《设计模式(三)之——装饰者模式》

一、情景：

老王在卖的煎饼摊子开张了，主营产品有煎饼、鸡蛋、火腿。 问题：1、有顾客想买加了鸡蛋的煎饼、或者加了火腿、鸡蛋的煎饼，那么该如何改造呢？

二、情景后续

设计模式大佬、推荐他用装饰者模式，进行包装、包装出带有鸡蛋的煎饼

三、装饰者模式（Decorator）

定义：动态的将责任附加到对象上若老扩展功能，装饰者提供老比继承更有弹性的替代方案重要组件Component 被包装的原始组件Decorator 实现与原始组件相同的接口，并且组合了一个原始组件的引用

四、代码演示

4.1类图

4.2 煎饼、鸡蛋、火腿公共接口(Component)

/** * @author liuzihao * @create 2020-10-07-11:45 * 煎饼摊子基本接口 */ public interface JbComponent { /** * 描述 * @return */ public String desc(); /** * 价格 * @return */ public BigDecimal price(); }

4.3 各自的实现(ConcreteComponent)

/** * @author liuzihao * @create 2020-10-07-11:48 * 鸡蛋 */ public class JbComponenteEgg implements JbComponent { @Override public String desc() { return "鸡蛋"; } @Override public BigDecimal price() { return new BigDecimal("1"); } } /** * @author liuzihao * @create 2020-10-07-11:47 */ public class JbComponentPancakes implements JbComponent{ @Override public String desc() { return "煎饼"; } @Override public BigDecimal price() { return new BigDecimal(2); } } /** * @author liuzihao * @create 2020-10-07-11:48 */ public class JbComponenthHotDog implements JbComponent { @Override public String desc() { return "火腿"; } @Override public BigDecimal price() { return new BigDecimal(1); } }

4.4 装饰者——带有鸡蛋煎饼(ConcreteDecoratoerA)

/** * @author liuzihao * @create 2020-10-07-11:51 */ public class DecoratorPancakesWithEgg implements JbComponent { private JbComponent jbComponent; public DecoratorPancakesWithEgg(JbComponent jbComponent) { this.jbComponent = jbComponent; } @Override public String desc() { return jbComponent.desc() + "\t" + "加鸡蛋"; } @Override public BigDecimal price() { return jbComponent.price().add(BigDecimal.ONE); } }

4.5 装饰者——带有鸡蛋、火腿的煎饼(ConcreteDecoratoerB)

/** * @author liuzihao * @create 2020-10-07-11:54 */ public class DecoratorPancakesWithHotDog implements JbComponent{ // 传入已经加了鸡蛋的煎饼 private JbComponent jbComponent; public DecoratorPancakesWithHotDog(JbComponent jbComponent) { this.jbComponent = jbComponent; } @Override public String desc() { return jbComponent.desc() +"\t" + "加热狗"; } @Override public BigDecimal price() { return jbComponent.price().add(BigDecimal.ONE); } }

4.6 代码测试

public class DemoMain { public static void main(String[] args) { JbComponent jbComponent1 = new JbComponenthHotDog(); System.out.println("卖" + jbComponent1.desc() + "\t 价格" +jbComponent1.price()); JbComponent jbComponent2 = new JbComponenteEgg(); System.out.println("卖" + jbComponent2.desc() + "\t 价格" +jbComponent2.price()); JbComponent jbComponent3 = new JbComponentPancakes(); System.out.println("卖" + jbComponent3.desc() + "\t 价格" +jbComponent3.price()); System.out.println("==开始卖包装产品=="); JbComponent jbComponent4 = new DecoratorPancakesWithEgg(jbComponent3); System.out.println("卖" + jbComponent4.desc() + "\t 价格" +jbComponent4.price()); JbComponent jbComponent5 = new DecoratorPancakesWithEgg(jbComponent4); System.out.println("卖" + jbComponent5.desc() + "\t 价格" +jbComponent5.price()); } }

五、小结

5.1改造之后的煎饼摊子

5.2总结

装饰者模式同继承一样，可以扩展行为，这符合开闭原则，但是有一个不好的地方就是这样会到账设计中出现很多小对象，程序的复杂性会增加。装饰者可以不断的叠加，例如上述案例中，可以用完包装完鸡蛋后，继续包装鸡蛋、不断重复包装行为。