Stream流分组，统计，求和

public class Test {

public static void main(String[] args) {
List<OrdersDO> list = new ArrayList<>();//查询昨天一天的所有交易
OrdersDO o1 = new OrdersDO();
o1.setAppId(1L);
o1.setTradeAmount(100L);
o1.setStatus(1);
list.add(o1);
OrdersDO o2 = new OrdersDO();
o2.setAppId(5L);
o2.setTradeAmount(300L);
o2.setStatus(2);
list.add(o2);
OrdersDO o3 = new OrdersDO();
o3.setAppId(1L);
o3.setTradeAmount(100L);
o3.setStatus(3);
list.add(o3);
OrdersDO o4 = new OrdersDO();
o4.setAppId(5L);
o4.setTradeAmount(300L);
o4.setStatus(4);
list.add(o4);
OrdersDO o5 = new OrdersDO();
o5.setAppId(5L);
o5.setTradeAmount(300L);
o5.setStatus(4);
list.add(o5);
//统计每个应用实际支付总额
Map<Long, Long> tradeAmountMap = list.stream().filter(o->o.getStatus()==2)
.collect(Collectors.groupingBy(OrdersDO::getAppId,
Collectors.summingLong(OrdersDO::getTradeAmount)));
System.out.println(tradeAmountMap);

//统计每个应用取消总额
Map<Long, Long> cancelAmountMap = list.stream()
.collect(Collectors.groupingBy(OrdersDO::getAppId,
Collectors.summingLong(OrdersDO::getTradeAmount)));
System.out.println(cancelAmountMap);

//统计每个应用下交易笔数
Map<Long, Long> appTradeNum = list.stream().collect(Collectors.groupingBy(OrdersDO::getAppId, Collectors.counting()));
System.out.println(appTradeNum);

//统计每个应用每种状态下交易笔数
Map<Long, Map<Integer, Long>> tradeNumMap = list.stream().
collect(Collectors.groupingBy(OrdersDO::getAppId,
Collectors.groupingBy(OrdersDO::getStatus,
Collectors.counting())));
System.out.println(tradeNumMap);

//每个应用下交易笔数按数量排序
Map<Long,Long> finalMap = new LinkedHashMap<>();
appTradeNum.entrySet().stream().sorted(Map.Entry.<Long, Long>comparingByValue().reversed()).forEachOrdered(e->finalMap.put(e.getKey(),e.getValue()));
System.out.println(finalMap);
}
}

原文地址：https://www.cnblogs.com/tubashu/p/11994535.html