Chapter 70: Iterator in Java

for-each loops over Iterable types compile down to iterator() calls under the hood. Understanding Iterator explains why you sometimes see ConcurrentModificationException and how to remove during traversal without hand-index bugs.

Comparable / Comparator sort theory is Chapter 71.

1. Topic title

Iterator: step through a collection with permission to delete the current item

2. What it means

Iterator<E> exposes boolean hasNext(), E next(), and optional void remove() (removes the last returned element). Collection.iterator() hands you a fresh iterator object; each next() advances a cursor.

ListIterator extends the idea for List with previous(), add, set, and index hints—useful for editing while walking both directions.

3. Why it is used

Safe removal while scanning—list.remove(i) inside a classic for (int i=0; …) loop is easy to off-by-one; Iterator.remove() tracks structure for you on ArrayList.

Library interoperability—many APIs accept Iterable because streaming starts from iteration.

4. Mental picture

The iterator is a bookmark: next() flips the page you read; remove() tears that page out before you move on again.

5. Java code example

import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;
 
public class IteratorDemo {
    public static void main(String[] args) {
        List<Integer> nums = new ArrayList<>(List.of(3, 7, 8, 9));
        Iterator<Integer> it = nums.iterator();
        while (it.hasNext()) {
            int n = it.next();
            if (n % 2 == 0) {
                it.remove();
            }
        }
        System.out.println(nums);
 
        for (String s : List.of("a", "b", "c")) {
            System.out.println(s);
        }
    }
}

After the loop, nums drops 8—even numbers removed without index math.

6. Explanation of code

Enhanced for on List.of(...) uses an iterator internally but does not expose remove—you cannot delete safely that way; use an explicit iterator or removeIf.

7. Common mistakes

Calling remove() before next()—IllegalStateException.

Modifying the collection through other means while an iterator is open on ArrayList—fail-fast checks throw ConcurrentModificationException (best-effort, not a concurrency guarantee).

Confusing iterator order with PriorityQueue priority order (Chapter 69)—iterator on PriorityQueue does not drain by poll priority.

8. Best practices

Prefer removeIf when you only need a predicate filter—less code than manual iterators.

When you need index while looping lists, use ListIterator or a for (int i = 0; …) with care—pick the least surprising style for teammates.

9. Small practice task

Build List<String> with mixed blank strings. Remove blanks with an iterator (not removeIf this time). Print the cleaned list.

Beginner tip

If the debugger shows “modCount” in a stack trace, you changed a structurally backed collection while something else was iterating—clone first, or collect deletions in a second list, then apply.