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
构造外观模式
举一个经典的例子：
- CPU
public class CPU {
public void startup(){
System.out.println("cpu startup");
}
public void shutdown(){
System.out.println("cpu shutdown");
}
}
- Mainboard
public class Mainboard {
public void startup(){
System.out.println("mainboard startup");
}
public void shutdown(){
System.out.println("mainboard shutdown");
}
}
- Disk
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
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
分析
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！
