org.apache.commons.collections4.iterators

## Class PeekingIterator<E>

- java.lang.Object
- - org.apache.commons.collections4.iterators.PeekingIterator<E>

- All Implemented Interfaces:
    Iterator<E>

      public class PeekingIterator<E>
      extends Object
      implements Iterator<E>

  Decorates an iterator to support one-element lookahead while iterating.

  The decorator supports the removal operation, but an `IllegalStateException` will be thrown if `remove()` is called directly after a call to `peek()` or `element()`.

  Since:
    4.0
  Version:
    $Id: PeekingIterator.java 1686855 2015-06-22 13:00:27Z tn $

- - ### Constructor Summary

    Constructors
    Constructor and Description
    `PeekingIterator(Iterator<? extends E> iterator)` Constructor.

  - ### Method Summary

    Methods
    Modifier and Type | Method and Description
    --- | ---
    `E` | `element()` Returns the next element in iteration without advancing the underlying iterator.
    `boolean` | `hasNext()`
    `E` | `next()`
    `E` | `peek()` Returns the next element in iteration without advancing the underlying iterator.
    `static <E> PeekingIterator<E>` | `peekingIterator(Iterator<? extends E> iterator)` Decorates the specified iterator to support one-element lookahead.
    `void` | `remove()`

    - ### Methods inherited from class java.lang.Object

      `clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait`

- - ### Constructor Detail

    - #### PeekingIterator

          public PeekingIterator(Iterator<? extends E> iterator)

      Constructor.
      Parameters:
        `iterator` - the iterator to decorate

  - ### Method Detail

    - #### peekingIterator

          public static <E> PeekingIterator<E> peekingIterator(Iterator<? extends E> iterator)

      Decorates the specified iterator to support one-element lookahead.

      If the iterator is already a `PeekingIterator` it is returned directly.

      Type Parameters:
        `E` - the element type
      Parameters:
        `iterator` - the iterator to decorate
      Returns:
        a new peeking iterator
      Throws:
        `NullPointerException` - if the iterator is null

    - #### hasNext

          public boolean hasNext()

      Specified by:
        `hasNext` in interface `Iterator<E>`

    - #### peek

          public E peek()

      Returns the next element in iteration without advancing the underlying iterator. If the iterator is already exhausted, null will be returned.

      Note: this method does not throw a `NoSuchElementException` if the iterator is already exhausted. If you want such a behavior, use `element()` instead.

      The rationale behind this is to follow the `Queue` interface which uses the same terminology.

      Returns:
        the next element from the iterator

    - #### element

          public E element()

      Returns the next element in iteration without advancing the underlying iterator. If the iterator is already exhausted, null will be returned.
      Returns:
        the next element from the iterator
      Throws:
        `NoSuchElementException` - if the iterator is already exhausted according to `hasNext()`

    - #### next

          public E next()

      Specified by:
        `next` in interface `Iterator<E>`

    - #### remove

          public void remove()

      Specified by:
        `remove` in interface `Iterator<E>`
      Throws:
        `IllegalStateException` - if `peek()` or `element()` has been called prior to the call to `remove()`

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