Decorator Pattern in Design Pattern

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Let's begin to learn what's Decorator Pattern.

装饰器模式

Overview

装饰器模式（Decorator Pattern）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。

这种类型的设计模式属于结构型模式，它是作为现有的类的一个包装。

这种模式创建了一个装饰类，用来包装原有的类，并在保持类方法签名完整性的前提下，提供了额外的功能。

主要解决

一般的，我们为了扩展一个类经常使用继承方式实现，由于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。

何时使用

在不想增加很多子类的情况下扩展类。

应用实例

  • 给画，添加上画框

优点

  • 装饰类和被装饰类可以独立发展，不会相互耦合，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。

实现

Decorator Pattern

Shape

Shape Interface

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package individual.cy.learn.pattern.structural.decorator;

/**
 * @author mystic
 */
public interface Shape {
    /**
     * to draw a geometric shape
     */
    void draw();
}

Circle

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package individual.cy.learn.pattern.structural.decorator;

/**
 * @author mystic
 */
public class Circle implements Shape {
    @Override
    public void draw() {
        System.out.println("Circle.draw");
    }
}

Rectangle

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package individual.cy.learn.pattern.structural.decorator;

/**
 * @author mystic
 */
public class Rectangle implements Shape {
    @Override
    public void draw() {
        System.out.println("Rectangle.draw");
    }
}

Decorator

AbstractShapeDecorator

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package individual.cy.learn.pattern.structural.decorator;

/**
 * @author mystic
 */
public abstract class AbstractShapeDecorator implements Shape {
    protected Shape decoratedShape;

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

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

RedShapeDecorator

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package individual.cy.learn.pattern.structural.decorator;

/**
 * @author mystic
 */
public class RedShapeDecorator extends AbstractShapeDecorator {
    public RedShapeDecorator(Shape decoratedShape) {
        super(decoratedShape);
    }

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

    private void setRedBorder(Shape decoratedShape) {
        System.out.println("RedShapeDecorator.setRedBorder");
    }
}

VioletShapeDecorator

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package individual.cy.learn.pattern.structural.decorator;

/**
 * @author mystic
 */
public class VioletShapeDecorator extends AbstractShapeDecorator {
    public VioletShapeDecorator(Shape decoratedShape) {
        super(decoratedShape);
    }

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

    public void setVioletBorder(Shape decoratedShape) {
        System.out.println("VioletShapeDecorator.setVioletBorder");
    }
}

Tester

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package individual.cy.learn.pattern.structural.decorator;

/**
 * @author mystic
 */
public class DecoratorPatternTester {
    public static void main(String[] args) {
        Shape circle = new Circle();
        Shape redCircle = new RedShapeDecorator(new Circle());
        Shape redRectangle = new RedShapeDecorator(new Rectangle());
        Shape violetCircle = new VioletShapeDecorator(new Circle());
        Shape violetRectangle = new VioletShapeDecorator(new Rectangle());
//        Shape redCircle = new RedShapeDecorator(Circle::new);
//        Shape redRectangle = new RedShapeDecorator(Rectangle::new);
//        Shape violetCircle = new VioletShapeDecorator(Circle::new);
//        Shape violetRectangle = new VioletShapeDecorator(Rectangle::new);
        circle.draw();
        redCircle.draw();
        redRectangle.draw();
        violetCircle.draw();
        violetRectangle.draw();
    }
}
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Circle.draw
Circle.draw
RedShapeDecorator.setRedBorder
Rectangle.draw
RedShapeDecorator.setRedBorder
Circle.draw
VioletShapeDecorator.setVioletBorder
Rectangle.draw
VioletShapeDecorator.setVioletBorder

如果执行注释的代码,即使用::new创建对象,则输出

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Circle.draw
RedShapeDecorator.setRedBorder
RedShapeDecorator.setRedBorder
VioletShapeDecorator.setVioletBorder
VioletShapeDecorator.setVioletBorder