Why aren't Enumerations Iterable?

Question

In Java 5 and above you have the foreach loop, which works magically on anything that implements Iterable:

for (Object o : list) {
  doStuff(o);         


        

Answer 1

It doesn't make sense for Enumeration to implement Iterable. Iterable is a factory method for Iterator. Enumeration is analogous to Iterator, and only maintains state for a single enumeration.

So, be careful trying to wrap an Enumeration as an Iterable. If someone passes me an Iterable, I will assume that I can call iterator() on it repeatedly, creating as many Iterator instances as I want, and iterating independently on each. A wrapped Enumeration will not fulfill this contract; don't let your wrapped Enumeration escape from your own code. (As an aside, I noticed that Java 7's DirectoryStream violates expectations in just this way, and shouldn't be allowed to "escape" either.)

Enumeration is like an Iterator, not an Iterable. A Collection is Iterable. An Iterator is not.

You can't do this:

Vector list = …
Iterator i = list.iterator();
for (X x : i) {
    x.doStuff();
}

So it wouldn't make sense to do this:

Vector list = …
Enumeration i = list.enumeration();
for (X x : i) {
    x.doStuff();
}

There is no Enumerable equivalent to Iterable. It could be added without breaking anything to work in for loops, but what would be the point? If you are able to implement this new Enumerable interface, why not just implement Iterable instead?