lambda 分组表达式

lamda分组表达式

   // 单条件分组，
   Map<String, List<YourBean>> mapByOne = yourBeanList.stream().collect(Collectors.groupingBy(YourBean::getBillDateStr));
 
   // 分组后，统计每组中数据量
   Map<String, Long> count = yourBeanList.stream().collect(Collectors.groupingBy(YourBean::getBillDateStr, Collectors.counting()));
 
   // 分组后，求出每组中某属性的平均值
   Map<String, Double> avg = yourBeanList.stream().filter(i -> i.getGoodAmount() != null).
          collect(Collectors.groupingBy(YourBean::getBillDateStr, Collectors.averagingDouble(YourBean::getGoodAmount)));
 
   // 分组，某属性求和
   Map<String, Double> sum = yourBeanList.stream().filter(i -> i.getGoodAmount() != null).
                collect(Collectors.groupingBy(YourBean::getBillDateStr, Collectors.summingDouble(YourBean::getGoodAmount)));
 
        //对求和的结果集进行从大到小排序
   Map<String, Double> finalMap = new LinkedHashMap<>();
   sum.entrySet().stream().sorted(Map.Entry.<String, Double>comparingByValue().reversed()).forEachOrdered(e -> finalMap.put(e.getKey(), e.getValue()));
 
   // 分组后，通过join组成新的map
   Map<String, String> joinNewMap = yourBeanList.stream().filter(i -> i.getGoodAmount() != null)
                .collect(Collectors.groupingBy(YourBean::getBillDateStr,
                        Collectors.mapping(i -> i.getGoodAmount().toString(), Collectors.joining(", ", "Post titles: [", "]"))));
   // 2022-04-23
   // Post titles: [3.0, 1.0, 2.0, 3.0]
 
   // 转换分组结果List -> Set
   Map<String, Set<String>> namesByName = yourBeanList.stream()
                .collect(Collectors.groupingBy(YourBean::getBillDateStr, Collectors.mapping(YourBean::getGoodName, Collectors.toSet())));
   Set<String> x = namesByCity.keySet();
 
   // 两个条件分组
   Map<String, Map<String, List<YourBean>>> mapByTwo = yourBeanList.stream()
                .collect(Collectors.groupingBy(YourBean::getBillDateStr, Collectors.groupingBy(YourBean::getGoodName)));
 
        // 使用java8 stream groupingBy 操作,按日期分组list,将List转化为name的List
        Map<String, List<String>> mapList = yourBeanList.stream()
                .collect(Collectors.groupingBy(YourBean::getBillDateStr, Collectors.mapping(YourBean::getGoodName, Collectors.toList())));