Iterator and ListIterator in Java

An Iterator in Java is an object that is used to traverse a collection, one element at a time. It is also the safe way to remove elements while looping. A ListIterator is a more powerful iterator for lists: it can move forward and backward, and it can add and replace elements. In this lesson, you will learn the methods of both, with examples.

How an Iterator Moves

An iterator keeps a cursor that sits between elements. next() steps over one element and returns it.

Methods of Iterator

Example: Removing Safely with an Iterator

Let's see an example that removes long names while looping.

import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;
 
public class SafeRemove {
    public static void main(String[] args) {
        List<String> names = new ArrayList<>(
            List.of("Navika", "Srihitha", "Advitha"));
 
        Iterator<String> it = names.iterator();
        while (it.hasNext()) {
            String name = it.next();
            if (name.length() > 6) {
                it.remove();
            }
        }
        System.out.println(names);
    }
}

[Navika]

Explanation: The loop asks hasNext() before every step, and next() returns each name in turn. "Srihitha" (8 letters) and "Advitha" (7 letters) are longer than 6, so it.remove() deletes them. Removing through the iterator is safe; removing through names.remove(...) inside the loop would usually throw ConcurrentModificationException (see "ConcurrentModificationException in Java").

Methods of ListIterator

ListIterator extends Iterator and is available from list.listIterator().

Example: Changing a List with ListIterator

import java.util.ArrayList;
import java.util.List;
import java.util.ListIterator;
 
public class ListIter {
    public static void main(String[] args) {
        List<String> names = new ArrayList<>(List.of("Navika", "Advitha"));
 
        ListIterator<String> li = names.listIterator();
        while (li.hasNext()) {
            String name = li.next();
            li.set(name.toUpperCase());
            if (name.equals("Navika")) {
                li.add("SRIHITHA");
            }
        }
        System.out.println(names);
 
        StringBuilder backward = new StringBuilder();
        while (li.hasPrevious()) {
            backward.append(li.previous()).append(' ');
        }
        System.out.println(backward.toString().strip());
    }
}

[NAVIKA, SRIHITHA, ADVITHA]
ADVITHA SRIHITHA NAVIKA

Explanation: set replaces each name with its upper-case version. After "Navika", add inserts "SRIHITHA" at the cursor, so it lands between the first and second names. Then the second loop walks backward with previous(), printing the list in reverse order.

Iterator vs ListIterator

Common Mistakes

Calling remove() before next(), or twice in a row. It throws IllegalStateException.

Calling next() without checking hasNext(). At the end it throws NoSuchElementException.

Removing through the list while iterating. Use the iterator's own remove.

Interview Corner

Practice

1. Easy: Use an Iterator to remove all marks below 40 from a list of marks.
2. Harder: Use a ListIterator to double every even number in a list and insert a 0 after each odd number.

Key Takeaways