Iterator

Learn to iterate collections with Iterator.

The Iterator<E> interface

Iterator<E> is the universal interface for walking a collection one element at a time. hasNext() asks if there is a next element; next() returns the next element and advances the cursor.

You obtain an Iterator from a List with Iterator<String> it = names.iterator();. The loop pattern is while (it.hasNext()) { String s = it.next(); }. An Iterator does not modify the collection by itself; it only reads. Removal uses a separate remove method.

The traversal pattern

The standard Iterator pattern is: get the iterator with iterator(), open a while (it.hasNext()) loop, read elements with it.next() inside. This pattern is what for-each does behind the scenes; Java implements for-each with an Iterator.

If next() is called without hasNext() and the cursor is at the end, NoSuchElementException is thrown. Always call hasNext before next in a loop.

Exercise Walk the list with an Iterator

Declare List<String> names = new ArrayList<>();; add "Ayşe" and "Alex". Obtain an Iterator with Iterator<String> it = names.iterator();. Inside a while (it.hasNext()) loop, print each element with System.out.println(it.next());.

Show example solution

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

public class Main {
    public static void main(String[] args) {
        List<String> names = new ArrayList<>();
        names.add("Ayşe");
        names.add("Alex");
        Iterator<String> it = names.iterator();
        while (it.hasNext()) {
            System.out.println(it.next());
        }
    }
}

Test yourself

When does it.hasNext() return true?
- When the cursor reached the end
- When the collection is empty
- When remove is called
- When a next element exists
Show answer

When a next element exists — hasNext returns true when a next element exists.

Complete the loop condition

while (it.____()) {
    String s = it.next();
}

Show answer

while (it.hasNext()) {
    String s = it.next();
}

Match the Iterator methods

hasNext()

Has next?
next()

Returns the next element
iterator()

Gets an Iterator
remove()

Deletes last read

Next lesson: remove

We Are Enhancing Your Experience

Cookie Preferences

Strictly Necessary Cookies

Analytical Cookies