Why cant I add to ListIterator while iterating over it?

Question

Im trying to add to a ListIterator while iterating over it but when checking if it worked hasNext() always returns false.

As javadoc states that a ListIterator is "An iterator for lists that allows the programmer to traverse the list in either direction, modify the list during iteration, and obtain the iterator's current position in the list." however when Im trying to add I'm neither getting an Error message nor is the ListIterator getting a new Element.

public void foo() {
    List<String> bar = new ArrayList<String>();
    bar.add("foobar");
    ListIterator<String> quux = bar.listIterator();
    while(quux.hasNext()) {
        if(quux.next().equals("foobar")) {
            quux.add("baz");
        }
    }
}

At the end of this funtion I expect the ListIterator to contain the new String "baz". However when I call the hasNext() method it returns false. The program even got past the if-Statement it just turns out, that the add-method doesnt do what its supposed to do.

Possible duplicate of [How do I compare strings in Java?](https://stackoverflow.com/questions/513832/how-do-i-compare-strings-in-java) — Ben R., Oct 16 '19 at 09:02
I absolutely love persons who downvote Q/A without the slightest explanation. Continue like this, this is so useful. — Arnaud Denoyelle, Oct 16 '19 at 09:05
@ArnaudDenoyelle I agree with you, I received a downvote without explanation. — user2342558, Oct 16 '19 at 09:06
@LucienNestler You should not compare String with `==` but with `String.equals` instead. In this use case, it might work by chance (because of vm optimization) but the code is not valid — Arnaud Denoyelle, Oct 16 '19 at 09:08
Im sorry but I didnt downvote anyone, I just clicked the button that said that this is not a duplicate. — Lucien Nestler, Oct 16 '19 at 09:11
Also @ArnaudDenoyelle I wanted to answer your comment but you deleted it beforehand, so I answered on user2342558 's answer — Lucien Nestler, Oct 16 '19 at 09:13
@LucienNestler I am sorry, my comment was not against you. I know it was not you because you were downvoted also :p — Arnaud Denoyelle, Oct 16 '19 at 09:33

score 0 · Answer 1 · answered Oct 16 '19 at 09:02

0

Your code works (after fixed the missed () after new ArrayList<String> and replaced == with the correct method to compare strings, equals:

import java.util.*;

public class test
{
    public static void main(String[] args) {
        List<String> bar = new ArrayList<String>();
        bar.add("foobar");
        ListIterator<String> quux = bar.listIterator();
        while(quux.hasNext()) {
            if(quux.next().equals("foobar")) {
                quux.add("baz");
            }
        }
        System.out.println(bar);
    }
}

returns:

[foobar, baz]

As you can see, baz were inserted.

answered Oct 16 '19 at 09:02

user2342558

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Thanks for the fast answer, I added the missing () in the code for this site, but the objects Im comparing are not really Strings, I just used Strings to simplify the problem as I am using a self defined class and in fact Im getting inside the if-Branch where I call the add function just to see that nothing changes in the Iterator. – Lucien Nestler Oct 16 '19 at 09:10
@LucienNestler You're welcome. Please flag my answer as accepted and upvote it. If you'll open a new question about this with the reral types I'll answer it – user2342558 Oct 28 '19 at 13:01

score 0 · Answer 2 · edited Jun 20 '20 at 09:12

It seems that you expect to be able to fetch the added element with next():

nor is the ListIterator getting a new Element

[...]

At the end of this funtion I expect the ListIterator to contain the new String "baz". However when I call the hasNext() method it returns false

However, the JavaDoc for ListIterator.add() explicitly states that this is not the case:

The new element is inserted before the implicit cursor: a subsequent call to next would be unaffected

If you want to include the element in the iteration you can rewind the list iterator by one position:

    if(quux.next().equals("foobar")) {
        quux.add("baz");
        quux.previous();
    }

Oh thanks, looks like I overread that, is there a way to iterate over also the new element? — Lucien Nestler, Oct 17 '19 at 10:25

Why cant I add to ListIterator while iterating over it?

2 Answers2