未经测试，但是您可以执行以下操作：
mapIntToStrings.entrySet().stream() .flatMap(entryIntToStrings -> entryIntToStrings.getValue().stream() .map(str -> new AbstractMap.SimpleEntry<>(entryIntToStrings.getKey(), str))) .collect(groupingBy(Map.Entry::getValue, mapping(Map.Entry::getKey, toList())))
