通过有效地连接几个Java Map键集来迭代

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

public class MapIteration {
    public static void main(String[] args) {
        Map<String, Integer> map1 = new HashMap<>();
        map1.put("key1", 1);
        map1.put("key2", 2);

        Map<String, Integer> map2 = new HashMap<>();
        map2.put("key3", 3);
        map2.put("key4", 4);

        Map<String, Integer> map3 = new HashMap<>();
        map3.put("key5", 5);
        map3.put("key6", 6);

        List<Map<String, Integer>> maps = Arrays.asList(map1, map2, map3);

        Set<String> keys = maps.stream()
                .flatMap(map -> map.keySet().stream())
                .collect(Collectors.toSet());

        keys.forEach(System.out::println);
    }
}

key1
key2
key3
key4
key5
key6