java8 groupingby_Java8stream中利用groupingBy进行多字段分组求和

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.asList(wr.getFiveMinuteWindow(), wr.getCdn(),

wr.getIsp(),

wr.getResultCode(), wr.getTxnTime());

Map, 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。