Understand the idea

An Iterator separates traversal from the collection implementation. hasNext checks whether another element exists; next consumes it. When supported, iterator.remove removes the most recently returned element in a controlled way.

Explore the details

Java Iterator

An Iterator is an object that can be used to loop through collections, like ArrayList and HashSet.

It is called an "iterator" because "iterating" is the technical term for looping.

To use an Iterator, you must import it from the java.util package.

Getting an Iterator

The iterator() method can be used to get an Iterator for any collection:

Looping Through a Collection

To loop through a collection, use the hasNext() and next() methods of the Iterator:

Removing Items from a Collection

Iterators are designed to easily change the collections that they loop through. The remove() method can remove items from a collection while looping.

The var Keyword with Iterators

You can also use the var keyword with iterators. This avoids repeating the long type name Iterator<String>, since the compiler already knows the type from the collection.

This makes code shorter, but many developers still use the full type for clarity. Since var is valid from Java version 10, you may see it in other code, so it's good to know that it exists:

ArrayList<String> cars = new ArrayList<>();
cars.add("Volvo");
cars.add("BMW");
cars.add("Ford");

// Get an iterator and loop
Iterator<String> it = cars.iterator();
while (it.hasNext()) {
    System.out.println(it.next());
}
// Volvo, BMW, Ford

// Safe removal during iteration
ArrayList<Integer> nums = new ArrayList<>(List.of(1, 2, 3, 4, 5));
Iterator<Integer> iter = nums.iterator();
while (iter.hasNext()) {
    if (iter.next() < 3) iter.remove();  // removes 1 and 2
}
System.out.println(nums);  // [3, 4, 5]

Here, var makes the iterator declaration shorter, but the actual type is still Iterator<String>.

A complete runnable example

This complete Main.java program focuses on the lesson idea. Reference fragments above may demonstrate individual statements; this example includes the imports, entry point, and supporting types it needs.

import java.util.*;
public class Main {
    public static void main(String[] args) throws Exception {
        ArrayList<Integer> numbers = new ArrayList<>(Arrays.asList(1, 2, 3, 4));
        Iterator<Integer> iterator = numbers.iterator();
        while (iterator.hasNext()) {
            if (iterator.next() % 2 == 0) iterator.remove();
        }
        System.out.println(numbers);
    }
}

[1, 3]

Change it. Run it. Understand it.

Load this lesson example into the editor. Change one value or statement, predict the result, then press RUN to execute Java.

Make it your own

Your learning checkpoint

• Explain the example without reading the output first.
• Make the change requested in the practice task.
• Describe why the result changed—or why compilation failed.

Ready to move on? Follow the next lesson in course order, or return to the curriculum to review a prerequisite.