6. What’s New?

tags:java, oop category:java

Contents

• What’s New?
  • PizzaD

---

6.1. PizzaD

abstract class PizzaD { //比萨饼
    abstract PizzaD remA(); //去除比萨饼中的凤尾鱼顶料(因为太咸了)
    abstract PizzaD topAwC(); //在凤尾鱼顶料上加上奶酪顶料(这样会盖住凤尾鱼的咸味)
    abstract PizzaD subAbC(); //将所有的凤尾鱼顶料换成奶酪顶料
}

class Crust extends PizzaD { //面包皮
    PizzaD subAbC(){
        return new Crust();
    }
    PizzaD topAwC(){
        return new Crust();
    }
    PizzaD subAbC(){
        return new Crust();
    }
}

// 下面是各种顶料
class Cheese extends PizzaD { //奶酪
    PizzaD p;
    Cheese (PizzaD _p) {
        p = _p;
    }
    PizzaD remA(){
        return new Cheese(p.remA());
    }
    PizzaD topAwC(){
        return new Cheese(p.topAwC());
    }
    PizzaD subAbC(){
        return new Cheese(p.subAbC());
    }
}

class Olive extends PizzaD { //橄榄
    PizzaD p;
    Olive (PizzaD _p) {
        p = _p;
    }
    PizzaD remA(){
        return new Olive(p.remA());
    }
    PizzaD topAwC(){
        return new Olive(p.topAwC());
    }
    PizzaD subAbC(){
        return new Olive(p.subAbC());
    }
}

class Anchovy extends PizzaD { //凤尾鱼
    PizzaD p;
    Anchovy (PizzaD _p) {
        p = _p;
    }
    PizzaD remA(){
        return p.remA();
    }
    PizzaD topAwC(){
        return new Cheese(new Anchovy(p.topAwC()));
    }
    PizzaD subAbC(){
        return new Cheese(p.subAbC());
    }
}

class Sausage extends PizzaD { //香肠
    PizzaD p;
    Sausage (PizzaD _p) {
        p = _p;
    }
    PizzaD remA(){
        return new Sausage(p.remA());
    }
    PizzaD topAwC(){
        return new Sausage(p.topAwC());
    }
    PizzaD subAbC(){
        return new Sausage(p.subAbC());
    }
}

如果想要在比萨饼上面添加额外的顶料怎么办？

很简单，再从 PizzaD 扩展出一个新的子类型就可以了。

class Spinach extends PizzaD { //菠菜
    PizzaD p;
    Spinach (PizzaD _p) {
        p = _p;
    }
    PizzaD remA(){
        return new Spinach(p.remA());
    }
    PizzaD topAwC(){
        return new Spinach(p.topAwC());
    }
    PizzaD subAbC(){
        return new Spinach(p.subAbC());
    }
}

但是每添加一个新的变体类型都要加上三个方法，好累的说。

有什么比较好的办法解决这个问题呢？

下一章节给你答案。

第三条建议

When writing a function that returns values of a datatype,

use new to create these values.