collectingAndThen

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

import com.alibaba.fastjson.JSON;


class Scratch {

    public static void main(String[] args) {
        Map<String, String> collect = Stream.<String>empty().collect(Collectors.groupingBy(String::intern, Collectors.collectingAndThen(Collectors.toList(), JSON::toJSONString)));
    }
}