Iterator and fail-fast

Definition

An Iterator walks a collection one item at a time and is the safe way to remove items while looping. Most collections are fail-fast: if you change them during a for-each loop, they throw ConcurrentModificationException.

Example code

The wrong way to remove while looping, and two right ways

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

class Demo {
    public static void main(String[] args) {
        List<Integer> numbers = new ArrayList<>(List.of(1, 2, 3, 4, 5, 6));

        try {
            for (Integer n : numbers) {
                if (n % 2 == 0) numbers.remove(n);        // wrong: changes the list while looping
            }
        } catch (java.util.ConcurrentModificationException e) {
            System.out.println("fail-fast: ConcurrentModificationException");
        }

        Iterator<Integer> iterator = numbers.iterator();
        while (iterator.hasNext()) {
            if (iterator.next() % 2 == 0) iterator.remove();   // right: the iterator removes safely
        }
        System.out.println(numbers);

        numbers.removeIf(n -> n > 3);                          // also right, and shortest
        System.out.println(numbers);
    }
}

// Output:
// fail-fast: ConcurrentModificationException
// [1, 3, 5]
// [1, 3]