java - Java使用流按月获取订购产品的数量

我想准备一个简单的报告，显示按月显示的订购产品数量（降序）：

Month         Number of products
2018-10       8
2018-11       3

我的资料：

  class Orders {

        private List<Order> orders = new ArrayList<>();

        public void prepareData() {

            Product socks = new ProductBuilder()
                    .setPrice(new BigDecimal("23"))
                    .setCategory(Category.C)
                    .setName("SOCKS")
                    .build();

            Product jacket = new ProductBuilder()
                    .setPrice(new BigDecimal("199"))
                    .setCategory(Category.A)
                    .setName("JACKET")
                    .build();

            Product watch = new ProductBuilder()
                    .setPrice(new BigDecimal("100"))
                    .setCategory(Category.B)
                    .setName("WATCH CASIO")
                    .build();


            Customer john = new CustomerBuilder()
                    .setAge(18)
                    .setName("JOHN")
                    .setSurname("JOHNSON")
                    .setEmail("[email protected]")
                    .build();

            Customer mike = new CustomerBuilder()
                    .setAge(20)
                    .setName("MIKE")
                    .setSurname("MAX")
                    .setEmail("[email protected]")
                    .build();


            Order orderJohn = new OrderBuilder()
                    .setQuantity(2)
                    .setCustomer(john)
                    .setProduct(watch)
                    .setOrderDate(LocalDate.now())
                    .build();

            Order orderJohn2 = new OrderBuilder()
                    .setQuantity(4)
                    .setCustomer(john)
                    .setProduct(socks)
                    .setOrderDate(LocalDate.now())
                    .build();


             Order orderMike = new OrderBuilder()
                    .setQuantity(2)
                    .setCustomer(mike)
                    .setProduct(jacket)
                    .setOrderDate(LocalDate.now())
                    .build();

            orders.add(orderJohn);
            orders.add(orderJohn2);


            orders.add(orderMike);
    }
}

现在，我使用groupingBy按月份获取订单数

System.out.println(orders.stream()
       .collect(Collectors
               .groupingBy(e -> e.getOrderDate().withDayOfMonth(1), Collectors.counting())));

但是如何按月增加产品数量？ Quantity类中的Order字段。

public class Order {

    private Customer customer;
    private Product product;
    private int quantity;
    private LocalDate orderDate;
   //get/set
}

public class Customer {

    private String name;
    private String surname;
    private int age;
    private String email;
    //get/set
}

public class Product {
    private String name;
    private BigDecimal price;
    private Category category;
    //get/set
}

和建设者

public class CustomerBuilder {

    private Customer customer = new Customer();

    public CustomerBuilder setName(String name){
        customer.setName(name);
        return this;
    }

    public CustomerBuilder setSurname(String surname){
        customer.setSurname(surname);
        return this;
    }

    public CustomerBuilder setAge(int age){
        customer.setAge(age);
        return this;
    }

    public CustomerBuilder setEmail(String email){
        customer.setEmail(email);
        return this;
    }

    public Customer build()  {

        return customer;
    }

}
public class OrderBuilder {
   private Order order = new Order();

   public OrderBuilder setCustomer(Customer customer){
       order.setCustomer(customer);
       return this;
   }

   public OrderBuilder setProduct(Product product){
       order.setProduct(product);
       return this;
   }

   public OrderBuilder setQuantity(int quantity){
       order.setQuantity(quantity);
       return this;
   }

   public OrderBuilder setOrderDate(LocalDate orderDate){
       order.setOrderDate(orderDate);
       return this;
   }

   public Order build(){
       return order;
   }

}

public class ProductBuilder {

    private Product product = new Product();

    public ProductBuilder setCategory(Category category){
        product.setCategory(category);
        return this;
    }

    public ProductBuilder setName(String name){
        product.setName(name);
        return this;
    }

    public ProductBuilder setPrice(BigDecimal bigDecimal){
        product.setPrice(bigDecimal);
        return this;
    }

    public Product build() {
        return product;
    }
}

最佳答案

您可以将Collectors.counting()替换为：

Collectors.summingInt(Order::getQuantity)

（它的工作方式是：Collectors.reducing(0, Order::getQuantity, Integer::sum)）

编辑：

查看您对其他答案的评论，这是您需要的：

Map<LocalDate, IntSummaryStatistics> ord = orders.stream().collect(Collectors.groupingBy(e -> e.getOrderDate().withDayOfMonth(1), Collectors.summarizingInt(Order::getQuantity)));
ord.entrySet().stream().sorted(Map.Entry.comparingByValue((p1,p2) -> Long.compare(p2.getSum(), p1.getSum()))).forEach(System.out::println);