Iterators and Iterables

There is a close correspondence between the Java interface Iterable and C#'s IEnumerable; and similarly between Java's Iterator and C# IEnumerator. In both cases the interface is closely tied up with the ability to write a "for each" loop. If we're going to be able to translate this Java:

for (Attribute att : attributes) {...}

into this C#:

foreach(Attribute att in attributes) {...}

then the variable attributes, which is an Iterable in Java, had better become an IEnumerable in C#. We can handle that by rewriting class names; and we can also rewrite the method attributes.iterator() as attributes.GetEnumerable() so that it satisfies the C# interface. What now gets tricky is that Java's Iterator has two methods hasNext() and next() which don't correspond neatly to C# IEnumerator, which has MoveNext() and Current. Specifically, hasNext() is stateless, and can be called any number of times, while MoveNext() is state-changing and can only be called once. However, "sane" code that uses an iterator always makes one call on hasNext() followed by one call on next(), and that sequence translates directly to a call on MoveNext() followed by a call on Current. So the converter assumes that the code will follow this discipline – and if we find code that doesn't, then we have to change it[11].

[11] A benefit of having the parsed Java code in XML format is that it's easy to do queries to search for code that needs to be inspected.