How to Prevent ConcurrentModificationException While Modifying a Map in Java

How can I avoid ConcurrentModificationException when I am iterating over a map and simultaneously changing its values in Java?

Map<String, Integer> map = new HashMap<>();
map.put("one", 1);
map.put("two", 2);

for (String key : map.keySet()) {
    if (key.equals("two")) {
        // Incorrect: This line will throw ConcurrentModificationException
        map.remove(key);
    }
}

The `ConcurrentModificationException` in Java occurs when a collection is modified while it is being iterated. This can happen if you try to remove or add elements to a map or collection directly within a loop. To avoid this exception, you can use various methods such as using an iterator, the `forEach` method, or concurrent collections.

// Using an Iterator to avoid ConcurrentModificationException
Iterator<Map.Entry<String, Integer>> it = map.entrySet().iterator();
while (it.hasNext()) {
    Map.Entry<String, Integer> entry = it.next();
    if (entry.getKey().equals("two")) {
        it.remove(); // Safely remove the entry using Iterator's remove method
    }
}

Causes

• Modifying a collection directly while iterating over it using enhanced for-loop or forEach method.
• Using methods that are not thread-safe on shared collections in concurrent environments.

Solutions

• Use an Iterator to modify the collection safely by using its `remove()` method.
• Update the collection after finishing the iteration, or use `entrySet()` for maps and modify values safely.
• Utilize the `Map.forEach` method introduced in Java 8, which allows modifying values without causing a `ConcurrentModificationException`.
• Consider using concurrent collections, such as `ConcurrentHashMap`, which are designed for concurrent access.

Mistake: Using the enhanced for-loop to iterate and call remove directly on the map.

Solution: Switch to using an Iterator to avoid ConcurrentModificationException.

Mistake: Assuming the concurrent collections will automatically handle all cases of modification.

Solution: Always check your use case; use `ConcurrentHashMap` for thread-safe operations, but manage concurrent modifications carefully.