What is the difference between iterators in Java and C++?

Question

How is the implementation of Iterator in Java different from that in C++?

Answer 1

In the current C++ (98) standard library (in particular the portion formerly known as STL) defines a form of iterators that are very close to C pointers (including arithmetic). As such they just point somewhere. To be useful, you generally need two pointers so that you can iterate between them. I understand C++0x introduces ranges which act more like Java iterators.

Java introduced the Iterator (and ListIterator) interface in 1.2, largely taking over from the more verbose Enumerable. Java has no pointer arithmetic, so there is no need to behave like a pointer. They have a hasNext method to see if they have go to the end, instead of requiring two iterators. The downside is that they are less flexible. The system requires methods as subList rather than iterating between two iterators are specific points in the containing list.

A general difference in style is that whereas C++ use "static polymorphism" through templates, Java uses interfaces and the common dynamic polymorphism.

The concept of iterators is to provide the glue to allow separation of "algorithm" (really control flow) and data container. Both approaches do that reasonably well. In ideal situations "normal" code should barely see iterators.