装饰者模式

定义：动态的将责任附加到对象上。若要扩展功能，装饰着提供了比继承更有弹性的替代方法
类图

image.png
事例：
去饮品店点饮料和调料，计算所需要的价格。
定义饮料类(Component)

public abstract class Beverage {

    String description = "Unkown beverage";

    public String getDescription() {
        return description;
    }

    public abstract double cost();
}

定义调料类(Decorator)

public abstract class CondimentDecorator extends Beverage{
    public abstract String getDescription();
}

定义具体饮料类(ConcreteComponent)

public class Espresso extends Beverage {

    public Espresso(){
        description = "Espresso";
    }
    @Override
    public double cost() {
        return 1.99;
    }
}

public class HouseBlend extends Beverage{
    public HouseBlend(){
        description = "HouseBlend";
    }
    @Override
    public double cost() {
        return 0.89;
    }
}

定义调料实现类(ConcreteDecorator)

public class Mocha extends CondimentDecorator{
    Beverage beverage;
    public Mocha(Beverage beverage){
        this.beverage = beverage;
    }
    @Override
    public String getDescription() {
        return this.beverage.getDescription() + ",Mocha";
    }

    @Override
    public double cost() {
        return 0.20 + this.beverage.cost();
    }
}

public class Soy extends CondimentDecorator {
    Beverage beverage;
    public Soy(Beverage beverage){
        this.beverage = beverage;
    }
    @Override
    public String getDescription() {
        return this.beverage.getDescription() + ", Soy";
    }

    @Override
    public double cost() {
        return 0.15 + this.beverage.cost();
    }
}

public class Whip extends CondimentDecorator {
    Beverage beverage;
    public Whip(Beverage beverage){
        this.beverage = beverage;
    }
    @Override
    public String getDescription() {
        return this.beverage.getDescription() + ", Whip";
    }

    @Override
    public double cost() {
        return 0.30 + this.beverage.cost();
    }
}

测试类

public class Test {
    public static void main(String[] args){
        Beverage beverage = new Espresso();
        System.out.println(beverage.getDescription()+"$"+beverage.cost());

        Beverage beverage1 = new HouseBlend();
        beverage1 = new Mocha(beverage1);
        beverage1 = new Soy(beverage1);
        beverage1 = new Whip(beverage1);
        beverage1 = new Mocha(beverage1);
        System.out.println(beverage1.getDescription()+"$"+beverage1.cost());
    }
}

类似的java.io使用了装饰者模式。
例如：InputStream是抽象组件，FileInputStream是一个抽象装饰者，BufferedInputStream是具体的装饰者。
缺点：利用装饰模式，常常造成设计中出现大量小类。