分组<OrderItems>列表和计数

Question

我有OrderItems类

public class OrderItems {
public  int id;
public  double portion;

和List<OrderItems>如下:

List<OrderItems> orderItems = new ArrayList<>();
orderItems.add(new OrderItems(1,0.5));
orderItems.add(new OrderItems(2,1));
orderItems.add(new OrderItems(1,0.5));
orderItems.add(new OrderItems(1,1.5));
orderItems.add(new OrderItems(2,1));
orderItems.add(new OrderItems(2,0.5));

我的问题:我如何将此列表分组并将NewOrderItems类转换为流API:

public class NewOrderItems {
public int id;
public double portion;
public long amount;
}

结果应该是这样的:

NewItems{id=1, portion=0.5, amount=2}
NewItems{id=1, portion=1.5, amount=1}
NewItems{id=2, portion=0.5, amount=1}
NewItems{id=2, portion=1.0, amount=2}

Answer 1

我的解决方案是这样的:

Map<Integer, Map<Double, Long>> collect = orderItems.stream()
.collect(Collectors.groupingBy(OrderItems::getId, Collectors.groupingBy(OrderItems::getPortion, Collectors.counting())));
System.out.println(collect);
List<NewOrderItems> newOrderItems = new ArrayList<>();
collect.forEach((k,v)->{
v.forEach((x,y)->{
NewOrderItems newOrderitems=new NewOrderItems(k,x,y);
newOrderItems.add(newOrderitems);
});
});
newOrderItems.forEach(System.out::println);

控制台:

NewItems{id=1, portion=0.5, amount=2}
NewItems{id=1, portion=1.5, amount=1}
NewItems{id=2, portion=0.5, amount=1}
NewItems{id=2, portion=1.0, amount=2}

Answer 2

第一个分组。你会得到一张地图。OrderItems作为键，List作为值。然后数一数。最后排序。

在你听不懂的地方检查。

public static void main(String[] args) {
List<OrderItems> orderItems = new ArrayList<>();
orderItems.add(new OrderItems(1, 0.5));
orderItems.add(new OrderItems(2, 1));
orderItems.add(new OrderItems(1, 0.5));
orderItems.add(new OrderItems(1, 1.5));
orderItems.add(new OrderItems(2, 1));
orderItems.add(new OrderItems(2, 0.5));

final List<NewOrderItems> collect = orderItems.stream().collect(Collectors.groupingBy(x -> x))
.entrySet().stream().map(x -> {
NewOrderItems newOrderItems = new NewOrderItems();
newOrderItems.setId(x.getKey().id);
newOrderItems.setPortion(x.getKey().portion);
newOrderItems.setAmount(x.getValue().size());
return newOrderItems;
})
.sorted(new Comparator<NewOrderItems>() {
@Override
public int compare(NewOrderItems o1, NewOrderItems o2) {
return o1.id - o2.id;
}
})
.collect(Collectors.toList());
System.out.println(collect);
}

Answer 3

我们可以利用

• 收藏家。groupingBy→用于对订单项
进行分组
• Java record ->定义分组
的键
• 收藏家。减少→组合各组订单项目列表

public class GroupItem {
public static void main(String[] args) {
List<OrderItems> orderItems = new ArrayList<>();
orderItems.add(new OrderItems(1, 0.5));
orderItems.add(new OrderItems(2, 1));
orderItems.add(new OrderItems(1, 0.5));
orderItems.add(new OrderItems(1, 1.5));
orderItems.add(new OrderItems(2, 1));
orderItems.add(new OrderItems(2, 0.5));
// java 17 feature to define key easily
record ItemKey(int id, double portion) {
}
Map<ItemKey, GroupedOrderItems> m = orderItems.stream().collect(
Collectors.groupingBy(
// group by ItemKey
i -> new ItemKey(i.id, i.portion),
// reducing the OrderItems with same key
Collectors.reducing(
new GroupedOrderItems(0, 0.0, 0L),
a -> new GroupedOrderItems(a.id, a.portion, 1),
// combine 2 GroupedOrderItems
(b, c) -> new GroupedOrderItems(c.id,
c.portion,
b.amount + c.amount)
)
)
);
m.entrySet().forEach(e -> System.out.println(e.getValue()));
}
public static class OrderItems {
public int id;
public double portion;
public OrderItems(int id, double portion) {
this.id = id;
this.portion = portion;
}
}
public static class GroupedOrderItems {
public int id;
public double portion;
public long amount;
public GroupedOrderItems(int id, double portion, long amount) {
this.id = id;
this.portion = portion;
this.amount = amount;
}
@Override
public String toString() {
return "GroupedOrderItems{" +
"id=" + id +
", portion=" + portion +
", amount=" + amount +
'}';
}
}
}