Java generics: wildcards

So I'm reading generics to get familiar with these concepts again, especially when it comes to wildcards, because I hardly use them or encounter them From my reading, I can't understand why they use wildcards An example I keep coming across is as follows

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

Why don't you write this:

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

Another example from Oracle website:

public static double sumOfList(List<? extends Number> list) {
    double s = 0.0;
    for (Number n : list)
        s += n.doubleValue();
    return s;
}

Why isn't this written

public static <T extends Number> double sumOfList(List<T> list) {
    double s = 0.0;
    for (Number n : list)
        s += n.doubleValue();
    return s;
}

Did I miss anything?

Solution

Starting with Oracle:

interface Collection<E> {
     public boolean containsAll(Collection<?> c);
     public boolean addAll(Collection<? extends E> c);
 }

interface Collection<E> {
     public <T> boolean containsAll(Collection<T> c);
     public <T extends E> boolean addAll(Collection<T> c);
     // Hey,type variables can have bounds too!
 }

So for the first example, this is because the operation is not type dependent

For the second, this is because it depends only on the number class