Iterator

1. What is an Iterator?

An Iterator is an object used to go through the elements of a collection one by one, in a standard way, regardless of the specific type of collection being used (List, Set, or Map's key/value views).

2. Why is it used?

It provides a consistent, safe way to loop through any collection's elements, and it also allows safely removing elements while iterating — something a normal for loop can't do reliably on collections.

3. Real-Life Example

Think of flipping through pages of a photo album one at a time, always moving forward, and being able to remove a photo you're currently looking at without disturbing the rest of the album. An Iterator provides this same controlled, one-by-one movement through a collection.

4. Syntax

java
Iterator<DataType> it = collectionName.iterator();
while (it.hasNext()) {
    DataType value = it.next();
}

5. Example Program

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

public class IteratorDemo {
    public static void main(String[] args) {
        ArrayList<String> fruits = new ArrayList<>();
        fruits.add("Apple");
        fruits.add("Banana");

        Iterator<String> it = fruits.iterator();
        while (it.hasNext()) {
            System.out.println(it.next());
        }
    }
}

Output:

Apple
Banana

6. Key Points to Remember

• hasNext() checks if more elements exist; next() retrieves the next element.
• Iterator also provides a remove() method to safely delete the current element during iteration.
• Directly modifying a collection while using a regular for-each loop (instead of an Iterator's remove()) can cause a ConcurrentModificationException.