1 装饰器模式

装饰器模式允许向一个现有的对象添加新的功能，同时又不改变其结构，它是作为现有类的一个包装。

2 类图实现

这里最关键的是我们装饰器类（抽象类）既实现了原始功能接口（Shape接口）又和原始接口是关联关系（Shape接口作为这个抽象装饰类的成员变量）

3 代码实现

    public interface Shape {
       void draw();
    }

    public class Rectangle implements Shape {
     
       @Override
       public void draw() {
          System.out.println("Shape: Rectangle");
       }
    }

    public class Circle implements Shape {
     
       @Override
       public void draw() {
          System.out.println("Shape: Circle");
       }
    }

    public abstract class ShapeDecorator implements Shape {
       protected Shape decoratedShape;
     
       public ShapeDecorator(Shape decoratedShape){
          this.decoratedShape = decoratedShape;
       }
     
       public void draw(){
          decoratedShape.draw();
       }  
    }

    public class RedShapeDecorator extends ShapeDecorator {
     
       public RedShapeDecorator(Shape decoratedShape) {
          super(decoratedShape);     
       }
     
       @Override
       public void draw() {
          decoratedShape.draw();         
          setRedBorder(decoratedShape);
       }
     
       private void setRedBorder(Shape decoratedShape){
          System.out.println("Border Color: Red");
       }
    }

    public class DecoratorPatternDemo {
       public static void main(String[] args) {
     
          Shape circle = new Circle();
          ShapeDecorator redCircle = new RedShapeDecorator(new Circle());
          ShapeDecorator redRectangle = new RedShapeDecorator(new Rectangle());
          //Shape redCircle = new RedShapeDecorator(new Circle());
          //Shape redRectangle = new RedShapeDecorator(new Rectangle());
          System.out.println("Circle with normal border");
          circle.draw();
     
          System.out.println("\nCircle of red border");
          redCircle.draw();
     
          System.out.println("\nRectangle of red border");
          redRectangle.draw();
       }
    }

 
4 运行结果

    Circle with normal border
    Shape: Circle
     
    Circle of red border
    Shape: Circle
    Border Color: Red
     
    Rectangle of red border
    Shape: Rectangle
    Border Color: Red

 
 
5 优点

1）继承的缺点：如果我们用继承的方式增加新功能，继承为类引入静态特征，并且随着扩展功能的增多，子类会很膨胀

2）装饰器的有点：装饰类和被装饰类可以独立发展，不相互耦合，装饰模式是继承的一个替代模式，装饰模式可以动态扩展一个实现类的功能

部分参考：https://www.runoob.com/design-pattern/decorator-pattern.html