Java Generics - Class a Subclass of itself?

Question

I'm currently reading Java Generics, and I am a bit stuck when it comes to Wildcards.

I have been given this method from the Collections class:

public void <T> copy(List<? super T> dest, List<? extends T> src) {
    for(int i = 0; i < src.size(); i++) {
        dest.set(i, src.get(i));
     }
}

I have then been told that it is possible to call the method like this:

List<Object> objs = new ArrayList<Object>();
List<Integer> ints = new ArrayList<Integer>();
Collections.copy(objs, ints);

As the type parameter has been left to the compiler to determine, the book says the compiler chooses the type parameter to be Integer.

But how is that possible?

If it were taken to be Integer, this would mean that in the method declaration -
List<? extends T> would translate to List<Integer extends Integer>.

Is this a mistake, or are there different rules when regarding Generics? I have googled around and the majority of results say that a class cannot be a subclass of itself.

Answer 1

No, it's not an error.

? extends Integer means: any class that is or extends Integer (or implements Integer, if Integer was an interface).

The same goes for ? super Integer, which means: any class that is Integer or is a superclass or super-interface of Integer.