Im currently trying to study java generics and Im trying to understand wilcards . I was reading java docs and I encountered this information below. Can you please clarify why Collection is not the supertype of all collections and it is when we use Collection<?> ? Is this because , in example , we can pass a collection into a collection ?

thank you . The text from docs is below :

Consider the problem of writing a routine that prints out all the elements in a collection. Here's how you might write it in an older version of the language (that is, a pre-5.0 release):

void printCollection(Collection c) {
Iterator i = c.iterator();
for (k = 0; k < c.size(); k++) {
    System.out.println(i.next());
}

} And here is a naive attempt at writing it using generics (and the new for loop syntax):

     void printCollection(Collection<Object> c) {
      for (Object e : c) {
      System.out.println(e);
      }
        }

The problem is that this new version is much less useful than the old one. Whereas the old code could be called with any kind of collection as a parameter, the new code only takes Collection, which, as we've just demonstrated, is not a supertype of all kinds of collections!

So what is the supertype of all kinds of collections? It's written Collection<?> (pronounced "collection of unknown"), that is, a collection whose element type matches anything. It's called a wildcard type for obvious reasons. We can write:

    void printCollection(Collection<?> c) {
     for (Object e : c) {
    System.out.println(e);
    }
  }