Calling generic method

Question

I am getting following error when I execute the code mentioned below:

TestGenerics1.java:40: error: cannot find symbol
                              arr.get(i).eat();
                                        ^
symbol:   method eat()
location: class Object
1 error

The issue I am facing is reproduced with the help of following sample code:

import java.util.*;

abstract class Animal
{
    void eat() { System.out.println("animal eating"); }
}

class Dog extends Animal
{
    void bark() { }
}

class Cat extends Animal
{
    void meow() { }
}

class RedCat extends Cat { }

public class TestGenerics1
{
    public static void main(String[] args)
    {
        new TestGenerics1().go();
    }

    public void go()
    {
        List<Cat> arrAnimals = new ArrayList<Cat>(Arrays.asList(new RedCat(), new Cat()));

        takeAnimals(arrAnimals);
    }

    //public static void takeAnimals(List<Cat> arr)
    public static void takeAnimals(List<? super RedCat> arr)
    {
        for(int i=0; i<arr.size(); i++)
        {
            arr.get(i).eat();
        }
    }
}

If I uncomment public static void takeAnimals(List<Cat> arr) and comment out public static void takeAnimals(List<? super RedCat> arr) then it works good.

Why does it not work with public static void takeAnimals(List<? super RedCat> arr) ?

Possible duplicate of [What is PECS (Producer Extends Consumer Super)?](https://stackoverflow.com/questions/2723397/what-is-pecs-producer-extends-consumer-super) — luk2302, Jun 03 '18 at 12:40
Mehotd should be 'takeAnimals(List extends Animal> arr)' to solve compile error — janith1024, Jun 03 '18 at 12:48

score 5 · Accepted Answer · answered Jun 03 '18 at 12:40

5

List<? super RedCat> is list of some types which are supertype of RedCat.

The compiler cannot figure out which type is passed and does not guaranteed eat() method exists.

Instead, you should use extends:

List<? extends Cat>

answered Jun 03 '18 at 12:40

Mạnh Quyết Nguyễn

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I think it should be `List extends Animal>` since `Animal` defines method `eat()`. – Turing85 Jun 03 '18 at 12:46
Good point. But I think wildcard is good in situation where we want to restrict/narrow down type of input list – Mạnh Quyết Nguyễn Jun 03 '18 at 12:47
`List extends Cat>` allows `Cat` and suptypes of `Cat` only. In your program, they're `Cat` and `RedCat` – Mạnh Quyết Nguyễn Jun 03 '18 at 12:57

Calling generic method

1 Answers1