使用java流解析列表

Question

我的用例是我有餐馆对象的列表，其中包含属性作为城市，类型，评级，我想按照给定组合（城市，类型）的最高评级打印列表。 当我创建这个例子来混淆java流时，请回复如何使用流来实现：

import java.util.List;
import java.util.ArrayList;
import java.util.stream.Collectors;
import java.util.Map;
import java.util.Optional;
import static java.util.stream.Collectors.maxBy;
import static java.util.stream.Collectors.groupingBy;
import static java.util.Comparator.comparingLong;

class Restaurant {
  public String city; // Any city
  public String type; // Tier1, Tier2, etc. 
  public Long rating; // for simplicity sake, assume its a number from 1 to 5
  public Restaurant(String city, String type, Long rating) { 
    this.city = city;
    this.type = type;
    this.rating = rating;
  }
  public String getCity() { return city; }
  public String getType() { return type; }
  public Long getRating() { return rating; }
}

public class HelloWorld
{
  public static void main(String[] args)
  {
    List<Restaurant> mylist = new ArrayList<Restaurant>();
    mylist.add(new Restaurant("City1", "Tier1", 1L));
    mylist.add(new Restaurant("City1", "Tier1", 2L));
    mylist.add(new Restaurant("City1", "Tier2", 1L));
    mylist.add(new Restaurant("City2", "Tier1", 1L));
    mylist.add(new Restaurant("City2", "Tier1", 3L));
    mylist.add(new Restaurant("City2", "Tier3", 1L));
    // expected outcome: (City1, Tier1, 2), (City1, Tier2, 1), (City2, Tier1, 3), (City2, Tier3, 1)

    Map<String, Map<String, Optional<Restaurant>>> mymap =
   mylist.stream().collect(groupingBy(Restaurant::getCity, groupingBy(Restaurant::getType, maxBy(comparingLong(Restaurant::getRating)))));
   /* Upto here everything is fine, but I want to have everything stored back into a list. Tried flatMap(Map::values).collect(Collectors.toList()); but it fails*/
  /*My output list should have 4 objects of Restaurant*/
  /*(City1, Tier1, 2), (City1, Tier2, 1), (City2, Tier1, 3), (City2, Tier3, 1)*/
  }
}

Answer 1

首先，您可以创建复合键以摆脱嵌套映射。 Arrays.asList(r.getCity(), r.getType())就是您所需要的。 接下来， groupingBy不适合你的情况，因为你应该处理那些讨厌的选项。 而是使用带有自定义合并功能的toMap collector：

Map<List<String>, Restaurant> mymap = mylist.stream().collect(
        toMap(r -> Arrays.asList(r.getCity(), r.getType()), r -> r,
                BinaryOperator.maxBy(comparingLong(Restaurant::getRating))));

现在mymap.values()是你想要的平面餐馆集合。 如果你真的想要List ，你可以把它转储成一个：

List<Restaurant> list = new ArrayList<>(mymap.values());

Answer 2

下面的代码片段应该产生您所追求的结果：

    List<Optional<Restaurant>> resultList = mymap.values()
                                                 .stream()
                                                 .flatMap(map -> map.values().stream())
                                                 .collect(Collectors.toList());

mymap.values().stream()生成一个Stream<Map<String, Optional<Restaurant>>> 。 flatMap()然后将该流映射到Stream<Optional<Restaurant>> ，并且collect(Collectors.toList())将该流收集到List<Optional<Restaurant>> 。

Answer 3

我还没有使用Java 8，但我的猜测是

mylist.stream().sorted(new Comparator<Restaurant>() {
     public int compare(Restaurant r1, Restaurant r2) {
        // impl here how your restaurant should be compared
        // (by rating or smth. else)
     }
}).collect(groupingBy( .../ al the rest stuff here/ ...));

Answer 4

使用Java 8流API：

List<Restaurant> sortedList = mylist.stream()
.sorted((restaurant1, restaurant2) -> Long.compare(restaurant1.getRating(), restaurant2.getRating()))
.collect(Collectors.toList());