The Facade design pattern is often used when a system is very complex or difficult to understand because the system has a large number of interdependent classes or its source code is unavailable. This pattern hides the complexities of the larger system and provides a simpler interface to the client.
Intent
- Provide a unified interface to a set of interfaces in a subsystem. Facade defines a higher-level interface that makes the subsystem easier to use.
- Wrap a complicated subsystem with a simpler interface.
Implementation
Class Diagram of Facade Pattern
This is an abstract example of how a client interacts with a facade (the "computer") to a complex system (internal computer parts, like CPU and HardDrive).
// CPU.java
class CPU {
public void freeze() {
System.out.println("CPU: freezing...");
}
public void jump(long position) {
System.out.println("CPU: jumping...");
}
public void execute() {
System.out.println("CPU: executing...");
}
}
// HardDrive.java
public class HardDrive {
public byte[] read(long lba, int size) {
System.out.println("HardDrive: reading...");
return null;
}
}
// Memory.java
public class Memory {
public void load(long position, byte[] data) {
System.out.println("Memory: loading... " );
}
}
// ComputerFacade.java
public 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();
}
// Dummy values for simplicity
static long BOOT_ADDRESS = 0;
static long BOOT_SECTOR = 0;
static int SECTOR_SIZE = 0;
}
// ClientMain.java
public class ClientMain {
public static void main(String[] args) {
ComputerFacade computer = new ComputerFacade();
computer.start();
}
}
Output:
CPU: freezing...
HardDrive: reading...
Memory: loading...
CPU: jumping...
CPU: executing...
