设计模式之装饰器模式

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