🚦 Law of Demeter (LoD): “Don’t talk to strangers.”

// Too much knowledge about the object graph
String city = order.getCustomer().getAddress().getCity();
shippingService.shipTo(city);

// Ask the object that knows
shippingService.shipTo(order.shippingCity());

class Order {
    private Customer customer;
    public String shippingCity() {
        return customer.shippingCity();
    }
}

class Customer {
    private Address address;
    public String shippingCity() {
        return address.getCity();
    }
}

class Address {
    private final String city;
    public Address(String city) { this.city = city; }
    public String getCity() { return city; }
}

boolean hasLowStock = cart.getItems().stream()
    .map(Item::getProduct)
    .map(Product::getStock)
    .anyMatch(Stock::isLow);

if (cart.hasLowStockItems()) { /* ... */ }

class Cart {
    private List<Item> items;
    public boolean hasLowStockItems() {
        return items.stream().anyMatch(i -> i.getProduct().getStock().isLow());
    }
}