PECS: How to convert a Consumer to a Producer?

Question

I have a Restaurant that produces Meals. The kitchens is given plates that are Consumers.

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
    }
}

In the last line, I know that my OhPanda Restaurant only produces Bamboos. What is the best practice to convert my List<? super Food> without creating/copying an ArrayList in memory ?

A more complete Gist is written here: https://gist.github.com/nicolas-zozol/8c66352cbbad0ab67474a776cf007427

Maaaatt · Answer 1 · 2017-07-20T13:30:13.160

1

I think you make the wrong use of the Lower Bounded Wildcards. You use it as a turn around to the Upper Bounded Wildcards restrictions you may face by not specifying the Class of your wildcards in your implementation. I don't think you want to work with food and its supertypes. You only want to work with Food and its derivatives and you should find a solution with "? extends Food" or even get rid of the Wildcards and use List< Food >

edited Jul 20 '17 at 13:30

answered Jul 20 '17 at 10:55

Maaaatt

429
4
14

You can't do that. A `List` is NOT a `List super Food>`. It's captured as two different types. – Nicolas Zozol Jul 20 '17 at 11:16
Indeed, my bad. Edited my post. – Maaaatt Jul 20 '17 at 12:41

Tobias Otto · Accepted Answer · 2017-07-20T12:34:12.013

1

Or, maybe you could write a typed version of the restaurant and kitchen?

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;
    }
}

edited Jul 20 '17 at 12:34

answered Jul 20 '17 at 12:26

Tobias Otto

1,634
13
20

I have added another possible solution. – Tobias Otto Jul 20 '17 at 12:40
It works, I made just now a close solution where T and F extends Food : https://gist.github.com/nicolas-zozol/37748839e4bcfeaaf7cec914fb2442ce – Nicolas Zozol Jul 20 '17 at 12:45
But why `this.kitchen.build(bamboos);`compile ? How `void build(List super F> plate);` can accept bamboos when previously `void build(List super Food> plate);` did not accept these bamboos ? – Nicolas Zozol Jul 20 '17 at 12:46
I think my solution is better because you loose the ability to put anything on the plate in `void build(List plate);`. Using PECS, it should be void `build(List super F> plate);` – Nicolas Zozol Jul 20 '17 at 12:48

PECS: How to convert a Consumer to a Producer?

2 Answers2