如何使用Java Streams获取嵌套映射的所有键组合？

import java.util.*;
import java.util.stream.*;

public class Main {
    public static void main(String[] args) {
        Map<String, Object> nestedMap = new HashMap<>();
        nestedMap.put("key1", "value1");
        
        Map<String, Object> innerMap = new HashMap<>();
        innerMap.put("key2", "value2");
        innerMap.put("key3", "value3");
        
        nestedMap.put("key4", innerMap);
        
        List<String> keys = getAllKeys(nestedMap);
        System.out.println(keys);
    }
    
    public static List<String> getAllKeys(Map<String, Object> map) {
        return map.entrySet().stream()
            .flatMap(entry -> {
                if (entry.getValue() instanceof Map) {
                    return Stream.concat(
                        Stream.of(entry.getKey()),
                        getAllKeys((Map<String, Object>) entry.getValue()).stream()
                            .map(subKey -> entry.getKey() + "." + subKey)
                    );
                } else {
                    return Stream.of(entry.getKey());
                }
            })
            .collect(Collectors.toList());
    }
}