深入理解Java POJO：业务逻辑的边界与应用

import java.math.BigDecimal;
import java.time.LocalDateTime;
import java.util.ArrayList;
import java.util.List;
import java.util.Objects;

public class Order {
    private Long id;
    private String customerId;
    private LocalDateTime orderDate;
    private OrderStatus status;
    private List<OrderItem> items;
    private BigDecimal totalAmount;

    public Order(Long id, String customerId, LocalDateTime orderDate) {
        this.id = id;
        this.customerId = customerId;
        this.orderDate = orderDate;
        this.status = OrderStatus.PENDING; // 初始状态
        this.items = new ArrayList<>();
        this.totalAmount = BigDecimal.ZERO;
    }

    // Getter methods
    public Long getId() { return id; }
    public String getCustomerId() { return customerId; }
    public LocalDateTime getOrderDate() { return orderDate; }
    public OrderStatus getStatus() { return status; }
    public List<OrderItem> getItems() { return new ArrayList<>(items); } // 返回副本以保证不可变性
    public BigDecimal getTotalAmount() { return totalAmount; }

    // Business logic methods
    public void addItem(OrderItem item) {
        Objects.requireNonNull(item, "Order item cannot be null");
        if (status != OrderStatus.PENDING) {
            throw new IllegalStateException("Cannot add items to an order that is not PENDING.");
        }
        this.items.add(item);
        this.totalAmount = this.totalAmount.add(item.getPrice().multiply(BigDecimal.valueOf(item.getQuantity())));
    }

    public void placeOrder() {
        if (status != OrderStatus.PENDING) {
            throw new IllegalStateException("Only PENDING orders can be placed.");
        }
        if (items.isEmpty()) {
            throw new IllegalStateException("Cannot place an empty order.");
        }
        this.status = OrderStatus.PLACED;
        // 可以在这里触发事件通知或进行其他业务操作
        System.out.println("Order " + id + " placed successfully.");
    }

    public void cancelOrder() {
        if (status == OrderStatus.DELIVERED || status == OrderStatus.CANCELLED) {
            throw new IllegalStateException("Cannot cancel an order that is DELIVERED or already CANCELLED.");
        }
        this.status = OrderStatus.CANCELLED;
        // 可以在这里处理退款逻辑等
        System.out.println("Order " + id + " cancelled.");
    }

    // Enum for Order Status
    public enum OrderStatus {
        PENDING, PLACED, SHIPPED, DELIVERED, CANCELLED
    }

    // Nested POJO for OrderItem
    public static class OrderItem {
        private String productId;
        private int quantity;
        private BigDecimal price;

        public OrderItem(String productId, int quantity, BigDecimal price) {
            this.productId = productId;
            this.quantity = quantity;
            this.price = price;
        }

        // Getter methods for OrderItem
        public String getProductId() { return productId; }
        public int getQuantity() { return quantity; }
        public BigDecimal getPrice() { return price; }
    }
}

import java.time.LocalDate;

public record Employee(String firstName, String lastName, LocalDate hired) {
    // Records可以有紧凑构造函数或自定义方法，但通常用于简洁的数据持有
    public String getFullName() {
        return firstName + " " + lastName;
    }
}