Iterator (Java SE 15 & JDK 15)
Description: declaration: module: java.base, package: java.util, interface: Iterator
Keywords: java.util.Iterator interface

Module java.base
Package java.util

# Interface Iterator<E>

Type Parameters:
  `E` - the type of elements returned by this iterator

All Known Subinterfaces:
  `EventIterator`, `ListIterator<E>`, `PrimitiveIterator<T,​T_CONS>`, `PrimitiveIterator.OfDouble`, `PrimitiveIterator.OfInt`, `PrimitiveIterator.OfLong`, `XMLEventReader`

All Known Implementing Classes:
  `BeanContextSupport.BCSIterator`, `EventReaderDelegate`, `Scanner`

    public interface Iterator<E>

An iterator over a collection. `Iterator` takes the place of `Enumeration` in the Java Collections Framework. Iterators differ from enumerations in two ways:

- Iterators allow the caller to remove elements from the underlying collection during the iteration with well-defined semantics.
- Method names have been improved.

This interface is a member of the Java Collections Framework.

API Note:
  An `Enumeration` can be converted into an `Iterator` by using the `Enumeration.asIterator()` method.
Since:
  1.2
See Also:
  `Collection`, `ListIterator`, `Iterable`

- ## Method Summary

  Modifier and Type | Method | Description
  --- | --- | ---
  `default void` | `forEachRemaining​(Consumer<? super E> action)` | Performs the given action for each remaining element until all elements have been processed or the action throws an exception.
  `boolean` | `hasNext()` | Returns `true` if the iteration has more elements.
  `E` | `next()` | Returns the next element in the iteration.
  `default void` | `remove()` | Removes from the underlying collection the last element returned by this iterator (optional operation).

- ## Method Details

  - ### hasNext

    boolean hasNext()
    Returns `true` if the iteration has more elements. (In other words, returns `true` if `next()` would return an element rather than throwing an exception.)
    Returns:
      `true` if the iteration has more elements

  - ### next

    E next()
    Returns the next element in the iteration.
    Returns:
      the next element in the iteration
    Throws:
      `NoSuchElementException` - if the iteration has no more elements

  - ### remove

    default void remove()
    Removes from the underlying collection the last element returned by this iterator (optional operation). This method can be called only once per call to `next()`.

    The behavior of an iterator is unspecified if the underlying collection is modified while the iteration is in progress in any way other than by calling this method, unless an overriding class has specified a concurrent modification policy.

    The behavior of an iterator is unspecified if this method is called after a call to the `forEachRemaining` method.

    Implementation Requirements:
      The default implementation throws an instance of `UnsupportedOperationException` and performs no other action.
    Throws:
      `UnsupportedOperationException` - if the `remove` operation is not supported by this iterator
      `IllegalStateException` - if the `next` method has not yet been called, or the `remove` method has already been called after the last call to the `next` method

  - ### forEachRemaining

    default void forEachRemaining​(Consumer<? super E> action)
    Performs the given action for each remaining element until all elements have been processed or the action throws an exception. Actions are performed in the order of iteration, if that order is specified. Exceptions thrown by the action are relayed to the caller.

    The behavior of an iterator is unspecified if the action modifies the collection in any way (even by calling the `remove` method or other mutator methods of `Iterator` subtypes), unless an overriding class has specified a concurrent modification policy.

    Subsequent behavior of an iterator is unspecified if the action throws an exception.

    Implementation Requirements:

      The default implementation behaves as if:

               while (hasNext())
                   action.accept(next());

    Parameters:
      `action` - The action to be performed for each element
    Throws:
      `NullPointerException` - if the specified action is null
    Since:
      1.8