7. Come to Our Carousel

tags:java, oop category:java

Contents

• Come to Our Carousel
  • ShishD
  • PizzaD

---

这一章节就针对上一章节暴露的问题，提出解决方法：

访问者模式

不过作者没有一上来就给出完美的实现，它是通过一步步来引导的。

下面的两个例子只是第一步：

先将类似的方法聚合到访问者类中

7.1. ShishD

// 访问者
class OnlyOnionsV {
    boolean forSkewer() {
        return true;
    }
    boolean forOnion(ShishD s) {
        return s.onlyOnions();
    }
    boolean forLamb(ShishD s) {
        return false;
    }
    boolean forTomato(ShishD s) {
        return false;
    }
}

class IsVegetarianV {
    boolean forSkewer() {
        return ture;
    }
    boolean forOnion(ShishD s) {
        return s.IsVegetarian();
    }
    boolean forLamb(ShishD s) {
        return false;
    }
    boolean forTomato(ShishD s) {
        return s.IsVegetarian();
    }
}

// 使用访问者模式的ShishD
// 这里可以对比一下第二章节的ShishD代码
abstract class ShishD { //羊肉
    OnlyOnionsV ooFn = new OnlyOnionsV();
    IsVegetarianV ivFn = new IsVegetarianV();
    abstract boolean onlyOnions();
    abstract boolean IsVegetarian();
}

class Skewer extends ShishD { //串
    boolean onlyOnions() {
        return ooFn.forSkewer();
    }
    boolean IsVegetarian() {
        return ivFn.forSkewer();
    }
}

class Onion extends ShishD { //洋葱
    ShishD s;
    Onion (ShishD _s) {
        s = _s;
    }
    boolean onlyOnions() {
        return ooFn.forOnion(s);
    }
    boolean IsVegetarian() {
        return ivFn.forOnion(s);
    }
}

class Lamb extends ShishD { //羔羊肉
    ShishD s;
    Lamb (ShishD _s) {
        s = _s;
    }
    boolean onlyLambs() {
        return ooFn.forLamb(s);
    }
    boolean IsVegetarian() {
        return ivFn.forLamb(s);
    }
}

class Tomato extends ShishD { //西红柿
    ShishD s;
    Tomato (ShishD _s) {
        s = _s;
    }
    boolean onlyTomatos() {
        return ooFn.forTomato(s);
    }
    boolean IsVegetarian() {
        return ivFn.forTomato(s);
    }
}

第四条建议

When writing several functions for the

same self-referential datatype, use

visitor protocols so that all methods for

a function can be found in a single class.

7.2. PizzaD

abstract class PizzaD { //披萨饼
    RemAV remFn = new RemAV();
    TopAwCV topFn = new TopAwCV();
    SubAbCV subFn = new SubAbCV();
    abstract PizzaD remA();
    abstract PizzaD topAwC();
    abstract PizzaD subAbC();
}

class Crust extends PizzaD { //面包皮
    PizzaD remA() {
        return remFn.forCrust();
    }
    PizzaD topAwC() {
        return topFn.forCrust();
    }
    PizzaD subAbC() {
        return subFn.forCrust();
    }
}

class Cheese extends PizzaD { //奶酪
    PizzaD p;
    Cheese(PizzaD _p) {
        p = _p;
    }
    PizzaD remA() {
        return remFn.forCheese(p);
    }
    PizzaD topAwC() {
        return topFn.forCheese(p);
    }
    PizzaD subAbC() {
        return subFn.forCheese(p);
    }

}

Classr Olive extends PizzaD { //橄榄
    PizzaD p;
    Olive(PizzaD _p) {
        p = _p;
    }
    PizzaD remA() {
        return remFn.forOlive(p);
    }
    PizzaD topAwC() {
        return topFn.forOlive(p);
    }
    PizzaD subAbC() {
        return subFn.forOlive(p);
    }
}

class Anchovy extends PizzaD { //凤尾鱼
    PizzaD p;
    Anchovy(PizzaD _p) {
        p = _p;
    }
    PizzaD remA() {
        return remFn.forAnchovy(p);
    }
    PizzaD topAwC() {
        return topFn.forAnchovy(p);
    }
    PizzaD subAbC() {
        return subFn.forAnchovy(p);
    }
}

class Sausage extends PizzaD { //香肠
    PizzaD p;
    Sausage(PizzaD _p) {
        p = _p;
    }
    PizzaD remA() {
        return remFn.forSausage(p);
    }
    PizzaD topAwC() {
        return topFn.forSausage(p);
    }
    PizzaD subAbC() {
        return subFn.forSausage(p);
    }
}

class RemAV {
    PizzaD forCrust() {
        return new Crust();
    }
    PizzaD forCheese(PizzaD p) {
        return new Cheese(p.remA());
    }
    PizzaD forOlive(PizzaD p) {
        return new Olive(p.remA());
    }
    PizzaD forAnchovy(PizzaD p) {
        return p.remA();
    }
    PizzaD forSausage(PizzaD p) {
        return new Sausage(p.remA());
    }
}

class TopAwCV {
    PizzaD forCrust() {
        return new Crust();
    }
    PizzaD forCheese(PizzaD p) {
        return new Cheese(p.topAwC());
    }
    PizzaD forOlive(PizzaD p) {
        return new Olive(p.topAwC());
    }
    PizzaD forAnchovy(PizzaD p) {
        return new Cheese(new Anchovy(p.topAwC()));
    }
    PizzaD forSausage(PizzaD p) {
        return new Sausage(p.topAwC());
    }
}

class SubAbCV {
    PizzaD forCrust() {
        return new Crust();
    }
    PizzaD forCheese(PizzaD p) {
        return new Cheese(p.subAbC());
    }
    PizzaD forOlive(PizzaD p) {
        return new Olive(p.subAbC());
    }
    PizzaD forAnchovy(PizzaD p) {
        return new Cheese(p.subAbC());
    }
    PizzaD forSausage(PizzaD p) {
        return new Sausage(p.subAbC());
    }
}