Iterator

In Java, an Iterator is an interface that provides a way to traverse or iterate over a collection of objects, such as lists, sets, or other data structures. It is part of the java.util package and allows us to access elements of a collection one by one.

Here are some key points about the Iterator interface:

Basic Methods

• boolean hasNext(): Returns true if there are more elements to iterate over, and false otherwise.
• E next(): Returns the next element in the iteration.
• void remove(): Removes the last element returned by the iterator from the underlying collection. This is an optional operation and may not be supported by all collections.

Usage Example

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

public class IteratorExample {
    public static void main(String[] args) {
        // Create a collection
        ArrayList<String> fruits = new ArrayList<>();
        fruits.add("Apple");
        fruits.add("Banana");
        fruits.add("Mango");

        // Get an Iterator
        Iterator<String> iterator = fruits.iterator();

        // Iterate over the collection
        while (iterator.hasNext()) {
            String fruit = iterator.next();
            System.out.println(fruit);

            // Optional remove operation
            if (fruit.equals("Banana")) {
                iterator.remove();
            }
        }

        // After removal
        System.out.println(fruits);
    }
}

Advantages

• Iterator allows the user to traverse the collection without exposing the underlying representation.
• It provides a fail-safe mechanism to remove elements during iteration.

Limitations

• Iterator only moves forward and cannot be reset or move backward.
• Removing an element using remove() can only be done once per call to next(). If called multiple times consecutively without a next(), it will throw IllegalStateException.

Enhanced for-loop (for-each)

While an Iterator is useful, Java provides an enhanced for loop (for-each loop) as a more convenient way to iterate through collections, although we can’t modify the collection during iteration with this method.

for (String fruit : fruits) {
    System.out.println(fruit);
}