Facade

The Goal

Hide a complex, ugly system behind a simple, clean method call.

Imagine you are placing an online order. Behind the scenes, the system has to check inventory, charge a credit card, and generate a shipping label.

• You don’t want the client application to have to orchestrate all of that. You build a Facade to wrap it structurally.

1. THE COMPLEX SUBSYSTEM

public class InventoryService {
    public boolean checkStock(String item) { 
    return true; 
    }
}

public class PaymentGateway {
    public void chargeCard(double amount) {
    System.out.println("Card charged."); 
    }
}

public class ShippingService {
    public void dispatch(String item) {
    System.out.println("Item shipped."); 
     }
}

2. THE STRUCTURAL WRAPPER (Facade)

public class OrderProcessingFacade {
    private final InventoryService inventory = new InventoryService();
    private final PaymentGateway payment = new PaymentGateway();
    private final ShippingService shipping = new ShippingService();

    // The client only calls this ONE simple method
    public void placeOrder(String item, double price) {
        if (inventory.checkStock(item)) {
            payment.chargeCard(price);
            shipping.dispatch(item);
            System.out.println("Order complete!");
        }
    }
}

Why this is Structural

• Look at the subsystem classes (InventoryService, PaymentGateway, etc.).

• They don’t have an OrderProcessingFacade passed into their constructors.

• They don’t know the Facade exists.

• The Facade is just a structural container built on top of them to make the client’s life easier.

• Communication flows in one direction: Top-Down.