Why is Java's Iterator not an Iterable?

Why does the Iterator interface not extend Iterable?

The iterator() method could simply return this.

Is it on purpose or just an oversight of Java's designers?

It would be convenient to be able to use a for-each loop with iterators like this:

for(Object o : someContainer.listSomeObjects()) {
    ....
}

where listSomeObjects() returns an iterator.

Solution 1:

An iterator is stateful. The idea is that if you call Iterable.iterator() twice you'll get independent iterators - for most iterables, anyway. That clearly wouldn't be the case in your scenario.

For example, I can usually write:

public void iterateOver(Iterable<String> strings)
{
    for (String x : strings)
    {
         System.out.println(x);
    }
    for (String x : strings)
    {
         System.out.println(x);
    }
}

That should print the collection twice - but with your scheme the second loop would always terminate instantly.

Solution 2:

Because an iterator generally points to a single instance in a collection. Iterable implies that one may obtain an iterator from an object to traverse over its elements - and there's no need to iterate over a single instance, which is what an iterator represents.