Avoid Removing Elements While Iterating: Java ConcurrentModificationException Example and Fix

I encountered a bug in a company product where removing an element from a Map while iterating caused a crash due to a ConcurrentModificationException.

Important reminder: do not delete elements during iteration.

Reproduction code:

Map
abc = new HashMap<>();
abc.put(1, 2);
abc.put(2, 3);
abc.put(3, 4);
Set
keys = abc.keySet();
for (int i : keys) {
    if (i == 2) {
        abc.remove(i);
    }
}

The exception thrown is:

Exception in thread "main" java.util.ConcurrentModificationException
    at java.util.HashMap$HashIterator.nextNode(HashMap.java:1429)
    at java.util.HashMap$KeyIterator.next(HashMap.java:1453)
    at source.Practise.testDemo(Practise.java:23)
    at source.Practise.main(Practise.java:12)

Correct approach: first collect the keys to be removed, then delete them after the iteration completes.

Map
abc = new HashMap<>();
abc.put(1, 2);
abc.put(2, 3);
abc.put(3, 4);
Set
keys = abc.keySet();
List
toRemove = new ArrayList<>();
for (int i : keys) {
    if (i == 2) {
        toRemove.add(i);
    }
}
for (int i : toRemove) {
    abc.remove(i);
}

This method avoids modifying the collection during iteration, preventing the ConcurrentModificationException.