使用reduce方法在流中添加BigDecimal

Question

目标：练习“.reduce”方法

有3类：x类枚举（A,B,C,D）,x类产品（字符串名称,BigDecimal价格,BigDecimal折扣,Category Category）x类订单-产品数量

问题：类App1流操作

public class Product {

    private String name;
    private BigDecimal price;
    private BigDecimal discount;
    private Category category;

    public Product(String name, BigDecimal price, BigDecimal discount, Category category) {
        this.name = name;
        this.price = price;
        this.discount = discount;
        this.category = category;
    }


public class OrderPosition {

    private Product product;
    private int quantity;

    public OrderPosition() {
    }

    public OrderPosition(Product product, int quantity) {
        this.product = product;
        this.quantity = quantity;
    }


public enum Category {
    A, B, C, D
}

public class App1 {
    public static void main(String[] args) {

        List<OrderPosition> orderPositions = List.of(
                new OrderPosition(new Product("Szynka", new BigDecimal("25"),BigDecimal.ZERO, Category.A), 10),
                new OrderPosition(new Product("Ser", new BigDecimal("25.5"),BigDecimal.ZERO, Category.B), 20),
                new OrderPosition(new Product("Chleb", new BigDecimal("15.5"),BigDecimal.ZERO, Category.C), 10),
                new OrderPosition(new Product("Nabial", new BigDecimal("8"),BigDecimal.ZERO, Category.D), 2),
                new OrderPosition(new Product("Kielbasa", new BigDecimal("15"),BigDecimal.ZERO, Category.A), 10),
                new OrderPosition(new Product("Jajko", new BigDecimal("5"),BigDecimal.ZERO, Category.B), 5)
        );

        Optional<BigDecimal> op = orderPositions
                .stream()
                .filter(orderPosition -> orderPosition.getProduct().getCategory().equals(Category.A))
                .reduce((x,y) -> x.add.y);

Answer 1

映射到大的十进制流，然后缩小

BigDecimal op = orderPositions
                .stream()
                .filter(orderPosition -> Category.A.equals(orderPosition.getProduct().getCategory()))
.map(orderPosition1 -> orderPosition1.getProduct().getPrice())
.reduce(BigDecimal.ZERO, BigDecimal::add);

链接：https://mkyong.com/java8/java-8-how-to-sum-bigdecimal-using-stream/stream为空时，第一个值（bigdecimal.zero）是默认值

注意，为了避免nullPointerException我还将枚举的顺序改为相等