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。
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<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：
