java按某个字段对数据分组（场景：可应用于在java中对集合进行二次处理）

场景：可应用于在java中对集合进行二次处理

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;


import com.alibaba.fastjson.JSONArray;
import com.alibaba.fastjson.JSONObject;

public class JsonTest {
    
    public static void main(String[] args) {
        String msg = "[{"serviceCode":"MCY000003","serviceName":"床位费","discount_rate":"80%","minoutdays":"0","maxoutdays":"5"},{"serviceCode":"MCY000002","serviceName":"床位费","discount_rate":"80%","minoutdays":"0","maxoutdays":"5"},{"serviceCode":"MCY000001","serviceName":"床位费","discount_rate":"80%","minoutdays":"0","maxoutdays":"5"},{"serviceCode":"MCY000001","serviceName":"床位费","discount_rate":"80%","minoutdays":"0","maxoutdays":"5"},{"serviceCode":"MCY000003","serviceName":"床位费","discount_rate":"80%","minoutdays":"0","maxoutdays":"5"},{"serviceCode":"MCY000005","serviceName":"床位费","discount_rate":"80%","minoutdays":"0","maxoutdays":"5"}]";
        groupByCode(msg);
    }

    private static Map<String, List<?>> groupByCode(String msg) {
        JSONArray data = JSONArray.parseArray(msg);
        Map<String, List<?>> dataMap = new HashMap<String, List<?>>();
        //数据分组算法
        for (int i = 0; i < data.size(); i++) {
            JSONObject json = (JSONObject)data.get(i);
            List<Object> tempList = (List<Object>) dataMap.get(json.get("serviceCode"));
            if(tempList == null) { //说明是第一次，则创建list
                tempList = new ArrayList<>();
                tempList.add(json);
                dataMap.put(json.get("serviceCode").toString(), tempList);//放入map中
            }else {
                tempList.add(json);
            }
        }
        //遍历
        for (String serviceCode : dataMap.keySet()) {
            System.out.println(serviceCode+":"+dataMap.get(serviceCode));
        }
        return dataMap;
    }

}