Generic

[meeta gaur]

import java.util.ArrayList;
import java.util.List;
class Animal{}

class BigCat extends Animal{}
class Cat extends BigCat{}
class Dog extends Animal{}

class AnimalDoctor6{
	void addAll(List<? super BigCat> animals){ 
		animals.add(new BigCat());		
		animals.add(new Object());		
	}
	public static void main(String[] args){
		List<BigCat> bigcats=new ArrayList<>();
		bigcats.add(new BigCat());
		bigcats.add(new BigCat());
		
		AnimalDoctor6 obj=new AnimalDoctor6();
		obj.addAll(bigcats);
	}
}

AnimalDoctor6.java:12: error: no suitable method found for add(Object)
animals.add(new Object());
^
method List.add(int,CAP#1) is not applicable
(actual and formal argument lists differ in length)
method List.add(CAP#1) is not applicable
(actual argument Object cannot be converted to CAP#1 by method invocation conversion)
where CAP#1 is a fresh type-variable:
CAP#1 extends Object super: BigCat from capture of ? super BigCat
1 error

Why i can't add "Object" ? ,that is a super of "BigCat".

[Paul Clapham]

You have the usual misunderstanding regarding generic wildcards. When you write "List<? super BigCat>" that means that at runtime, an instance of that type will be an instance of List<X> where X is some supertype of BigCat. It does NOT mean that an instance of that type can contain any object of any supertype of BigCat.

So in your example, an instance of List<? super BigCat> could be a List<Animal>, since Animal is a supertype of BigCat. And you can't add an Object to a List<Animal>. So the compiler rejects that line of code.