Java Iterators: Iterator, ListIterator and Fail-Fast vs Fail-Safe

An Iterator is an object that implements the Iterator Design Pattern, allowing sequential traversal across any Collection while safely abstracting away the underlying data structure.

1. The Iterator Interface (Forward Only)

import java.util.ArrayList;
import java.util.Iterator;

public class IteratorDemo {
    public static void main(String[] args) {
        ArrayList<String> cities = new ArrayList<>();
        cities.add("Mumbai");
        cities.add("New York");
        cities.add("London");
        cities.add("Tokyo");

        Iterator<String> it = cities.iterator();
        while (it.hasNext()) {
            String city = it.next();
            // Safe removal during iteration:
            if (city.equals("London")) {
                it.remove(); // Safely removes without ConcurrentModificationException!
            }
        }
        System.out.println("Remaining cities: " + cities); // [Mumbai, New York, Tokyo]
    }
}

2. The ListIterator Interface (Bidirectional Traversal)

Available exclusively for List implementations:

import java.util.List;
import java.util.ListIterator;

public class ListIteratorDemo {
    public static void main(String[] args) {
        List<Integer> nums = List.of(10, 20, 30);
        ListIterator<Integer> lit = nums.listIterator(nums.size()); // Starts at end!

        System.out.print("Reverse Traversal: ");
        while (lit.hasPrevious()) {
            System.out.print(lit.previous() + " "); // Output: 30 20 10
        }
    }
}

Fail-Fast vs. Fail-Safe Iterators