Java8新特性整理记录

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 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 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));

    }