最近使用java的stream功能有點多，理由有2：

1）少寫了不少程式碼

2）在效能可以接受的範圍內

在巨大的collection基礎上使用stream,沒有什麼經驗。所以非關鍵業務上，樂於使用stream。

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工作中常常使用toMap，而且嚐嚐需要把ListMap轉為Map<String,List<Map<String,Object>>>

之所以這麼做，主要兩個理由：

1）業務需要

2）願意使用key去尋找，而不是重複遍歷陣列。完成的程式碼比較友好

例子：

package com.hc.demomain.base.stream;

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

import com.alibaba.fastjson.JSONObject;

public class StreamTest {

    public static void main(String[] args) {
        List<Map<String,Object>> stuList=new ArrayList<Map<String,Object>>();
        
        Map
 
<String,Object> a1=new HashMap<String,Object>();
        a1.put("className", "fujian");
        a1.put("name", "a1");
        a1.put("sex", "男");
        
        stuList.add(a1);
        
        Map<String,Object> a2=new HashMap<String,Object>();
        a2.put("className", "fujian");
        a2.put(
 
"name", "a2");
        a2.put("sex", "女");
        stuList.add(a2);
        
        Map<String,Object> b1=new HashMap<String,Object>();
        b1.put("className", "beijing");
        b1.put("name", "b1");
        b1.put("sex", "男");
        stuList.add(b1);
        
        Map<String,Object> b2=new HashMap<String,Object>();
        b2.put("className", "beijing");
        b2.put("name", "b2");
        b2.put("sex", "女");
        stuList.add(b2);
        
        Map<String,Object> c1=new HashMap<String,Object>();
        c1.put("className", "烏魯木齊");
        c1.put("name", "c1");
        c1.put("sex", "女");
        stuList.add(c1);
        
        Map<String,Object> c2=new HashMap<String,Object>();
        c2.put("className", "烏魯木齊");
        c2.put("name", "c2");
        c2.put("sex", "男");
        stuList.add(c2);
        //使用className分組
        Map<String,List<Map<String,Object>>> destMap= stuList.stream().collect(Collectors.toMap(
                  item->item.get("className").toString(),v->{
                      List<Map<String,Object>> list=new ArrayList<Map<String,Object>>();
                      Map<String,Object> m=new HashMap<String,Object>();
                      m.put("name",v.get("name"));
                      m.put("sex",v.get("sex"));
                      list.add(m);
                      return list;
                  },
                  //重複處理
                  (List<Map<String,Object>> oldList, List<Map<String,Object>> newList) -> {
                        oldList.addAll(newList);
                        return oldList;
                    }
                  )
                );
        JSONArray srcObject = (JSONArray)JSONObject.toJSON(stuList);
  System.out.println(JSONObject.toJSONString(srcObject, true));


        JSONObject jsonObject = (JSONObject)JSONObject.toJSON(destMap);
        System.out.println(JSONObject.toJSONString(jsonObject, true));
        
        
    }
}

以下是原始的ListMap和Map<String,List<Map<String,Object>>> 的對比（為了方便閱讀，使用fastJson格式化了下):

[
    {
        "name":"a1",
        "className":"fujian",
        "sex":"男"
    },
    {
        "name":"a2",
        "className":"fujian",
        "sex":"女"
    },
    {
        "name":"b1",
        "className":"beijing",
        "sex":"男"
    },
    {
        "name":"b2",
        "className":"beijing",
        "sex":"女"
    },
    {
        "name":"c1",
        "className":"烏魯木齊",
        "sex":"女"
    },
    {
        "name":"c2",
        "className":"烏魯木齊",
        "sex":"男"
    }
]
{
    "烏魯木齊":[
        {
            "name":"c1",
            "sex":"女"
        },
        {
            "name":"c2",
            "sex":"男"
        }
    ],
    "beijing":[
        {
            "name":"b1",
            "sex":"男"
        },
        {
            "name":"b2",
            "sex":"女"
        }
    ],
    "fujian":[
        {
            "name":"a1",
            "sex":"男"
        },
        {
            "name":"a2",
            "sex":"女"
        }
    ]
}

下次有機會，轉麼比較下，不同寫法的效率！

不過已經有人做過了試驗，https://www.cnblogs.com/jpfss/p/11262231.html