四、装饰者模式

一、模式定义

  在不改变原有对象的前提下（不改变ConcreteComponent情况下），给原有对象扩展功能（利用ConcreteDecorator扩展功能，在ConcreteDecorator中注入ConcreteComponent）。如下所示：

二、应用场景

  扩展一个类的功能，或者给一个类添加一些附加职责

三、优点：

①、灵活改变组合；
②、符合开闭原则；

四、装饰者模式的实现方式

4.1、方式1

package decorator;

public class DecoratorDesignPattern {
   public static void main(String[] args) {
      Decorator decoratorA = new ConcreteDecoratorA(new ConcreteComponent());
      decoratorA.operation();

      Decorator decoratorB = new ConcreteDecoratorB(new ConcreteDecoratorA(new ConcreteComponent()));
      decoratorB.operation();
   }
}

interface Component{
   void operation();
}

class ConcreteComponent implements Component{

   @Override
   public void operation() {
      System.out.println("拍照...\n");
   }
}

abstract class Decorator implements Component{
   Component component;

   public Decorator(Component component) {
      this.component = component;
   }
}

class ConcreteDecoratorA extends Decorator{

   public ConcreteDecoratorA(Component component) {
      super(component);
   }

   @Override
   public void operation() {
      System.out.println("添加美颜....");
      component.operation();
   }
}

class ConcreteDecoratorB extends Decorator{
   public ConcreteDecoratorB(Component component) {
      super(component);
   }

   @Override
   public void operation() {
      System.out.println("添加滤镜");
      component.operation();
   }
}

上面代码的运行结果如下：

4.2、方式2

package decorator;

public class DecoratorPattern {
   public static void main(String[] args) {
      Circle circle = new Circle();
      RedShapeDecorator redCircle = new RedShapeDecorator(circle);
      Rectangle rectangle = new Rectangle();
      RedShapeDecorator redRectangle = new RedShapeDecorator(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();

   }
}


interface Shape {
   void draw();
}

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

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


abstract class ShapeDecorator implements Shape {
   public Shape decoratedShape;

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

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

class RedShapeDecorator extends ShapeDecorator {

   public RedShapeDecorator(Shape decoratorShape) {
      super(decoratorShape);
   }

   @Override
   public void draw() {
      decoratedShape.draw();
      setRedBorder(decoratedShape);

   }

   private void setRedBorder(Shape decoratedShape){
      System.out.println("Border Color: Red");
   }
}

上面代码的运行结果如下：

五、装饰者模式的应用

  Java IO 库采用了装饰器模式（Decorator Pattern）和适配器模式（Adapter Pattern）的组合设计模式，其中InputStream是装饰器模式中顶层的抽象类，FilterInputStream是装饰器基类，BufferedInputStream是带有缓冲区的装饰器类，ObjectInputStream是可以读取对象的装饰器类，SequenceInputStream是可以顺序读取多个输入Stream的装饰器类，FileInputStream、ByteArrayInputStream则是被装饰的类。 这些类的UML图，如下所示：