// 抽象类：被abstract修饰的类
public abstract class Shape {
// 抽象方法：被abstract修饰的方法，没有方法体
abstract public void draw();
abstract void calcArea();
// 抽象类也是类，也可以增加普通方法和属性
public double getArea(){
  return area;
}
protected double area; // 面积
}

 public abstract class Shape {
        abstract public void draw();
        abstract void calcArea();
        public double getArea(){
             return area;
        }
        protected double area; // 面积
 }
// 矩形类
public class Rect extends Shape {
  private double length;
  private double width;
  Rect(double length, double width){
    this.length = length;
    this.width = width;
  }
  public void draw(){
    System.out.println("矩形: length= "+length+" width= " + width);
  }
  public void calcArea(){
    area = length * width;
  }
}
// 圆类：
public class Circle extends Shape{
  private double r;
  final private static double PI = 3.14;
  public Circle(double r){
    this.r = r;
  }
  public void draw(){
    System.out.println("圆：r = "+r);
  }
  public void calcArea(){
    area = PI * r * r;
  }
}
// 三角形类：
public abstract class Triangle extends Shape {
  private double a;
  private double b;
  private double c;
@Override
  public void draw() {
    System.out.println("三角形：a = "+a + " b = "+b+" c = "+c);
  }
//@Override
//double calcArea(); // 编译失败：要么实现该抽象方法，要么将三角形设计为抽象类
}

public interface 接口名称{
// 抽象方法
  public abstract void method1(); // public abstract 是固定搭配，可以不写
  public void method2();
  abstract void method3();
  void method4();
// 注意：在接口中上述写法都是抽象方法，跟推荐方式4，代码更简洁
}

public class 类名称 implements 接口名称{
  // ...
}

// 测试类：测试代码正确性
public class TestUSB {
    public static void main(String[] args) {
        Computer computer = new Computer();
        computer.powerOn();
// 使用鼠标设备
        computer.useDevice(new Mouse());
// 使用键盘设备
        computer.useDevice(new KeyBoard());
        computer.powerOff();
    }
}

// USB接口
public interface USB {
    void openDevice();
    void closeDevice();
}

// 笔记本类：使用USB设备
public class Computer {
    public void powerOn(){
        System.out.println("打开笔记本电脑");
    }
    public void powerOff(){
        System.out.println("关闭笔记本电脑");
    }
    public void useDevice(USB usb){
        usb.openDevice();
        if(usb instanceof Mouse){
            Mouse mouse = (Mouse)usb;
            mouse.click();
        }else if(usb instanceof KeyBoard){
            KeyBoard keyBoard = (KeyBoard)usb;
            keyBoard.inPut();
        }
        usb.closeDevice();
    }
}

// 鼠标类，实现USB接口
public class Mouse implements USB {
    @Override
    public void openDevice() {
        System.out.println("打开鼠标");
    }
    @Override
    public void closeDevice() {
        System.out.println("关闭鼠标");
    }
    public void click(){
        System.out.println("鼠标点击");
    }
}

// 键盘类，实现USB接口
public class KeyBoard implements USB {
    @Override
    public void openDevice() {
        System.out.println("打开键盘");
    }
    @Override
    public void closeDevice() {
        System.out.println("关闭键盘");
    }
    public void inPut(){
        System.out.println("键盘输入");
    }
}

public interface USB {
  double brand = 3.0; // 默认被：final public static修饰
  void openDevice();
  void closeDevice();
}

public class TestUSB {
  public static void main(String[] args) {
    System.out.println(USB.brand); // 可以直接通过接口名访问，说明是静态的
    // 编译报错： 无法为最终变量brand分配值
    USB.brand = 2.0; // 说明brand具有final属性
  }
}

public interface USB {
  // 编译失败
  public USB(){
  }
  {} // 编译失败
  void openDevice();
  void closeDevice();
}