使用Java Stream高效处理Map：按值排序并提取键名教程

import java.util.Comparator;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;

Map<String, Integer> cities = new HashMap<>();
cities.put("Minsk", 1999234);
cities.put("Mogilev", 1599234);
cities.put("Vitebsk", 3999231);
cities.put("Brest", 4999234);

// 错误的尝试：先映射到值，再排序，导致键信息丢失
List<Integer> collect = cities.entrySet().stream()
    .map(o -> o.getValue()) // 此时已经失去了城市名称（键）的信息
    .sorted(Comparator.reverseOrder())
    .collect(Collectors.toList());

System.out.println("仅按人口排序（丢失城市名）: " + collect);
// 预期输出：[4999234, 3999231, 1999234, 1599234]
// 但无法获取对应的城市名

import java.util.Comparator;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;

Map<String, Integer> cities = new HashMap<>();
cities.put("Minsk", 1999234);
cities.put("Mogilev", 1599234);
cities.put("Vitebsk", 3999231);
cities.put("Brest", 4999234);

// 正确的方法：先对Map.Entry进行排序，再提取键
List<String> sortedCityNames = cities.entrySet().stream()
    .sorted(Map.Entry.comparingByValue(Comparator.reverseOrder())) // 按值降序排序Map.Entry
    .map(Map.Entry::getKey) // 提取排序后的Map.Entry的键
    .toList(); // 或者 .collect(Collectors.toList());

System.out.println("按人口降序排序的城市名: " + sortedCityNames);
// 预期输出：[Brest, Vitebsk, Minsk, Mogilev]

// Java 14+ record
record City(String name, int population) {}

// 或者传统的类
/*
class City {
    private String name;
    private int population;

    public City(String name, int population) {
        this.name = name;
        this.population = population;
    }

    public String getName() { return name; }
    public int getPopulation() { return population; }

    // 建议重写equals, hashCode, toString
}
*/

import java.util.Comparator;
import java.util.List;
import java.util.stream.Collectors;

// 定义City记录
record City(String name, int population) {}

List<City> citiesList = List.of(
    new City("Minsk", 1999234),
    new City("Mogilev", 1599234),
    new City("Vitebsk", 3999231),
    new City("Brest", 4999234)
);

// 使用自定义类进行排序和提取
List<String> sortedCityNamesFromList = citiesList.stream()
    .sorted(Comparator.comparingInt(City::population).reversed()) // 按人口降序排序City对象
    .map(City::name) // 提取排序后的City对象的名称
    .toList(); // 或者 .collect(Collectors.toList());

System.out.println("使用City记录按人口降序排序的城市名: " + sortedCityNamesFromList);
// 预期输出：[Brest, Vitebsk, Minsk, Mogilev]