Iterate with for loop or while loop?

Question

I often see code like:

Iterator i = list.iterator();
while(i.hasNext()) {
    ...
}

but I write that (when Java 1.5 isn\'t available or for

Answer 1

Academia tends to prefer the while-loop as it makes for less complicated reasoning about programs. I tend to prefer the for- or foreach-loop structures as they make for easier-to-read code.

Answer 2

I think scope is the biggest issue here, as you have pointed out.

In the "while" example, the iterator is declared outside the loop, so it will continue to exist after the loop is done. This may cause issues if this same iterator is used again at some later point. E. g. you may forget to initialize it before using it in another loop.

In the "for" example, the iterator is declared inside the loop, so its scope is limited to the loop. If you try to use it after the loop, you will get a compiler error.

Answer 3

There are appropriate uses for the while, the for, and the foreach constructs:

• while - Use this if you are iterating and the deciding factor for looping or not is based merely on a condition. In this loop construct, keeping an index is only a secondary concern; everything should be based on the condition

• for - Use this if you are looping and your primary concern is the index of the array/collection/list. It is more useful to use a for if you are most likely to go through all the elements anyway, and in a particular order (e.g., going backwards through a sorted list, for example).

• foreach - Use this if you merely need to go through your collection regardless of order.

Obviously there are exceptions to the above, but that's the general rule I use when deciding to use which. That being said I tend to use foreach more often.

Answer 4

I prefer the for loop because it also sets the scope of the iterator to just the for loop.

Answer 5

Both are fine, but remember that sometimes access to the Iterator directly is useful (such as if you are removing elements that match a certain condition - you will get a ConcurrentModificationException if you do collection.remove(o) inside a for(T o : collection) loop).

I prefer to write the for(blah : blah) [foreach] syntax almost all of the time because it seems more naturally readable to me. The concept of iterators in general don't really have parallels outside of programming

Answer 6

Either is fine. I use for () myself, and I don't know if there are compile issues. I suspect they both get optimized down to pretty much the same thing.