JAVA设计模式之装饰模式

装饰模式：动态地将责任附加到对象上。若要扩展功能，装饰者提供了比继承更有弹性的替代方案。

例子：你要替一家咖啡店设计beverage类。beverage有很多种，并且可以添加许多的调料，比如蒸奶、豆浆、摩卡等。

public abstract class Beverage 
{
    String description = "Unknown Beverage";
    
    public String getDescription()
    {
        return description;
    }
    
    public abstract double cost();
}

public class Espresso extends Beverage
{
    public Espresso()
    {
        description = "Espresso";
    }

    @Override
    public double cost() 
    {
        return 1.99;
    }
}

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

public class Mocha extends CondimentDecorator
{
    Beverage beverage;
    
    Mocha(Beverage beverage)
    {
        this.beverage = beverage;
    }

    @Override
    public String getDescription() 
    {
        return beverage.getDescription() + ", Mocha";
    }

    @Override
    public double cost() 
    {
        return .20 + beverage.cost();
    }
}

public class Whip extends CondimentDecorator
{
    Beverage beverage;
    
    Whip(Beverage beverage)
    {
        this.beverage = beverage;
    }

    @Override
    public String getDescription() 
    {
        return beverage.getDescription() + ", Whip";
    }

    @Override
    public double cost() 
    {
        return .35 + beverage.cost();
    }
}

public class test 
{
    public static void main(String[] args)
    {
        Beverage espresso = new Espresso();
        Beverage espresso_mocha = new Mocha(espresso);
        Beverage espresso_mocha_whip = new Whip(espresso_mocha);
        output(espresso_mocha_whip);
    }
    
    public static void output(Beverage beverage)
    {
        System.out.println(beverage.getDescription() + " $: " + beverage.cost());
    }
}

注：JAVA I/O中利用了装饰模式。