如何理解Java中的装饰者模式

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

// 具体组件
class Espresso extends Beverage {
    public Espresso() {
        description = "Espresso";
    }
    public double cost() {
        return 1.99;
    }
}

// 装饰者基类
abstract class CondimentDecorator extends Beverage {
    Beverage beverage; // 持有被装饰的对象
    public abstract String getDescription();
}

// 具体装饰者
class Milk extends CondimentDecorator {
    public Milk(Beverage beverage) {
        this.beverage = beverage;
    }
    public String getDescription() {
        return beverage.getDescription() + ", Milk";
    }
    public double cost() {
        return .20 + beverage.cost();
    }
}

// 使用方式
public class Main {
    public static void main(String[] args) {
        Beverage drink = new Espresso();     // 基础咖啡
        drink = new Milk(drink);             // 加牛奶
        System.out.println(drink.getDescription()); // 输出: Espresso, Milk
        System.out.println("Total: $" + drink.cost());
    }
}

InputStream input = new BufferedInputStream(
                    new FileInputStream("data.txt"));