设计模式——装饰者模式 Java源代码

最后更新于:2022-04-01 20:02:06

装饰者模式,可以动态地把职责附加到已有的对象上面去。又称 Wrapper Pattern,在代码实现的时候,确实有“包装”的意思。 # 类图 ![装饰者](https://docs.gechiui.com/gc-content/uploads/sites/kancloud/2016-02-22_56cab10d6335a.jpg "") 图:来自《Head First Design Patterns》 可以看出,装饰者模式里面,有4个角色:Component抽象类,ConcreteComponent具体类,Decorator抽象类,ConcreteDecorator具体类 ![装饰者模式](https://docs.gechiui.com/gc-content/uploads/sites/kancloud/2016-02-22_56cab10d8b732.jpg "") 图:我的Java源代码的类图 总共10个类 一个Component抽象类 两个ConcreteComponent具体类 一个Decorator抽象类 五个ConcreteDecorator具体类 一个Main测试类 ConcreteDecorator **继承**了Decorator抽象类, **组合**了Component抽象类。Decorator抽象类继承了Component抽象类,这个继承的目的是:it’s vital that the decorators have the same type as the object they are going to decorate。简而言之,就是为了获得相同的类型,并不是为了获得行为。组合是为了让所有的concreteDecorator可以“互相包装、装饰” # talk is cheap, show me the code 去食堂打菜,经济套餐,假设总共只有5种食物可供选择分别是:手撕包菜,茄子豆角,麻婆豆腐,青椒肉丝,红烧里脊。 ### 一个Component抽象类 ~~~ package decorator; public abstract class Meal { String description = "Unknown Meal"; public String getDescription() { return description; } public abstract double cost(); } ~~~ ### 两个ConcreteComponent具体类 ~~~ package decorator; public class ChineseMeal extends Meal { public ChineseMeal() { description = "Chinese Meal"; } @Override public double cost() { return 1.0; // 一次性餐具费用 } } ~~~ ~~~ package decorator; public class WesternMeal extends Meal { public WesternMeal() { description = "Western meal"; } @Override public double cost() { return 5.0; // 不锈钢刀、叉费用 } } ~~~ ### 一个Decorator抽象类 ~~~ package decorator; public abstract class Decorator extends Meal { public abstract String getDescription(); } ~~~ ### 五个ConcreteDecorator具体类 ~~~ package decorator; // 茄子豆角 public class EggplantBean extends Decorator { Meal meal; public EggplantBean(Meal meal) { this.meal = meal; } @Override public String getDescription() { return meal.getDescription() + ", 茄子豆角"; } @Override public double cost() { return 2 + meal.cost(); } } ~~~ ~~~ package decorator; // 青椒炒肉 public class GreenPepperPork extends Decorator { Meal meal; public GreenPepperPork(Meal meal) { this.meal = meal; } @Override public String getDescription() { return meal.getDescription() + ", 青椒炒肉"; } @Override public double cost() { return 3 + meal.cost(); } } ~~~ ~~~ package decorator; // 手撕包菜 public class HandCabbage extends Decorator { Meal meal; public HandCabbage(Meal meal) { this.meal = meal; } @Override public String getDescription() { return meal.getDescription() + ", 手撕包菜"; } @Override public double cost() { return 2 + meal.cost(); } } ~~~ ~~~ package decorator; // 麻婆豆腐 public class MapoTofu extends Decorator { Meal meal; public MapoTofu(Meal meal) { this.meal = meal; } @Override public String getDescription() { return meal.getDescription() + ", 麻婆豆腐"; } @Override public double cost() { return 4 + meal.cost(); } } ~~~ ~~~ package decorator; // 红烧里脊 public class BraiseTenderloin extends Decorator { Meal meal; public BraiseTenderloin(Meal meal) { this.meal = meal; } @Override public String getDescription() { return meal.getDescription() + ", 红烧里脊"; } @Override public double cost() { return 5 + meal.cost(); } } ~~~ ### 一个Main测试类 ~~~ package decorator; public class Main { public static void main(String[] args) { Meal meal = new ChineseMeal(); meal = new GreenPepperPork(meal); meal = new HandCabbage(meal); meal = new BraiseTenderloin(meal); System.out.println(meal.getDescription() + " ¥" + meal.cost()); Meal meal2 = new ChineseMeal(); meal2 = new EggplantBean(new MapoTofu(new BraiseTenderloin(meal2))); System.out.println(meal2.getDescription() + " ¥" + meal2.cost()); } } ~~~ # 运行结果 直接从eclipse复制过来 ~~~ Chinese Meal, 青椒炒肉, 手撕包菜, 红烧里脊 ¥11.0 Chinese Meal, 红烧里脊, 麻婆豆腐, 茄子豆角 ¥12.0 ~~~ # 分析讨论 去食堂打菜,经济套餐,假设总共只有5种食物可供选择分别是:手撕包菜,茄子豆角,麻婆豆腐,青椒肉丝,红烧里脊。如果可以随意打菜,就可以有C15+C25+C35+C45+C55=25−1=31 种选择。如果写代码把这120种选择都分别弄成一个类,就有31个类。这样不仅代码重复率太高,而且如果食堂**更改**了上述五样菜中一样菜的**价格**,那么**31个类的代码需要全部修改**。这不符合开闭原则,不利于代码的维护和扩展。 装饰者模式就可以解决上述这种“类的爆炸式增长”问题,各种各样的排列组合太多,不能在代码中一个类一个。 装饰者模式的特点是“继承(inheritance) + 组合(Composition)”:继承是为了让component和decorator拥有相同的类型,组合是为了让所有的concreteDecorator可以“互相包装、装饰”。装饰者又名Wrapper大概就是这样来的。 # Java. IO Java.IO 中应用了装饰者模式 ~~~ FileInputStream fiStream = null; InputStreamReader iStreamReader = null; BufferedReader bReader = null; fiStream = new FileInputStream("C:\\xxxx"); // InputStreamReader 是字节流通向字符流的桥梁、 iStreamReader = new InputStreamReader(fiStream); // 从字符输入流中读取文件中的内容,装饰了一个InputStreamReader的对象 bReader = new BufferedReader(iStreamReader("C:\\xxxx")); ~~~ ~~~ 一句代码版本,把3个new放到一起 BufferedReader bReader = new BufferedReader(new InputStreamReader(new FileInputStream("C:\\xxxx")); ~~~
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