An Iterator is an interface in Java used to traverse collections one element at a time. It provides a uniform way to access elements regardless of the collection type.

• hasNext() → returns true if more elements exist.
• next() → returns the next element.
• remove() → removes the last element returned by next().

• Enumeration is legacy and only allows element traversal.
• Iterator is modern, allows element removal, and is safer with fail-fast behavior.

Iterators of most Java collections (like ArrayList, HashMap) are fail-fast, meaning they throw ConcurrentModificationException if the collection is modified while iterating. Some classes (like CopyOnWriteArrayList) provide fail-safe iterators.

• Iterator → works with all collections, forward-only traversal.
• ListIterator → works only with List, allows bidirectional traversal, and provides more methods like previous(), set(), and add().