在Java 8中从多级映射构造列表

import java.util.*;

public class Main {
    public static void main(String[] args) {
        // 创建一个多级映射
        Map<String, Map<String, List<String>>> multiMap = new HashMap<>();

        // 向多级映射中添加数据
        multiMap.computeIfAbsent("Category1", k -> new HashMap<>())
                .computeIfAbsent("Subcategory1", k -> new ArrayList<>())
                .add("Item1");
        multiMap.computeIfAbsent("Category1", k -> new HashMap<>())
                .computeIfAbsent("Subcategory1", k -> new ArrayList<>())
                .add("Item2");
        multiMap.computeIfAbsent("Category1", k -> new HashMap<>())
                .computeIfAbsent("Subcategory2", k -> new ArrayList<>())
                .add("Item3");
        multiMap.computeIfAbsent("Category2", k -> new HashMap<>())
                .computeIfAbsent("Subcategory3", k -> new ArrayList<>())
                .add("Item4");

        // 打印多级映射中的数据
        for (Map.Entry<String, Map<String, List<String>>> categoryEntry : multiMap.entrySet()) {
            String category = categoryEntry.getKey();
            System.out.println("Category: " + category);
            for (Map.Entry<String, List<String>> subcategoryEntry : categoryEntry.getValue().entrySet()) {
                String subcategory = subcategoryEntry.getKey();
                System.out.println("  Subcategory: " + subcategory);
                List<String> items = subcategoryEntry.getValue();
                System.out.println("    Items: " + items);
            }
        }
    }
}

Category: Category1
  Subcategory: Subcategory1
    Items: [Item1, Item2]
  Subcategory: Subcategory2
    Items: [Item3]
Category: Category2
  Subcategory: Subcategory3
    Items: [Item4]