1、list实体集合根据某个属性分组后求和

方法一： 

list.stream().collect(Collectors.groupingBy(e -> e.getId())).values().stream().map(d -> {DemoEntity sampleData = d.get(0);sampleData.setPremium(d.stream().map(s -> new BigDecimal(s.getPremium())).reduce(BigDecimal.ZERO, BigDecimal::add).longValue());return sampleData;}).collect(Collectors.toList());

方法二： 

List orderTwo = list.stream().collect(Collectors.toMap(DemoEntity::getId,e->e,(o1,o2)->{o1.setPremium(o1.getPremium()+o2.getPremium());return o1;})).values().stream().collect(Collectors.toList());System.out.println(JSON.toJSON(orderTwo));

 例子：

@Data
public class DemoEntity {private int id;private Long premium;
}运行main方法
public static void main(String[] args) {List<DemoEntity> list = new ArrayList<>();DemoEntity demo = new DemoEntity();demo.setId(1);demo.setPremium(23L);DemoEntity demo1 = new DemoEntity();demo1.setId(2);demo1.setPremium(13L);list.add(demo);list.add(demo1);List<DemoEntity> list1 = new ArrayList<>();DemoEntity demo4 = new DemoEntity();demo4.setId(1);demo4.setPremium(12L);DemoEntity demo5 = new DemoEntity();demo5.setId(2);demo5.setPremium(45L);list1.add(demo4);list1.add(demo5);list.addAll(list1);System.out.println(JSON.toJSON(list));List orders = list.stream().collect(Collectors.groupingBy(e -> e.getId())).values().stream().map(d -> {DemoEntity sampleData = d.get(0);sampleData.setPremium(d.stream().map(s -> new BigDecimal(s.getPremium())).reduce(BigDecimal.ZERO, BigDecimal::add).longValue());return sampleData;}).collect(Collectors.toList());System.out.println(JSON.toJSON(orders));List orderTwo = list.stream().collect(Collectors.toMap(DemoEntity::getId,e->e,(o1,o2)->{o1.setPremium(o1.getPremium()+o2.getPremium());return o1;})).values().stream().collect(Collectors.toList());System.out.println(JSON.toJSON(orderTwo));}