Java Stream 在单循环中以声明方式分别按多个字段进行分组

我用谷歌搜索了它，但我主要发现了按聚合字段分组或改变流响应的情况，但没有找到下面的情况：

我有课User有字段category and marketingChannel.

我必须以声明式风格编写一个方法，该方法接受用户列表并根据category并且还基于marketingChannel单独（即不groupingBy(... ,groupingBy(..)) ).

我无法在一个循环中完成它。这就是我必须达到的目标。

我编写了一些方法如下：

import java.util.*;
import java.util.stream.*;
public class Main
{
    public static void main(String[] args) {
        List<User> users = User.createDemoList();
        imperative(users);
        declerativeMultipleLoop(users);
        declerativeMultipleColumn(users);
    }
    
    public static void imperative(List<User> users){
        Map<String, Integer> categoryMap = new HashMap<>();
        Map<String, Integer> channelMap = new HashMap<>();
        for(User user : users){
           Integer  value = categoryMap.getOrDefault(user.getCategory(), 0);
           categoryMap.put(user.getCategory(), value+1);
           value = channelMap.getOrDefault(user.getMarketingChannel(), 0);
           channelMap.put(user.getMarketingChannel(), value+1);
        }
        System.out.println("imperative");
        System.out.println(categoryMap);
        System.out.println(channelMap);
    }
    
    public static void declerativeMultipleLoop(List<User> users){
        Map<String, Long> categoryMap = users.stream()
        .collect(Collectors.groupingBy(
            User::getCategory, Collectors.counting()));
        Map<String, Long> channelMap = users.stream()
        .collect(Collectors.groupingBy(
            User::getMarketingChannel, Collectors.counting()));
        System.out.println("declerativeMultipleLoop");
        System.out.println(categoryMap);
        System.out.println(channelMap);
    }
    
    public static void declerativeMultipleColumn(List<User> users){
        Map<String, Map<String, Long>> map = users.stream()
        .collect(Collectors.groupingBy(
            User::getCategory,
            Collectors.groupingBy(User::getMarketingChannel, 
            Collectors.counting())));
       
        System.out.println("declerativeMultipleColumn");
        System.out.println("groupingBy category and marketChannel");
        System.out.println(map);
        
        Map<String, Long> categoryMap = new HashMap<>();
        Map<String, Long> channelMap = new HashMap<>();
        
        for (Map.Entry<String, Map<String, Long>> entry: map.entrySet()) {
            String category = entry.getKey();
            Integer count = entry.getValue().size();
            Long value = categoryMap.getOrDefault(category,0L);
            categoryMap.put(category, value+count);
            for (Map.Entry<String, Long> channelEntry : entry.getValue().entrySet()){
                String channel = channelEntry.getKey();
                Long channelCount = channelEntry.getValue();
                Long channelValue = channelMap.getOrDefault(channel,0L);
                channelMap.put(channel, channelValue+channelCount);
            }
        }
        System.out.println("After Implerative Loop on above.");
        System.out.println(categoryMap);
        System.out.println(channelMap);
    }
}
class User{
    private String name;
    private String category;
    private String marketChannel;
    
    public User(String name, String category, String marketChannel){
        this.name = name;
        this.category = category;
        this.marketChannel = marketChannel;
    }
    public String getName(){
        return this.name;
    }
    public String getCategory(){
        return this.category;
    }
    public String getMarketingChannel(){
        return this.marketChannel;
    }
    
     @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        User user = (User) o;
        return Objects.equals(name, user.name) &&
                Objects.equals(category, user.category) &&
                Objects.equals(marketChannel, user.marketChannel);
    }

    @Override
    public int hashCode() {
        return Objects.hash(name, category, marketChannel);
    }
    public static List<User> createDemoList(){
        return Arrays.asList(
            new User("a", "student","google"),
            new User("b", "student","bing"),
            new User("c", "business","google"),
            new User("d", "business", "direct")
            );
    }

方法declerativeMultipleLoop是声明性的，但每个字段都有一个单独的循环。复杂度：O(noOfFields * 用户数量)

问题在于declerativeMultipleColumn方法，因为我最终编写了命令式代码和多个循环。

我想以完全声明性且尽可能高效的方式编写上述方法。即复杂度：O（用户数量）

示例输出：

至关重要的
{企业=2，学生=2}
{直接=1、谷歌=2、必应=1}
声明式多重循环
{企业=2，学生=2}
{直接=1、谷歌=2、必应=1}
声明式多列
按类别和市场渠道分组
{business={direct=1, google=1}, Student={google=1, bing=1}}
在上面的命令循环之后。
{企业=2，学生=2}
{直接=1、谷歌=2、必应=1}

如果我理解您的要求，那就是使用单个流操作来生成 2 个单独的映射。这将需要一个结构来保存地图和一个收集器来构建该结构。像下面这样：

class Counts {
    public final Map<String, Integer> categoryCounts = new HashMap<>();
    public final Map<String, Integer> channelCounts = new HashMap<>();

    public static Collector<User,Counts,Counts> countsCollector() {
        return Collector.of(Counts::new, Counts::accept, Counts::combine, CONCURRENT, UNORDERED);
    }

    private Counts() { }

    private void accept(User user) {
        categoryCounts.merge(user.getCategory(), 1, Integer::sum);
        channelCounts.merge(user.getChannel(), 1, Integer::sum);
    }

    private Counts combine(Counts other) {
        other.categoryCounts.forEach((c, v) -> categoryCounts.merge(c, v, Integer::sum));
        other.channelCounts.forEach((c, v) -> channelCounts.merge(c, v, Integer::sum));
        return this;
    }
}

然后可以将其用作收集器：

Counts counts = users.stream().collect(Counts.countsCollector());
counts.categoryCounts.get("student")...

（仅供参考：在这种情况下，命令式和声明式之间的区别是相当任意的。对我来说，定义流操作感觉非常程序化（与 Haskell 中的等效操作相反））。