使用多态

interface Usb {
    abstract public void on();

    abstract public void off();
}

class Mouse implements Usb {
    public void on() {
        System.out.println("鼠标上电 23v");
    }

    public void off() {
        System.out.println("鼠标关电 23v");
    }
}

class Keyback implements Usb {
    public void on() {
        System.out.println("键盘上电 223v");
    }

    public void off() {
        System.out.println("键盘关电 223v");
    }
}

class Light implements Usb {
    public void on() {
        System.out.println("灯上电 223v");
    }

    public void off() {
        System.out.println("灯关电 223v");
    }
}

class Notebook {
    public void shibieUsb(Usb u) {
        u.on();
    }

    public void run(Mouse m, Keyback k, Light l) {
        shibieUsb(m);
        shibieUsb(k);
        shibieUsb(l);
        System.out.println("电脑可以运行起来了");
    }
}

class Demo1 {
    public static void main(String[] args) {
        Notebook n = new Notebook();
        n.run(new Mouse(), new Keyback(), new Light());

    }
}

这应征了多态的三个特性：
引用名是父类或者接口，子类对它们进行了重写。父类，接口的引用指向子类。