从Map<String、Map<String、Integer>>流中收集Map<String、Integer>>

import java.util.HashMap;
import java.util.Map;
import java.util.stream.Collectors;
import java.util.stream.Stream;

public class Main {
    public static void main(String[] args) {
        // 创建一个包含嵌套结构的流
        Stream<Map<String, Map<String, Integer>>> stream = Stream.of(
                createNestedMap("A", createMap("A1", 1), createMap("A2", 2)),
                createNestedMap("B", createMap("B1", 3), createMap("B2", 4))
        );

        // 从流中收集Map<String, Integer>类型的元素
        Map<String, Integer> collectedMap = stream
                .flatMap(map -> map.entrySet().stream()) // 将嵌套结构展平为Stream<Map.Entry<String, Map<String, Integer>>>
                .collect(Collectors.toMap(
                        Map.Entry::getKey, // 获取外层Map的键
                        entry -> sumValues(entry.getValue()) // 计算内层Map的值之和
                ));

        // 打印收集到的Map<String, Integer>类型的元素
        collectedMap.forEach((key, value) -> System.out.println(key + ": " + value));
    }

    // 创建一个具有指定键值对的Map<String, Integer>
    private static Map<String, Integer> createMap(String key, int value) {
        Map<String, Integer> map = new HashMap<>();
        map.put(key, value);
        return map;
    }

    // 创建一个具有指定键和嵌套的Map<String, Map<String, Integer>>
    private static Map<String, Map<String, Integer>> createNestedMap(String key, Map<String, Integer> nestedMap1, Map<String, Integer> nestedMap2) {
        Map<String, Map<String, Integer>> map = new HashMap<>();
        map.put(key, nestedMap1);
        map.put(key, nestedMap2);
        return map;
    }

    // 计算Map<String, Integer>的值之和
    private static int sumValues(Map<String, Integer> map) {
        return map.values().stream().mapToInt(Integer::intValue).sum();
    }
}