【JavaSE专栏64】抽象类和接口，不能被实例化的类有什么用？

// 抽象类
abstract class Animal {
   // 抽象方法
   public abstract void sound();
   // 非抽象方法
   public void sleep() {
      System.out.println("Animal is sleeping.");
   }
}

// 子类
class Dog extends Animal {
   // 实现抽象方法
   public void sound() {
      System.out.println("Dog is barking.");
   }
}

// 子类
class Cat extends Animal {
   // 实现抽象方法
   public void sound() {
      System.out.println("Cat is meowing.");
   }
}

public class Main {
   public static void main(String[] args) {
      Animal dog = new Dog();
      dog.sound(); // 输出: Dog is barking.
      dog.sleep(); // 输出: Animal is sleeping.

      Animal cat = new Cat();
      cat.sound(); // 输出: Cat is meowing.
      cat.sleep(); // 输出: Animal is sleeping.
   }
}

// 定义一个 Shape 接口
interface Shape {
    double getArea();  // 获取形状的面积
    double getPerimeter();  // 获取形状的周长
}

// 实现 Shape 接口的圆形类
class Circle implements Shape {
    private double radius;
    
    public Circle(double radius) {
        this.radius = radius;
    }
    
    public double getArea() {
        return Math.PI * radius * radius;
    }
    
    public double getPerimeter() {
        return 2 * Math.PI * radius;
    }
}

// 实现 Shape 接口的矩形类
class Rectangle implements Shape {
    private double width;
    private double height;
    
    public Rectangle(double width, double height) {
        this.width = width;
        this.height = height;
    }
    
    public double getArea() {
        return width * height;
    }
    
    public double getPerimeter() {
        return 2 * (width + height);
    }
}

public class Main {
    public static void main(String[] args) {
        Circle circle = new Circle(5.0);
        System.out.println("圆形的面积：" + circle.getArea());
        System.out.println("圆形的周长：" + circle.getPerimeter());
        
        Rectangle rectangle = new Rectangle(3.0, 4.0);
        System.out.println("矩形的面积：" + rectangle.getArea());
        System.out.println("矩形的周长：" + rectangle.getPerimeter());
    }
}