Generics: problem with Lists when using wildcards

[Simon Joseph Aquilina]

Hello,

I have read about Generics and used them in my code as well. I am now trying to use Wildcards and have a problem. I have the following example.

class abstract Animal {}
class Dog extends Animal {}
class Cat extends Animal {}

class Clinic {

 List<? extends Animal> animals = new ArratList<? extends Animal>(); <--- error: Cannot instantiate the type ArrayList<? extends Animal>

 public void registerAnimal(Animal animal){
  this.animals.add(animal); <--- The method add(capture#1-of ? extends Animal) in the type List<capture#1-of ? extends Animal> is not applicable for the arguments (Animal)
 }
}

I solved the first error as follows:

List<? extends Animal> animals = new ArrayList<>();

However I cannot understand how to solve the second one. What I want is to be have a list that can contain any Animal (Dogs and Cats).

I must be doing something wrong but I cannot understand what!

[Simon Joseph Aquilina]

Solved ...

Was over complicating things. Here is my working version.

class Clinic{
	List<Animal> animals = new ArrayList<>();
	
	public void register(Animal animal){
		animals.add(animal);
	}
	
	public void register(List<? extends Animal> animals){
		for(Animal animal : animals){
			this.animals.add(animal);
		}
	}
}

[Matthew Brown]

If you've got a List<? extends Animal> reference, then you know it references a List of a specific type that is Animal or a subclass. Which means it can be a List<Animal>, a List<Dog>, a List<Cat>, a List<Snake> etc. So what can you safely add into all those? Nothing.

If you want a List that you can add any Animal to, then what you want is simply a List<Animal>. If you want a reference that can hold any List that can definitely contain any Animal, then the most general type is List<? super Animal> (which can reference either a List<Object> or List<Animal>).

The important point here - and it explains the problem you were originally having - is that wildcards only apply to references. The actual List has a specific type. When you wrote new ArrayList<>(), it would actually be interpreted as new ArrayList<Animal>().