Adding BigDecimal in Stream using reduce method

Task: count the sum of products prices from the List<OrderPosition> orderPositions that have category A.

Goal: to practice the .reduce method.

There are 3 classes:

• Category enum (A, B, C, D),
• Product (String name, BigDecimal price, BigDecimal discount, Category category)
• OrderPosition - quantity of the product

Problem: class App1 stream op

Code below

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);

Map to big decimal stream then reduce

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

Link: https://mkyong.com/java8/java-8-how-to-sum-bigdecimal-using-stream/ The first value(BigDecimal.ZERO) is the default value in case stream is empty

Note that I also change to equal order of the Enum, to avoid nullPointerException