java 8流收集最大对象并按属性区分

Question

这里有一些列表

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

每一个元素

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

此代码仅排序。

Answer 1

首先你可以在 Book 上进行分组。命名并将它们收集到Map<String, List<Book>> , 然后从 map.values()收集每种类型的最高价格书

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

@Holger 使用 Collectors.toMap 建议的其他解决方案与收集和查找最大元素相比会更有效

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())));