How to get the current loop index when using Itera

问题:

I am using an Iterator to iterate through a collection and I want to get the current element's index.

How can I do that?

回答1:

Use your own variable and increment it in the loop.

回答2:

I had the same question and found using a ListIterator worked. Similar to the test above:

List<String> list = Arrays.asList("zero", "one", "two");

ListIterator iter = list.listIterator();

while (iter.hasNext()) {
    System.out.println("index: " + iter.nextIndex() + " value: " + iter.next());
}

Make sure you call the nextIndex BEFORE you actually get the next().

回答3:

Here's a way to do it using your own variable and keeping it concise:

List<String> list = Arrays.asList("zero", "one", "two");

int i = 0;
for (Iterator<String> it = list.iterator(); it.hasNext(); i++) {
    String s = it.next();
    System.out.println(i + ": " + s);
}

Output (you guessed it):

0: zero
1: one
2: two

The advantage is that you don't increment your index within the loop (although you need to be careful to only call Iterator#next once per loop - just do it at the top).

回答4:

You can use ListIterator to do the counting:

final List<String> list = Arrays.asList("zero", "one", "two", "three");

for (final ListIterator<String> it = list.listIterator(); it.hasNext();) {
    final String s = it.next();
    System.out.println(it.previousIndex() + ": " + s);
}

回答5:

What kind of collection? If it's an implementation of the List interface then you could just use it.nextIndex() - 1.

回答6:

Use a ListIterator to iterate through the Collection. If the Collection is not a List to start with use Arrays.asList(Collection.toArray()) to turn it into a List first.

回答7:

Use an int and increment it within your loop.

回答8:

just do something like this:

        ListIterator<String> it = list1.listIterator();
        int index = -1;
        while (it.hasNext()) {
            index++;
            String value = it.next();
            //At this point the index can be checked for the current element.

        }

回答9:

All you need to use it the iterator.nextIndex() to return the current index that the iterator is on. This could be a bit easier than using your own counter variable (which still works also).

public static void main(String[] args) {    
    String[] str1 = {"list item 1", "list item 2", "list item 3", "list item 4"};
    List<String> list1 = new ArrayList<String>(Arrays.asList(str1));

    ListIterator<String> it = list1.listIterator();

    int x = 0;

    //The iterator.nextIndex() will return the index for you.
    while(it.hasNext()){
        int i = it.nextIndex();
        System.out.println(it.next() + " is at index" + i); 
    }
}

This code will go through the list1 list one item at a time and print the item's text, then "is at index" then it will print the index that the iterator found it at. :)

回答10:

See here.

iterator.nextIndex() would provide index of element that would be returned by subsequent call to next().

回答11:

Though you already had the answer, thought to add some info.

As you mentioned Collections explicitly, you can't use listIterator to get the index for all types of collections.

List interfaces - ArrayList, LinkedList, Vector and Stack.

Has both iterator() and listIterator()

Set interfaces - HashSet, LinkedHashSet, TreeSet and EnumSet.

Has only iterator()

Map interfaces - HashMap, LinkedHashMap, TreeMap and IdentityHashMap

Has no iterators, but can be iterated using through the keySet() / values() or entrySet() as keySet() and entrySet() returns Set and values() returns Collection.

So its better to use iterators() with continuous increment of a value to get the current index for any collection type.