设计模式——装饰器模式

本质：装饰器模式可以在不使用继承的前提下，对于一个对象添加新的功能。是不是太像iOS的catagory了？

最大的好处，当然是不使用继承的好处。（高耦合）

看看代码：

我们将创建一个Shape接口和实现了Shape接口的实体类。然后我们创建一个实现了Shape接口的抽象装饰类ShapeDecorator，并把Shape对象作为它的实例变量。

RedShapeDecorator是实现了ShapeDecorator的实体类。

DecoratorPatternDemo，我们的演示类使用RedShapeDecorator来装饰Shape对象。

创建一个接口

Shape.java

public interface Shape {
   void draw();
}

创建实现接口的实体类：

Rectangle.java

public class Rectangle implements Shape {

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

Circle.java

public class Circle implements Shape {

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

创建实现了Shape接口的抽象装饰类，ShapeDecorator.java，注意抽象装饰类，既实现了Shape接口，又包含一个Shape类型的属性，前者为了扩展Shape的方法，后者为了通过组合的方式将Shape引入。

public abstract class ShapeDecorator implements Shape {
   protected Shape decoratedShape;

   public ShapeDecorator(Shape decoratedShape){
      this.decoratedShape = decoratedShape;
   }

   public void draw(){
      decoratedShape.draw();
   }    
}

创建扩展了ShapeDecorator类的实体装饰类，RedShapeDecorator.java

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");
   }
}

使用RedShapeDecorator来装饰Shape对象，DecoratorPatternDemo.java

public class DecoratorPatternDemo {
   public static void main(String[] args) {

      Shape circle = new Circle();

      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();
   }
}

验证输出：

Circle with normal border
Shape: Circle

Circle of red border
Shape: Circle
Border Color: Red

Rectangle of red border
Shape: Rectangle
Border Color: Red