Java Collections

Iterator

Complete lesson for Iterator using LEARNCODEGURU’s 6-level learning model — from basics to advanced practice, with Java OOP and Collections where relevant.

Basics — Iterator

Section: Java Collections. Iterator, ListIterator, fail-fast, forEachRemaining, and ConcurrentModificationException.

Iterator is a cursor

Iterable.iterator() returns an Iterator. Enhanced for is compiled to this loop.
hasNext() / next() are the core. remove() removes the last element returned by next() — optional operation.
Calling next() without hasNext() throws NoSuchElementException. Calling remove() twice without next() throws IllegalStateException.

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

public class IteratorBasics {
    public static void main(String[] args) {
        List<String> list = new ArrayList<>();
        list.add("keep");
        list.add("drop");
        list.add("keep");
        Iterator<String> it = list.iterator();
        while (it.hasNext()) {
            if (it.next().equals("drop")) {
                it.remove();
            }
        }
        System.out.println(list);
    }
}

Iterator vs ListIterator vs for-each

API	Direction	Can set/add?	Safe remove?
`Iterator`	Forward	No	`remove()` only
`ListIterator`	Both (`hasPrevious`)	`set` / `add`	Yes
for-each	Forward	No	No — CME if you mutate
`forEachRemaining` (Java 8)	Rest of cursor	No	Depends on iterator

ListIterator extras

list.listIterator() or listIterator(index). nextIndex() / previousIndex() support algorithms that need positions.
set(e) replaces the last element returned. add(e) inserts before the implicit next.

Advanced — Iterator

Fail-fast, weakly consistent, Spliterator

ArrayList / HashMap iterators increment modCount. A mismatch throws ConcurrentModificationException — even on one thread.
This is not a lock. Two threads mutating a HashMap can still corrupt it without always seeing CME.
ConcurrentHashMap / ConcurrentLinkedQueue iterators are weakly consistent: they reflect some but not necessarily all concurrent updates, and they do not throw CME.
CopyOnWriteArrayList iterator is a snapshot — remove on the iterator is unsupported.
Streams use Spliterator (splitable iterator) for parallel decomposition. iterator() and spliterator() both come from Iterable.

Tricky Interview Points

Q1) How do you remove while iterating a List?

Answer Use Iterator.remove, ListIterator.remove, removeIf, or iterate indexes backward. Do not list.remove inside for-each.

Q2) What is fail-fast?

Answer The iterator detects that the collection’s modCount changed (structural change) and throws ConcurrentModificationException.

Q3) Iterator vs ListIterator?

Answer ListIterator is list-only, bidirectional, and can set/add. Iterator is the general Collection cursor.

Q4) Why does for-each throw CME when you add?

Answer for-each uses an Iterator. add is a structural change not done through that iterator.

Q5) Can you iterate a Map with Iterator?

Answer Yes — map.entrySet().iterator(), or keySet()/values(). Directly on Map there is no iterator() because Map is not a Collection.

Q6) Is ConcurrentModificationException guaranteed?

Answer No. It is fail-fast best effort. ConcurrentHashMap will not throw it for typical concurrent writes.

Simple Programs

ListIterator reverse print and set

import java.util.ArrayList;
import java.util.List;
import java.util.ListIterator;

public class ListIteratorDemo {
    public static void main(String[] args) {
        List<String> words = new ArrayList<>();
        words.add("one");
        words.add("two");
        words.add("three");
        ListIterator<String> it = words.listIterator(words.size());
        while (it.hasPrevious()) {
            String w = it.previous();
            if (w.equals("two")) it.set("TWO");
            System.out.println(w);
        }
        System.out.println(words);
    }
}

Problem Solving Tricks

removeIf is clearer than a manual iterator for predicate deletes.
When teaching CME, mutate in for-each on purpose once so the stack trace is familiar.
For maps, iterate entrySet — not keys then get.
Iterator.forEachRemaining(Consumer) is handy after you already consumed a prefix.
Do not store an iterator and reuse it after the collection changes.

Practice Programs List

Delete every string shorter than 3 using an Iterator.
Use ListIterator to insert "x" before each "a".
Reproduce CME, then fix the same loop.
Iterate a HashMap with entrySet().iterator() and remove keys starting with _.
Write a method that returns an Iterator over 1..n without storing the whole list (custom Iterator).