Why Deleting Elements from a Java List Inside a For Loop Is Unsafe and How to Do It Correctly

Conclusion: never delete List elements while iterating with a for loop, regardless of the scenario, because it leads to logical bugs and runtime exceptions.

The Alibaba development manual also explicitly forbids using foreach to add or remove List elements.

Correct ways to remove elements include using an Iterator :

List<String> list = new ArrayList<>();
Iterator<String> iterator = list.iterator();
while (iterator.hasNext()) {
    // delete element
    iterator.remove();
}

Or, with Java 8+ lambda syntax:

list.removeIf(s -> s.contains("a"));

If you only want to know why the for-loop deletion is prohibited, you can stop reading now; otherwise, continue for the detailed reasons.

Example

Scenario: a List<String> with N elements, remove all elements containing the character "a".

List<String> list = new ArrayList<>(4);
list.add("a");
list.add("ab");
list.add("abc");
list.add("abcd");

Correct Solution

public static void main(String[] args) {
    List<String> list = new ArrayList<>(4);
    list.add("a");
    list.add("ab");
    list.add("abc");
    list.add("abcd");

    Iterator<String> iterator = list.iterator();
    while (iterator.hasNext()) {
        if (iterator.next().contains("a")) {
            // delete element
            iterator.remove();
        }
    }
    System.out.println(list);
}

Output:

[]

Wrong Answer 1: Classic for-loop

public static void main(String[] args) {
    List<String> list = new ArrayList<>(4);
    list.add("a");
    list.add("ab");
    list.add("abc");
    list.add("abcd");

    for (int i = 0; i < list.size(); i++) {
        if (list.get(i).contains("a")) {
            list.remove(i);
        }
    }
    System.out.println(list);
}

Output:

[ab, abcd]

Analysis: Deleting while iterating with an index causes the subsequent elements to shift, so the next element is skipped, leading to incorrect results.

Although the code runs without error, it is a bug because the logic misses elements after each deletion, making it unreliable in production.

Wrong Answer 2: Enhanced for-loop (foreach)

public static void main(String[] args) {
    List<String> list = new ArrayList<>(4);
    list.add("a");
    list.add("ab");
    list.add("abc");
    list.add("abcd");

    for (String str : list) {
        if (str.contains("a")) {
            list.remove(str);
        }
    }
    System.out.println(list);
}

Runtime error:

Exception in thread "main" java.util.ConcurrentModificationException
    at java.util.ArrayList$Itr.checkForComodification(ArrayList.java:909)
    at java.util.ArrayList$Itr.next(ArrayList.java:859)
    at ...

Explanation: The foreach loop does not support structural modifications (add/remove) of the collection during iteration, though it allows element modification.

Conclusion: Use an Iterator or Java 8+ removeIf for safe removal; avoid modifying a List inside any form of for-loop.