使用java流解析列表

2019年7月22日 205次阅读

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)*/
  }
}

最佳答案 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());
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