java generic and wild card

Question

In java generic I understood what are the meanign of wild card, super and extends, but didn't get why does not allow me to add anything, and why allows me to add upto SomeType in hierarchy, but not above in the hierarchy?

class Animal {}
class Cat extends Animal{}

following method can take list of Animal or sub of Animal i.e Cat, but nothing else and I am not allowed to add anything, if try to add, compiler stops me why ?

void addAminal(List<? extends Aminal> aList){
       aList.add(new Cat()); // compiler error
       aList.add(new Animal()); // compiler error
}

Now following method can take any list of Animal or any super type of Animal, but no sub type of Animal, and I can add objects upto Animal or lower in hierarchy, so when I try to add Object, compiler complains why ?

void addAnimal(List<? super Animal> aList){
     aList.add(new Animal()); // no error
     aList.add(new Cat());     // no error
     aList.add(new Object()); // compiler error why ?
}

Thanks Arya

Answer 1

Suppose you defined a new class:

class Tabby extends Cat {}

And then you did the following:

List<Tabby> aList = new ArrayList<Tabby>();
addAnimal(aList);

There's no surprise that this list should not have an Animal or even a Cat that isn't a Tabby, yet if the compiler didn't flag the error, that's what you would have.

The reason is that hou've specified addAnimal to take a list of something that extends Animal, but that something could be highly restrictive. This, however, would compile:

void addAnimal(List<Animal> aList){
    aList.add(new Cat()); // OK
    aList.add(new Animal()); // OK
}

The use of super also would work, because an instance of either Cat or Animal is an instance of any superclass of Animal.