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<String> items =
        Arrays.asList("apple", "apple", "banana",
                "apple", "orange", "banana", "papaya");

// 分组
Map<String, List<String>> 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<String, Long> result2 = items.stream().collect(
        Collectors.groupingBy(
                Function.identity(), Collectors.counting()
        )
);
// {papaya=1, orange=1, banana=2, apple=3}
System.out.println(result2);
Map<String, Long> finalMap = new LinkedHashMap<>();


//分组, 计数和排序
result2.entrySet().stream()
        .sorted(Map.Entry.<String, Long>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<User> list = new ArrayList<User>();
    list.add(user1);
    list.add(user2);
    list.add(user3);
    Map<String, List<User>> 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<User> list = new ArrayList<User>();
list.add(user1);
list.add(user2);
list.add(user3);
Map<String, Map<String, List<User>>> 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<Date, Map<String, Map<String, Map<String, Map<String, Integer>>>>>>> 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。

 

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<WebRecord, List<Object>> keyExtractor = wr ->
    Arrays.<Object>asList(wr.getFiveMinuteWindow(), wr.getCdn(), wr.getIsp(),
             wr.getResultCode(), wr.getTxnTime());

Map<List<Object>, Integer> aggregatedData = webRecords.stream().collect(
      Collectors.groupingBy(keyExtractor, Collectors.summingInt(WebRecord::getReqBytes)));

在这种情况下，键是按固定顺序列出的5个元素。不是很面向对象，但很简单。或者，您可以定义自己的表示自定义键的类型，并创建适当的hashCode/equals实现。

参考链接：

• https://stackoverflow.com/questions/32531517/
• https://www.cnblogs.com/winner-0715/p/10246037.html