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) ?

? super RedCat allows e.g. List<Object> to be passed in. — luk2302
– luk2302, Commented Jun 3, 2018 at 12:39
Possible duplicate of What is PECS (Producer Extends Consumer Super)? — luk2302
– luk2302, Commented Jun 3, 2018 at 12:40
Mehotd should be 'takeAnimals(List<? extends Animal> arr)' to solve compile error — janith1024
– janith1024, Commented Jun 3, 2018 at 12:48

Mạnh Quyết Nguyễn · Accepted Answer · 2018-06-03 12:40:56Z

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 3, 2018 at 12:40

Mạnh Quyết Nguyễn

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3 Comments

Turing85 Over a year ago

I think it should be List<? extends Animal> since Animal defines method eat().

Mạnh Quyết Nguyễn Over a year ago

Good point. But I think wildcard is good in situation where we want to restrict/narrow down type of input list

Mạnh Quyết Nguyễn Over a year ago

List<? extends Cat> allows Cat and suptypes of Cat only. In your program, they're Cat and RedCat

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