Java8 stream 中利用 groupingBy 进行多字段分组求和案例

Java8的groupingBy实现集合的分组，类似Mysql的group by分组功能，注意得到的是一个map

对集合按照单个属性分组、分组计数、排序

List items =
    Arrays.asList("apple", "apple", "banana",
        "apple", "orange", "banana", "papaya");
 
// 分组
Map> result1 = items.stream().collect(
    Collectors.groupingBy(
        Function.identity()
    )
);
//{papaya=[papaya], orange=[orange], banana=[banana, banana], apple=[apple, apple, apple]}
System.out.println(result1);
 
 
// 分组计数
Map result2 = items.stream().collect(
    Collectors.groupingBy(
        Function.identity(), Collectors.counting()
    )
);
// {papaya=1, orange=1, banana=2, apple=3}
System.out.println(result2);
Map finalMap = new LinkedHashMap<>();
 
 
//分组, 计数和排序
result2.entrySet().stream()
    .sorted(Map.Entry.comparingByValue().reversed())
    .forEachOrdered(e -> finalMap.put(e.getKey(), e.getValue()));
// {apple=3, banana=2, papaya=1, orange=1}
System.out.println(finalMap);

集合按照多个属性分组

1.多个属性拼接出一个组合属性

public static void main(String[] args) {
  User user1 = new User("zhangsan", "beijing", 10);
  User user2 = new User("zhangsan", "beijing", 20);
  User user3 = new User("lisi", "shanghai", 30);
  List list = new ArrayList();
  list.add(user1);
  list.add(user2);
  list.add(user3);
  Map> collect = list.stream().collect(Collectors.groupingBy(e -> fetchGroupKey(e)));
  //{zhangsan#beijing=[User{age=10, name='zhangsan', address='beijing'}, User{age=20, name='zhangsan', address='beijing'}], 
  // lisi#shanghai=[User{age=30, name='lisi', address='shanghai'}]}
  System.out.println(collect);
} 
 
private static String fetchGroupKey(User user){
  return user.getName() +"#"+ user.getAddress();
}

2.嵌套调用groupBy

User user1 = new User("zhangsan", "beijing", 10);
User user2 = new User("zhangsan", "beijing", 20);
User user3 = new User("lisi", "shanghai", 30);
List list = new ArrayList();
list.add(user1);
list.add(user2);
list.add(user3);
Map>> collect
    = list.stream().collect(
        Collectors.groupingBy(
            User::getAddress, Collectors.groupingBy(User::getName)
        )
);
System.out.println(collect);

3. 使用Arrays.asList

我有一个与Web访问记录相关的域对象列表。这些域对象可以扩展到数千个。

我没有资源或需求将它们以原始格式存储在数据库中，因此我希望预先计算聚合并将聚合的数据放在数据库中。

我需要聚合在5分钟窗口中传输的总字节数，如下面的sql查询

select 
 round(request_timestamp, '5') as window, --round timestamp to the nearest 5 minute
 cdn, 
 isp, 
 http_result_code, 
 transaction_time, 
 sum(bytes_transferred)
from web_records
group by 
  round(request_timestamp, '5'), 
  cdn, 
  isp, 
  http_result_code, 
  transaction_time

在java 8中，我当前的第一次尝试是这样的，我知道这个解决方案类似于Group by multiple field names in java 8

Map>>>>>> aggregatedData =
webRecords
  .stream()
  .collect(Collectors.groupingBy(WebRecord::getFiveMinuteWindow,
        Collectors.groupingBy(WebRecord::getCdn,
         Collectors.groupingBy(WebRecord::getIsp,
          Collectors.groupingBy(WebRecord::getResultCode,
            Collectors.groupingBy(WebRecord::getTxnTime,
             Collectors.reducing(0,
                       WebRecord::getReqBytes(),
                       Integer::sum)))))));

这是可行的，但它是丑陋的，所有这些嵌套的地图是一个噩梦！要将地图“展平”或“展开”成行，我必须这样做

for (Date window : aggregatedData.keySet()) {
 for (String cdn : aggregatedData.get(window).keySet()) {
  for (String isp : aggregatedData.get(window).get(cdn).keySet()) {
   for (String resultCode : aggregatedData.get(window).get(cdn).get(isp).keySet()) {
    for (String txnTime : aggregatedData.get(window).get(cdn).get(isp).get(resultCode).keySet()) {
 
      Integer bytesTransferred = aggregatedData.get(window).get(cdn).get(distId).get(isp).get(resultCode).get(txnTime);
      AggregatedRow row = new AggregatedRow(window, cdn, distId...

如你所见，这是相当混乱和难以维持。

有谁知道更好的方法吗？任何帮助都将不胜感激。

我想知道是否有更好的方法来展开嵌套的映射，或者是否有一个库允许您对集合进行分组。

最佳答案

您应该为地图创建自定义密钥。最简单的方法是使用Arrays.asList：

Function> keyExtractor = wr ->
  Arrays.