Question

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.

Answer 1

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.