外观模式-Facade

外观模式是为了解决类与类之间依赖关系的，外观模式将类间关系放在一个Facade类中，降低了类类之间的耦合度，该模式中不涉及接口

举一个经典的例子：

CPU类：

public class CPU {  
    public void startup(){  
        System.out.println("cpu startup");  
    }  
      
    public void shutdown(){  
        System.out.println("cpu shutdown");  
    }  
}  

Mainboard类：

public class Mainboard {  
      
    public void startup(){  
        System.out.println("mainboard startup");  
    }  
      
    public void shutdown(){  
        System.out.println("mainboard shutdown");  
    }  
} 

Disk类：

public class Disk {  
      
    public void startup(){  
        System.out.println("disk startup");  
    }  
      
    public void shutdown(){  
        System.out.println("disk shutdown");  
    }  
} 

Computer类（外观类）：

public class Computer {  
    private CPU cpu;  
    private Mainboard mainboard;  
    private Disk disk;  
      
    public Computer(){  
        cpu = new CPU();  
        mainboard = new Mainboard();  
        disk = new Disk();  
    }  
      
    public void startup(){  
        System.out.println("starting the computer");  
        cpu.startup();  
        mainboard.startup();  
        disk.startup();  
        System.out.println("computer is started");  
    }  
      
    public void shutdown(){  
        System.out.println("closing the computer");  
        disk.shutdown();  
        mainboard.shutdown();  
        cpu.shutdown();  
        System.out.println("computer is closed");  
    }  
} 

测试类：

public class Main {  

    public static void main(String[] args) {  
        Computer computer = new Computer();  
        computer.startup();  
        computer.shutdown();  
    }  
}

测试结果：

starting the computer
cpu startup
mainboard startup
disk startup
computer is started
closing the computer
disk shutdown
mainboard shutdown
cpu shutdown
computer is closed

如果我们没有外观类（Computer），CPU、Mainboard、Disk之间将会相互持有实例对象，造成严重的依赖；而有了Computer，他们之间的关系被放在了Computer类里，这样就起到了解耦的作用。这就是外观模式！