Object is not super type of all kinds collections?

[suresh pilakal babu]

Java tutorial saying that
"Object is not super type of all kinds of collections"

Please clarify

Suresh.
[ November 01, 2008: Message edited by: Bear Bibeault ]

[Ken Truitt]

I think a little more text from your source would help
people understand what they meant. Obviously
Object is a supertype of all other objects.

[suresh pilakal babu]

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 (i.e., 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<Object>, 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);
}
}

[Anton Golovin]

Object is the supertype of any other object in Java. But this refers to Collection<Object> not being a supertype of Collection<Subobject> in the loop that you gave as an example. Object is the supertype of Subobject, though.