Any exception that is a subclass of Exception (or Exception itself), except for RuntimeException and it’s subclasses, is a checked exception.
The Java compiler forces you to catch checked exceptions (and either swallow or re-throw).
Some common examples of checked exceptions are:
The exception can be thrown while trying to remove an element from a Collection (e.g. an ArrayList) while iterating over it using a standard for loop.
Another way a ConcurrentModificationException can be thrown is if you forget to copy a Collection and rather take a reference to it, and then try and access the Collection.
For example, the following line won’t actually copy the entire list:
// This does not actually copy the list
ArrayList<Integer> newList = oldList.subList(0, 5);
oldList.get(2); // This will throw a ConcurrentModificationException
// Perform a deep copy of the list
ArrayList<Integer> newListSafe = new ArrayList<>(oldList.subList(0, 5));
oldList.get(2); // This is o.k., since newListSafe is a deep copy of the list