Narrowing doesn't work

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I can't get the point of this exercise. Why is it not possible to call the .onEat() method after narrowing selectable? Thanks ! ;)

1. Implement the Selectable interface in the Food class.
2. The onSelect() method should display "The food was selected".
3. Think which methods can be called with the variable food and which with the variable selectable.
4. In the foodMethods method, call the onSelect and onEat methods, if possible.
5. In the selectableMethods method, call the onSelect and onEat methods, if possible.
6. Don't use an explicit type conversion.

Requirements:

The Selectable interface must be implemented in the Food class.
The onSelect() method in the Food class should display "The food was selected".
In the foodMethods method, you need to call methods on a Food object.
In the selectableMethods method, you need to call methods accessible from any object that implements the Selectable interface.

package com.codegym.task.task14.task1405; /* Food 1. Implement the Selectable interface in the Food class. 2. The onSelect() method should display "The food was selected". 3. Think which methods can be called with the variable food and which with the variable selectable. 4. In the foodMethods method, call the onSelect and onEat methods, if possible. 5. In the selectableMethods method, call the onSelect and onEat methods, if possible. 6. Don't use an explicit type conversion. Requirements: 1. The Selectable interface must be implemented in the Food class. 2. The onSelect() method in the Food class should display "The food was selected". 3. In the foodMethods method, you need to call methods on a Food object. 4. In the selectableMethods method, you need to call methods accessible from any object that implements the Selectable interface. */ public class Solution { public static void main(String[] args) { Food food = new Food(); Selectable selectable = new Food(); Food newFood = (Food) selectable; foodMethods(food); selectableMethods(selectable); } public static void foodMethods(Food food) { food.onEat(); food.onSelect(); } public static void selectableMethods(Selectable selectable) { selectable.onSelect(); if(selectable instanceof Food){ ((Food) selectable).onEat(); } } interface Selectable { void onSelect(); } static class Food implements Selectable{ public void onEat() { System.out.println("The food was eaten"); } public void onSelect(){ System.out.println("The food was selected"); } } }

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Hoist

Level 32 , San Diego, United States

13 January 2023, 21:47

The comments below are very valuable and worthwhile. This is more vague / abstract for me .... THX

Nouser

Level 36 , Germany

7 December 2020, 06:47

useful

In addition to what Guadalupe said you have to know, that the methods you can call do not depend solely on the object. The objects and its parents together hold the maximum of possible methods that can be called. However you do not call method directly on an object but you use the reference varible for that. And the reference variable determins the possible methods. If you have a String object and assign that string to an Object reference variable (possible cause String a child of Object) then this reference just knows about object type methods and only can call these. All the specific String variables are unknown to Object.

Guadalupe Gagnon

Level 37 , Tampa, United States

7 December 2020, 03:10

solution

An interface is a guarantee that any class that implements the interface will implement the methods that are in the interface. The selectableMethods takes one argument, a Selectable object, which would be any class that implements the method in the Selectable class. Your code is attempting the convert that object into a Food object, but you don't have any idea what class could be passed into that method, only that it implements Selectable. So, line 36 is correct... the rest of the code in that method is not.

Isma

Level 41 , Madrid, Spain

7 December 2020, 08:56

Okay, so it improves the flexibility of that code! Thank you man

Guadalupe Gagnon

Level 37 , Tampa, United States

7 December 2020, 14:19

Interfaces are designed for flexibility. An interface example that is actually in the design of Java is Iterable, which has one method that returns an Iterator (another interface) object and allows the use of a foreach loop. You can design your own class (which is a little bit of a more complex task) that implements this method and it would work in a foreach loop. At this point in your learning you should have come in contact with foreach loops and Iterator objects. Iterator objects, also an interface, are very convenient in that you don't need to come up with code to go through each object contained in a collection (Maps, Lists, Sets). The Iterator object (remember that the one method of Iterable interface returns an Iterator object) has only 3 methods; hasNext(), next(), and remove(), which has standardized and simplified the process of going over each element.