What is the purpose of using the Fail-Fast concept when changing data collections?

The Fail-Fast concept when working with collections in Java is aimed at quickly detecting errors related to concurrent modification of the collection during its traversal (iteration). For example, if one part of the code iterates over the collection, and another modifies its structure (adds or removes elements), the Fail-Fast mechanism immediately throws a ConcurrentModificationException.

This helps to avoid hidden errors and unpredictable program behavior, as the error is detected at the moment it occurs, rather than later when the data may already be corrupted.

Example:

List<String> list = new ArrayList<>();
list.add("a");
list.add("b");

for (String s : list) {
    if (s.equals("a")) {
        list.remove(s); // will cause ConcurrentModificationException
    }
}

Fail-Fast ensures safety and predictability when working with collections in a multithreaded environment or during modifications while iterating.