Facade pattern

The facade pattern is a software engineering design pattern commonly used with Object-oriented programming. The name is by analogy to an architectural facade.

A facade is an object that provides a simplified interface to a larger body of code, such as a class library. A facade can:

• make a software library easier to use, understand and test, since the facade has convenient methods for common tasks;
• make code that uses the library more readable, for the same reason;
• reduce dependencies of outside code on the inner workings of a library, since most code uses the facade, thus allowing more flexibility in developing the system;
• wrap a poorly-designed collection of APIs with a single well-designed API (as per task needs).

An Adapter is used when the wrapper must respect a particular interface and must support a polymorphic behavior. On the other hand, a facade is used when one wants an easier or simpler interface to work with.

Structure

Facade

The facade class abstracts Packages 1, 2, and 3 from the rest of the application.

Clients

The objects using the Facade Pattern to access resources from the Packages.

Example

This is an abstract example of how a client ("you") interacts with a facade (the "computer") to a complex system (internal computer parts, like CPU and HardDrive).

/* Complex parts */
 
class CPU {
    public void freeze() { ... }
    public void jump(long position) { ... }
    public void execute() { ... }
}
 
class Memory {
    public void load(long position, byte[] data) { ... }
}
 
class HardDrive {
    public byte[] read(long lba, int size) { ... }
}
 
/* Facade */
 
class Computer {
    private CPU cpu;
    private Memory memory;
    private HardDrive hardDrive;
 
    public Computer() {
        this.cpu = new CPU();
        this.memory = new Memory();
        this.hardDrive = new HardDrive();
    }
 
    public void startComputer() {
        cpu.freeze();
        memory.load(BOOT_ADDRESS, hardDrive.read(BOOT_SECTOR, SECTOR_SIZE));
        cpu.jump(BOOT_ADDRESS);
        cpu.execute();
    }
}
 
/* Client */
 
class You {
    public static void main(String[] args) {
        Computer facade = new Computer();
        facade.startComputer();
    }
}

External links

• Description from the Portland Pattern Repository
• Description from the Net Objectives Repository