Mastering Java's Iterator Interface: Practical Guide with Code Example

Java Iterator Interface

Discover how the Iterator interface lets you navigate and manipulate Java collections efficiently. A hands‑on example demonstrates each method in action.

The Iterator interface, part of Java’s collections framework, provides a uniform way to traverse any collection. It is extended by ListIterator for lists.

Every collection class offers an iterator() method that returns an Iterator instance, enabling safe traversal and manipulation.

Iterator Methods

The interface defines four core methods:

• hasNext() – returns true if more elements exist.
• next() – retrieves the next element.
• remove() – deletes the last element returned by next().
• forEachRemaining() – applies a given action to all remaining elements.

Example: Using Iterator with ArrayList

The following snippet implements all four methods on an ArrayList<Integer>:

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

class Main {
    public static void main(String[] args) {
        // Create an ArrayList
        ArrayList<Integer> numbers = new ArrayList<>();
        numbers.add(1);
        numbers.add(3);
        numbers.add(2);
        System.out.println("ArrayList: " + numbers);

        // Obtain an Iterator
        Iterator<Integer> iterator = numbers.iterator();

        // Access the first element
        int number = iterator.next();
        System.out.println("Accessed Element: " + number);

        // Remove that element
        iterator.remove();
        System.out.println("Removed Element: " + number);

        System.out.print("Updated ArrayList: ");

        // Iterate over remaining elements
        while (iterator.hasNext()) {
            iterator.forEachRemaining(value -> System.out.print(value + ", "));
        }
    }
}

Output

ArrayList: [1, 3, 2]
Accessed Element: 1
Removed Element: 1
Updated ArrayList: 3, 2,

The lambda expression passed to forEachRemaining() demonstrates how the method processes each remaining element concisely.