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I don't claim to be authoritative with this answer. Instead, it is just my recollection of various discussions I read about iterators and/or my own thoughts... Unfortunately I do not have a list of sources (too long ago) nor can I easily produce one (because even googling up stuff takes lots of time sometimes). <code>StopIteration</code> The name <code>StopIteration</code> and a lot of the semantics originated from <a href="http://docs.python.org/2/library/exceptions.html#exceptions.StopIteration" rel="nofollow">python</a>. <a href="http://www.python.org/dev/peps/pep-0234/" rel="nofollow">PEP 234</a> has some remarks regarding alternative approaches: <blockquote> <ul> <li>It has been questioned whether an exception to signal the end of the iteration isn't too expensive. Several alternatives for the StopIteration exception have been proposed: a special value End to signal the end, a function end() to test whether the iterator is finished, even reusing the IndexError exception. <ul> <li>A special value has the problem that if a sequence ever contains that special value, a loop over that sequence will end prematurely without any warning. If the experience with null-terminated C strings hasn't taught us the problems this can cause, imagine the trouble a Python introspection tool would have iterating over a list of all built-in names, assuming that the special End value was a built-in name!</li> <li>Calling an end() function would require two calls per iteration. Two calls is much more expensive than one call plus a test for an exception. Especially the time-critical for loop can test very cheaply for an exception.</li> <li>Reusing IndexError can cause confusion because it can be a genuine error, which would be masked by ending the loop prematurely.</li> </ul></li> </ul> </blockquote> <code>hasNext()</code> Java's <code>hasNext()</code> is just a "bonus". Java's <code>next()</code>, however, has it's own flavor of <code>StopIteration</code> called <a href="http://docs.oracle.com/javase/7/docs/api/java/util/Iterator.html#next%28%29" rel="nofollow"><code>NoSuchElementException</code></a>. <code>hasNext()</code> is hard, if not impossible, to implement, which is best demonstrated with e.g. the following generator: <pre><code>var i = 0; function gen() { yield doSomethingDestructive(); if (!i) { yield doSomethingElse(); } } </code></pre> How would you implement <code>hasNext()</code> for that? You cannot simply "peek" into the generator, because that would actually execute <code>doSomethingDestructive()</code>, which you didn't intend to do or else you would have called <code>next()</code> in the first place. So you'd have to write some code analyzer that has to always reliably proof that a given code when run with a certain state will always result in either a yield or no yield (halting problem). So even if you could write something like that, the state may still change between <code>.hasNext()</code> and a subsequent <code>.next()</code>. What would <code>hasNext()</code> return in the following example: <pre><code>var g = gen(); g.next(); if (g.hasNext()) { ++i; alert(g.next()); } </code></pre> At that point of time <code>true</code> would be the right answer. However, the subsequent <code>.next()</code> would throw, and the user would probably have a hard time figuring out why... You could tell people: "Don't do the following things in your iterator implementation" (a contract). Which people will routinely break and complain. So in my humble opinion, <code>hasNext()</code> was ill-conceived and makes it too easy to write wrong code. <code>.MoveNext()</code> <code>C#</code> is pretty much the same as <code>.next()</code>, just that the return value indicates if there was actually a next item instead of or the lack of an exception. There is one important difference, however: You need to store the current item in the iterator itself, e.g. <code>.Current</code> in C#, which may prolong the life-time of the current item unnecessarily: <pre><code>var bufferCtor = (function gen() { for(;;) { yield new ArrayBuffer(1<<20); // buffer of size 1 megabyte } })(); setInterval(function() { // MoveNext scheme bufferCtor.MoveNext(); // Not actually Javascript var buf = bufferCtor.Current; // Not actually Javascript // vs. StopIteration scheme var buf = bufferCtor.next(); }, 60000); // Each minute </code></pre> OK, that example is a little contrived, but I'm sure there are actual use cases where a generator/iterator would return something talking up a lot of space or otherwise occupying resources. In the <code>StopIteration</code> scheme, <code>buf</code> can be garbage-collected as soon as it goes out of scope. In <code>.MoveNext()</code>, it cannot be garbage-collected until <code>.next()</code> is called again, because <code>.Current</code> will still hold a reference to it. Conclusion In my opinion, of the presented alternatives, the <code>StopIteration</code> one is the least error prone and least ambiguous approach. And if I recall correctly, the python and es-6 people thought about the same.
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