2.3 装饰器模式

装饰器的本质是包装一个已有对象，并增强它的行为。

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

public abstract class ToppingDecorator extends Beverage {
   public abstract double cost();
}

public class FruitTea extends Beverage {
    @Override
    public double cost() {
        return 5.6;
    }
}

public class Pudding extends ToppingDecorator {
    private static final int COST = 5;
    private final Beverage beverage;

    public Pudding(Beverage beverage) {
        this.beverage = beverage;
    }

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

public class Boba extends ToppingDecorator {
    private static final int COST = 7;
    private final Beverage beverage;

    public Boba(Beverage beverage) {
        this.beverage = beverage;
    }

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

public class Main {
    public static void main(String[] args) {
        Beverage milkTea = new MilkTea();
        milkTea = new Boba(milkTea);
        milkTea = new Boba(milkTea);
        milkTea = new Pudding(milkTea);
        System.out.println("Total cost: " + milkTea.cost());
    }
}