Iterator in Java

Iterator in Java

🔹 What is an Iterator?

An Iterator in Java is an object that allows you to traverse (iterate) through a collection (like ArrayList, HashSet, etc.) one element at a time.
It is part of the java.util package and is the standard way to loop through collections when you don’t want to use traditional loops.

🔹 Key Points

  • Iterator is a universal cursor for collections.
  • Belongs to java.util.Iterator interface.
  • Works with the Collection framework (List, Set, etc.).
  • Allows read and remove operations while iterating.
  • Unlike Enumeration, it is safer because it prevents ConcurrentModificationException if used properly.

🔹 Iterator Interface Methods

public interface Iterator<E> {
    boolean hasNext();   // Returns true if there are more elements
    E next();            // Returns the next element
    void remove();       // Removes the last element returned (optional)
}

🔹 Example: Using Iterator with ArrayList

import java.util.*;

public class IteratorExample {
    public static void main(String[] args) {
        // Create a list
        ArrayList<String> names = new ArrayList<>();
        names.add("Alice");
        names.add("Bob");
        names.add("Charlie");

        // Get the iterator
        Iterator<String> itr = names.iterator();

        // Iterate through the list
        while (itr.hasNext()) {
            String name = itr.next();
            System.out.println(name);

            // Example: remove "Bob"
            if (name.equals("Bob")) {
                itr.remove(); // Safe removal
            }
        }

        System.out.println("After removal: " + names);
    }
}

✅ Output:

Alice
Bob
Charlie
After removal: [Alice, Charlie]

🔹 Difference: Iterator vs. ListIterator

  • Iterator: Works with all collections, can only iterate forward.
  • ListIterator: Works only with List, can iterate both forward and backward, and allows adding/modifying elements.

🔹 For-each Loop vs Iterator

  • For-each loop internally uses an Iterator but does not allow element removal.
  • Iterator gives more control (removal while iterating).

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