Structural Patterns - Facade Pattern

Facade as the name suggests means the face of the building. The people walking past the road can only see this glass face of the building. They do not know anything about it, the wiring, the pipes and other complexities. The face hides all the complexities of the building and displays a friendly face.

This is how facade pattern is used. It hides the complexities of the system and provides an interface to the client from where the client can access the system. In Java, the interface JDBC can be called a facade. We as users or clients create connection using the "java.sql.Connection" interface, the implementation of which we are not concerned about. The implementation is left to the vendor of driver.

Let's try and understand the facade pattern better using a simple example. Let's consider a store. This store has a store keeper. In the storage, there are a lot of things stored e.g. packing material, raw material and finished goods.

You, as client want access to different goods. You do not know where the different materials are stored. You just have access to store keeper who knows his store well. Whatever you want, you tell the store keeper and he takes it out of store and hands it over to you on showing him the credentials. Here, the store keeper acts as the facade, as he hides the complexities of the system Store.

Let us see how the Store example works.

Store.java

package structural.facade; public interface Store {
	public Goods getGoods();
}// End of interface

The store can very well be an interface. This only returns Goods. The goods are of three types as discussed earlier in this document. RawMaterialGoods, FinishedGoods and PackagingMaterialsGoods. All these classes can implement the Goods interface.

Similarly, the stores are of three types and can implement the Store interface. Let's have a look at the code for one of the stores.

FinishedGoodsStore.java

package structural.facade; public class FinishedGoodsStore implements Store {
	public Goods getGoods() { FinishedGoods finishedGoods = new FinishedGoods(); return finishedGoods; }
}// End of class

Now let's consider the facade StoreKeeper.
StoreKeeper.java

package structural.facade; public class StoreKeeper {
	/** * The raw materials are asked for and * are returned * * @return raw materials */ public RawMaterialGoods getRawMaterialGoods() { RawMaterialStore store = new RawMaterialStore(); RawMaterialGoods rawMaterialGoods = (RawMaterialGoods)store.getGoods(); return rawMaterialGoods; } /** * The packaging materials are asked for and * are returned * * @return packaging materials */ public PackingMaterialGoods getPackingMaterialGoods() { PackingMaterialStore store = new PackingMaterialStore(); PackingMaterialGoods packingMaterialGoods = (PackingMaterialGoods)store.getGoods(); return packingMaterialGoods; } /** * The finished goods are asked for and * are returned * * @return finished goods */ public FinishedGoods getFinishedGoods() { FinishedGoodsStore store = new FinishedGoodsStore(); FinishedGoods finishedGoods = (FinishedGoods)store.getGoods(); return finishedGoods; }
}// End of class

This is clear that the complex implementation will be done by StoreKeeper himself. The client will just access the StoreKeeper and ask for either finished goods, packaging material or raw material.
How will the client program access this façade? Here is a simple code.

Client.java

package structural.facade; public class Client {
	/** * to get raw materials */ public static void main(String[] args) { StoreKeeper keeper = new StoreKeeper(); RawMaterialGoods rawMaterialGoods = keeper.getRawMaterialGoods(); }
}// End of class

In this way the implementation is left to the façade. The client is given just one interface and can access only that. This hides all the complexities.

There is another way of implementing this. We can have just one method in our StoreKeeper class getGoods(String goodsType).

Another version of StoreKeeper method is here.

StoreKeeper.java

package structural.facade; public class StoreKeeper {
	/** * The common method * * @return Goods */ public Goods getGoods(String goodsType) { if (goodsType.equals("Packaging")) { PackingMaterialStore store = new PackingMaterialStore(); PackingMaterialGoods packingMaterialGoods = (PackingMaterialGoods)store.getGoods(); return packingMaterialGoods; } else if (goodsType.equals("Finished")) { FinishedGoodsStore store = new FinishedGoodsStore(); FinishedGoods finishedGoods = (FinishedGoods)store.getGoods(); return finishedGoods; } else { RawMaterialStore store = new RawMaterialStore(); RawMaterialGoods rawMaterialGoods = (RawMaterialGoods)store.getGoods(); return rawMaterialGoods; }
}// End of class

The client program can now create an object of StoreKeeper class and call method getGoods() passing as parameter the type of goods required. This can be done as follows.
new StoreKeeper().getGoods("RawMaterials");

In this case, the type-casting ill be needed on client side to narrow down Goods to RawMaterialsGoods.

All in all, the Façade pattern hides the complexities of system from the client and provides a simpler interface. Looking from other side, the facade also provides the implementation to be changed without affecting the client code.