定义

装饰者模式动态的将责任附加在对象上. 若要拓展功能, 装饰者提供了比继承更有弹性的替代方案.

特点:

• 装饰者和被装饰对象有共同的超类型
• 可以用一个或多个装饰者包装一个对象
• 既然装饰者与被装饰对象有着相同的超类型, 所以在任何需要原始对象(被包装)的场合, 可以用装饰过的对象代替它
• 装饰者可以在所委托被装饰者的行为之前与/或之后, 加上自己的行为, 以达到特定的目的
• 对象可以在任何时候被装饰, 所以可以在运行时动态的, 不限量的用你喜欢的装饰者来装饰对象

设计原则

• 类应该对拓展开放, 对修改关闭

案例

背景: 酒吧卖酒, 酒Beverage是抽象类, 酒有很多子类,需要计算成本, 子类的调料并不相同
现将计算成本的方法cost()在父类中声明, 子类重写调料的方法

Beverage

public abstract class Beverage {
  protected abstract boolean hasMilk();
  protected abstract boolean hasSoy();
  private Integer money;
  public Integer getMoney() {
    return money;
  }
  public void setMoney(Integer money) {
    this.money = money;
  }
  public Integer cost() {
    if (hasMilk()) {
      this.money += 1;
    }
    if (hasSoy()) {
      this.money += 2;
    }
    return this.money;
  }
}

HouseBlend

public class HouseBlend extends Beverage {
  @Override
  protected boolean hasMilk() {
    return true;
  }
  @Override
  protected boolean hasSoy() {
    return false;
  }
}

测试

  @Test
  public void testDecorator() {
    HouseBlend houseBlend = new HouseBlend();
    houseBlend.setMoney(0);
    System.out.println("houseBlend的成本为: " + houseBlend.cost());
    DarkRoust darkRoust = new DarkRoust();
    darkRoust.setMoney(0);
    System.out.println("darkRoust的成本为: " + darkRoust.cost());
  }
结果:
houseBlend的成本为: 1
darkRoust的成本为: 2

问题:
出现新的调料, 需要在父类中添加新的方法, 并更改cost方法

改用装饰者模式后

被装饰者顶层抽象类

public abstract class Beverage {
  public abstract Integer cost();
}

被装饰者

public class DarkRoust extends Beverage {
  @Override
  public Integer cost() {
    return 10;
  }
}

public class HouseBlend extends Beverage {
  @Override
  public Integer cost() {
    return 20;
  }
}

装饰者
现有一个抽象类保证 装饰者与被装饰者类型一致

public abstract class CondimentDecorator extends Beverage {
}

装饰者持有被装饰者的属性, 在构造方法中传入,当然也可以动态set
Milk

public class Milk extends CondimentDecorator {
  private Beverage beverage;
  public Milk(Beverage beverage) {
    this.beverage = beverage;
  }
  @Override
  public Integer cost() {
    return 2 + beverage.cost();
  }
  public Beverage getBeverage() {
    return beverage;
  }
  public void setBeverage(Beverage beverage) {
    this.beverage = beverage;
  }
}
  @Override
  public Integer cost() {
    return 2 + beverage.cost();
  }
}

Soy, 注意可以”层层包装”

public class Soy extends CondimentDecorator {
  private Beverage beverage;
  public Soy(Beverage beverage) {
    this.beverage = beverage;
  }
  @Override
  public Integer cost() {
    return 1 + beverage.cost();
  }
  public Beverage getBeverage() {
    return beverage;
  }
  public void setBeverage(Beverage beverage) {
    this.beverage = beverage;
  }
}

测试

  @Test
  public void testDecorator() {
    //darkRoust成本10,houseBlend成本20
    //milk成本1,soy成本2
    // darkRoust加milk和soy
    DarkRoust darkRoust = new DarkRoust();
    Milk milk = new Milk(darkRoust);
    Soy soy = new Soy(milk);
    System.out.println("darkRoust的成本为: " + milk.cost());
    // houseBlend只加soy
    HouseBlend houseBlend = new HouseBlend();
    Soy soy1 = new Soy(houseBlend);
    System.out.println("houseBlend的成本为: " + soy.cost());
  }
结果:
darkRoust的成本为: 12
houseBlend的成本为: 13

java io装饰者模式