如何在ImmutableMap中扁平化ImmutableList

import com.google.common.collect.ImmutableList;
import com.google.common.collect.ImmutableMap;

public class FlattenImmutableMap {

    public static ImmutableMap<String, Object> flatten(ImmutableMap<String, Object> map) {
        ImmutableMap.Builder<String, Object> builder = ImmutableMap.builder();

        for (String key : map.keySet()) {
            Object value = map.get(key);

            if (value instanceof ImmutableList) {
                flattenList(builder, key, (ImmutableList) value);
            } else {
                builder.put(key, value);
            }
        }

        return builder.build();
    }

    private static void flattenList(ImmutableMap.Builder<String, Object> builder, String prefix, ImmutableList list) {
        for (int i = 0; i < list.size(); i++) {
            Object value = list.get(i);

            if (value instanceof ImmutableList) {
                flattenList(builder, prefix + "_" + i, (ImmutableList) value);
            } else {
                builder.put(prefix + "_" + i, value);
            }
        }
    }

    public static void main(String[] args) {
        ImmutableMap<String, Object> map = ImmutableMap.of(
                "key1", "value1",
                "key2", ImmutableList.of("value2", "value3"),
                "key3", ImmutableList.of(
                        ImmutableList.of("value4", "value5"),
                        "value6"
                )
        );

        ImmutableMap<String, Object> flattenedMap = flatten(map);
        System.out.println(flattenedMap);
    }
}