集合分组

一，List分组

// 1.java8之前 
// 按照Name分组,准备Map,此处使用LinkedHashMap的原因是为了保证后续遍历的时候,按照分组时的add顺序来操作
Map> groupMap = new LinkedHashMap>();
for(Vo groupVo : dataList) {
    String groupName = groupVo.getGroupName();
    //如果Map中的key包含此时的groupName,则取出Value,直接add此实体
    if(groupLimitNeedVO.containsKey(groupName)) {
        groupLimitNeedVO.get(groupName).add(groupVo);
        //否则的话,新建一个key为groupName的List,并将groupName作为key,list作为value放入map
    }else {
        List groupList = new LinkedList();
        groupList.add(groupVo);
        groupLimitNeedVO.put(groupName, groupList);
    }
}
// 遍历
Set keySet = groupMap.keySet();
    for(String groupName : keySet) {
        System.out.printf("groupName"+)groupName;
    }

// 2.java8的流分组
// 单条件
Map> groupMap = dataList.stream()
    .collect(Collectors.groupingBy(Vo::getGroupName));
     //.collect(Collectors.groupingBy( (Vo v) -> v.getGroupName() ))

// 多级
Map>>  groupMap = dataList.stream().collect(
Collectors.groupingBy(Vo::getGroupName1),Collectors.groupingBy(Vo::getGroupName2));

二，JSON分组

// 1.单条件
var map = {};
var dest = [];
for(var i = 0; i < datas.length; i++){
   var ai = datas[i];
   if(!map[ai.currency]){// 自定义分组
      dest.push({
         title: ai.currency,// 自定义分组
         name: i,
         content: [ai]
      });
      map[ai.currency] = ai; // 自定义分组
   }else{
      for(var j = 0; j < dest.length; j++){
         var data = dest[j];
         if(data.title == ai.currency){// 自定义分组
            data.content.push(ai);
            break;
         }
      }
   }
}

// 2.多条件 方法内传分组对象的数组，即可实现多次分组
function createTree(arr,keys) {
  function createObj(arr, arrORobj, keys, start) {
    for (let i = 0; i < arr.length; i++) {
      // 当不存在一个排序的key，返回[]
      if (start >= keys.length) {
        let newArray=Array.isArray(arrORobj)?arrORobj:[]
        return newArray.concat(arr[i])
      }
      let curKey = keys[start]
      let curVal = arr[i][curKey]
      if(!curVal)continue
      // 存在key对应的值存在，传入{}构造
      let newObj = arrORobj[curVal] ? arrORobj[curVal] : {}
      arrORobj[curVal] = createObj([arr[i]], newObj, keys, start + 1)
    }
    // 存在一个排序的key，返回{}
    return arrORobj
  }
  return createObj(arr,{},keys,0)
};
var obj1=[
    {
        "demp":"001",
        "line":"a",
        "id":"1",
    },
    {
        "demp":"001",
        "line":"b",
        "id":"2",
    },
    {
        "demp":"001",
        "line":"c",
        "id":"3",
    },
    {
        "demp":"002",
        "line":"a",
        "id":"4",
    },
    {
        "demp":"002",
        "line":"b",
        "id":"5",
    },
    {
        "demp":"002",
        "line":"b",
        "id":"6",
    }
];
createTree(obj1,['demp','line']);

参考：
https://cloud.tencent.com/developer/article/1446684
https://www.cnblogs.com/zhengyb/p/10106564.html