What is the facade design pattern?

Is facade a class which contains a lot of other classes?

What makes it a design pattern? To me, it is like a normal class.

Can you explain to me this Facade pattern?

Solution 1:

A design pattern is a common way of solving a recurring problem. Classes in all design patterns are just normal classes. What is important is how they are structured and how they work together to solve a given problem in the best possible way.

The Facade design pattern simplifies the interface to a complex system; because it is usually composed of all the classes which make up the subsystems of the complex system.

A Facade shields the user from the complex details of the system and provides them with a simplified view of it which is easy to use. It also decouples the code that uses the system from the details of the subsystems, making it easier to modify the system later.

http://www.dofactory.com/Patterns/PatternFacade.aspx

http://www.blackwasp.co.uk/Facade.aspx

Also, what is important while learning design patterns is to be able to recognize which pattern fits your given problem and then using it appropriately. It is a very common thing to misuse a pattern or trying to fit it to some problem just because you know it. Be aware of those pitfalls while learning\using design patterns.

Solution 2:

Wikipedia has a great example of Facade pattern.

/* 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 ComputerFacade {
    private CPU processor;
    private Memory ram;
    private HardDrive hd;

    public ComputerFacade() {
        this.processor = new CPU();
        this.ram = new Memory();
        this.hd = new HardDrive();
    }

    public void start() {
        processor.freeze();
        ram.load(BOOT_ADDRESS, hd.read(BOOT_SECTOR, SECTOR_SIZE));
        processor.jump(BOOT_ADDRESS);
        processor.execute();
    }
}

/* Client */

class You {
    public static void main(String[] args) {
        ComputerFacade computer = new ComputerFacade();
        computer.start();
    }
}