Add Strings through use of generic 'extends' causes compiler error

Question

Below code :

List genericNames = new ArrayList();
genericNames.add(\"John\");

Gives compiler error :

Answer 1

When you use wildcards with extends, you can't add anything in the collection except null. Also, String is a final class; nothing can extend String.

Reason: If it were allowed, you could just be adding the wrong type into the collection.

Example:

class Animal {

}

class Dog extends Animal {

}

class Cat extends Animal {

}

Now you have List

public static void someMethod(List list){
    list.add(new Dog()); //not valid
}

and you invoke the method like this:

List catList = new ArrayList(); 
someMethod(catList);

If it were allowed to add in the collection when using wildcards with extends, you just added a Dog into a collection which accepts only Cat or subtype type. Thus you can't add anything into the collection which uses wildcards with upper bounds.