PECS:如何将消费者转化为生产者?

Question

我有一家生产餐点的餐厅。厨房被给予消费者的盘子。

class Food{}
class Bamboo extends Food {}
interface Kitchen {
void build(List<? super Food> dessert);
}
abstract class Restaurant {
Kitchen kitchen;
public Restaurant(Kitchen kitchen) {
this.kitchen = kitchen;
}
List<? extends Food> getMeals() {
List<Food> food = new ArrayList<>();
this.kitchen.build(food);
return food;
}
}
class PandaKitchen implements Kitchen{
// Exact signature of the Kitchen
@Override
public void build(List<? super Food> plate) {
// the List IS a consumer of bamboos
plate.add(new Bamboo());
}
}
// Bamboo specialized restaurant
class OhPanda extends Restaurant {
public OhPanda() {
super(new PandaKitchen());
}
// Specialized signature of List<? extends Food>
@Override
List<Bamboo> getMeals() {
List<? super Food> bamboos = new ArrayList<>();
this.kitchen.build(bamboos);
// Obviously here, there is no information of having only Bamboos
return bamboos; // <==== FAIL
//return (List<Bamboo>) bamboos; // would not compile
}
}

在最后一行，我知道我的OhPanda餐厅只生产竹子。在不创建/复制内存中 ArrayList 的情况下转换List<? super Food>的最佳实践是什么？

这里写了一个更完整的要点：https://gist.github.com/nicolas-zozol/8c66352cbbad0ab67474a776cf007427

Answer 1

或者，也许你可以写一个餐厅和厨房的打字版本？

package kitchen;
import java.util.ArrayList;
import java.util.List;
class Food{}
class Bamboo extends Food {}
interface Kitchen<F> {
void build(List<F> dessert);
}
abstract class Restaurant<T> {
protected Kitchen kitchen;
Restaurant(Kitchen kitchen) {
this.kitchen = kitchen;
}
List<T> getMeals() {
List<T> food = new ArrayList<>();
kitchen.build(food);
return food;
}
}
class PandaKitchen implements Kitchen<Bamboo>{
@Override
public void build(List<Bamboo> dessert)
{
dessert.add(new Bamboo());
}
}
/** Bamboo specialized restaurant*/
class OhPanda extends Restaurant<Bamboo> {
OhPanda() {
super(new PandaKitchen());
}
@Override
List<Bamboo> getMeals() {
List<Bamboo> bamboos = new ArrayList<>();
kitchen.build(bamboos);
return bamboos;
}
}

Answer 2

我认为你错误地使用了下限通配符。您可以将其用作解决由于未在实现中指定通配符的类而可能面临的上限通配符限制。我不认为你想与食物及其超类型一起工作。您只想使用食品及其衍生物，并且应该找到"？扩展食物"，甚至摆脱通配符并使用列表<食物>