
                            BinaryOperator.maxBy(Comparator.comparingInt(Book::getCost))),
                    m -> new ArrayList<>(m.values())));