Java中的1混乱使迭代器关闭

Question

When using iterator in Java, the last iterator is pointing one past the last node right? And the value is null right? Then when using

iter = myList.listIterator();
sum = 0;
while (iter.hasNext()) {

sum += iter.next();

}

Shouldn't there be a off-by-one problem since the last value iterator is pointing at is null? I know in fact is there's no such off-by-one problem, but don't know why, please explain this for me. Thanks in advance!

Edit: sorry, too excited, didn't type complete code.

Summary： Hope this helps for future people, I was thinking about iterators as pointers in C++, but actually even if imagined as pointers, instead of pointing to nodes, they 'point' to the space between nodes (space before a node to be more precisce) as bmorris591 described.

Answer 1

next() moves the Iterator along, hasNext() only checks if there is a next. The Iterator doesn't "point", if anything it points at the spaces between elements.

You may want to read up on the Iterator Pattern .

Answer 2

Assuming your list contains Integer instances, you can do something like the following:

Iterator <Integer> iter = myList.listIterator();
int sum = 0;
while (iter.hasNext()) {
    sum += iter.next().intValue();
}

your code will add the result of each iter.next() to sum on each iteration of the loop.

When iter.hasNext() is called, it is not "at the first value", but just before the first value - the first call will then return that first value and update the internal state of the iterator to be just before the second value (just after the first value).

When you get to the end, the internal state is pointing to just after the last value, so iter.hasNext() returns false and your loop exits.

Now, if your iterator contains any null values, they are still returned by the calls to next() , and you'll end up with a NullPointerException (which you can avoid with a simple null check).

Answer 3

There is no null element returned from an Iterator when hasNext returns false .

Conceptually, when hasNext returns false , the iterator is pointing "past" the last element already, but there is nothing "past" the last element, not even null .

If you call next when hasNext returns false , then it won't return null -- it will throw a NoSuchElementException .

Answer 4

An iterator iterates over the list. The method hasNext() returns true when there are still elements left to iterate over and false when all elements have been processed. An example:

A list with contents [0,2,4]:

iter = myList.listIterator();
iter.hasNext(); // true
iter.next(); // returns 0
iter.hasNext(); // true
iter.next(); // returns 2
iter.hasNext(); // true
iter.next(); // returns 4
iter.hasNext(); // false

Thus there is no problem with off-by-one or something like that. Whenever hasNext() returns true, there are still elements left to iterate over; next() returns that element.