javautilIterator

The java.util.Iterator interface is part of the Java standard library and provides a uniform way to traverse the elements of a collection in a forward direction. It is generic, declared as Iterator<E>, and is implemented by most Java collection classes such as ArrayList, LinkedList, HashSet, and the views of maps.

The core methods of Iterator are hasNext(), next(), and remove() (where remove is optional). hasNext() returns true if there are more elements to traverse. next() returns the next element and advances the iterator; it throws NoSuchElementException if there are no further elements. remove() removes the last element returned by next; it is an optional operation and may throw IllegalStateException if next has not yet been called or if remove has already been called after the last next, or UnsupportedOperationException if the underlying iterator does not support removal.

Usage typically involves obtaining an iterator from a collection via collection.iterator(), then looping while hasNext() is true and calling next() to process each element. The remove method can be used to delete elements from the underlying collection during iteration. The enhanced for loop (for-each) uses an Iterator under the hood and provides a more concise way to iterate.

Many iterators are fail-fast: if the underlying collection is structurally modified after the iterator is created (except through the iterator’s own remove method), a ConcurrentModificationException may be thrown on a subsequent operation.

Notes: Iterator traverses elements forward only; to traverse bidirectionally, ListIterator offers additional capabilities. Implementations may vary in behavior and performance characteristics, but the contract remains consistent across standard Java collections.