org.apache.commons.collections4.iterators

## Class LoopingListIterator<E>

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

- All Implemented Interfaces:
    Iterator<E>, ListIterator<E>, OrderedIterator<E>, ResettableIterator<E>, ResettableListIterator<E>

      public class LoopingListIterator<E>
      extends Object
      implements ResettableListIterator<E>

  A ListIterator that restarts when it reaches the end or when it reaches the beginning.

  The iterator will loop continuously around the provided list, unless there are no elements in the collection to begin with, or all of the elements have been `removed`.

  Concurrent modifications are not directly supported, and for most collection implementations will throw a ConcurrentModificationException.

  Since:
    3.2
  Version:
    $Id: LoopingListIterator.java 1494333 2013-06-18 21:59:21Z sebb $

- - ### Constructor Summary

    Constructors
    Constructor and Description
    `LoopingListIterator(List<E> list)` Constructor that wraps a list.

  - ### Method Summary

    Methods
    Modifier and Type | Method and Description
    --- | ---
    `void` | `add(E obj)` Inserts the specified element into the underlying list.
    `boolean` | `hasNext()` Returns whether this iterator has any more elements.
    `boolean` | `hasPrevious()` Returns whether this iterator has any more previous elements.
    `E` | `next()` Returns the next object in the list.
    `int` | `nextIndex()` Returns the index of the element that would be returned by a subsequent call to `next()`.
    `E` | `previous()` Returns the previous object in the list.
    `int` | `previousIndex()` Returns the index of the element that would be returned by a subsequent call to `previous()`.
    `void` | `remove()` Removes the previously retrieved item from the underlying list.
    `void` | `reset()` Resets the iterator back to the start of the list.
    `void` | `set(E obj)` Replaces the last element that was returned by `next()` or `previous()`.
    `int` | `size()` Gets the size of the list underlying the iterator.

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

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

- - ### Constructor Detail

    - #### LoopingListIterator

          public LoopingListIterator(List<E> list)

      Constructor that wraps a list.

      There is no way to reset a ListIterator instance without recreating it from the original source, so the List must be passed in and a reference to it held.

      Parameters:
        `list` - the list to wrap
      Throws:
        `NullPointerException` - if the list it null

  - ### Method Detail

    - #### hasNext

          public boolean hasNext()

      Returns whether this iterator has any more elements.

      Returns false only if the list originally had zero elements, or all elements have been `removed`.

      Specified by:
        `hasNext` in interface `Iterator<E>`
      Specified by:
        `hasNext` in interface `ListIterator<E>`
      Returns:
        `true` if there are more elements

    - #### next

          public E next()

      Returns the next object in the list.

      If at the end of the list, returns the first element.

      Specified by:
        `next` in interface `Iterator<E>`
      Specified by:
        `next` in interface `ListIterator<E>`
      Returns:
        the object after the last element returned
      Throws:
        `NoSuchElementException` - if there are no elements in the list

    - #### nextIndex

          public int nextIndex()

      Returns the index of the element that would be returned by a subsequent call to `next()`.

      As would be expected, if the iterator is at the physical end of the underlying list, 0 is returned, signifying the beginning of the list.

      Specified by:
        `nextIndex` in interface `ListIterator<E>`
      Returns:
        the index of the element that would be returned if next() were called
      Throws:
        `NoSuchElementException` - if there are no elements in the list

    - #### hasPrevious

          public boolean hasPrevious()

      Returns whether this iterator has any more previous elements.

      Returns false only if the list originally had zero elements, or all elements have been `removed`.

      Specified by:
        `hasPrevious` in interface `ListIterator<E>`
      Specified by:
        `hasPrevious` in interface `OrderedIterator<E>`
      Returns:
        `true` if there are more elements

    - #### previous

          public E previous()

      Returns the previous object in the list.

      If at the beginning of the list, return the last element. Note that in this case, traversal to find that element takes linear time.

      Specified by:
        `previous` in interface `ListIterator<E>`
      Specified by:
        `previous` in interface `OrderedIterator<E>`
      Returns:
        the object before the last element returned
      Throws:
        `NoSuchElementException` - if there are no elements in the list

    - #### previousIndex

          public int previousIndex()

      Returns the index of the element that would be returned by a subsequent call to `previous()`.

      As would be expected, if at the iterator is at the physical beginning of the underlying list, the list's size minus one is returned, signifying the end of the list.

      Specified by:
        `previousIndex` in interface `ListIterator<E>`
      Returns:
        the index of the element that would be returned if previous() were called
      Throws:
        `NoSuchElementException` - if there are no elements in the list

    - #### remove

          public void remove()

      Removes the previously retrieved item from the underlying list.

      This feature is only supported if the underlying list's `iterator` method returns an implementation that supports it.

      This method can only be called after at least one `next()` or `previous()` method call. After a removal, the remove method may not be called again until another `next()` or `previous()` has been performed. If the `reset()` is called, then remove may not be called until `next()` or `previous()` is called again.

      Specified by:
        `remove` in interface `Iterator<E>`
      Specified by:
        `remove` in interface `ListIterator<E>`
      Throws:
        `UnsupportedOperationException` - if the remove method is not supported by the iterator implementation of the underlying list

    - #### add

          public void add(E obj)

      Inserts the specified element into the underlying list.

      The element is inserted before the next element that would be returned by `next()`, if any, and after the next element that would be returned by `previous()`, if any.

      This feature is only supported if the underlying list's `List.listIterator()` method returns an implementation that supports it.

      Specified by:
        `add` in interface `ListIterator<E>`
      Parameters:
        `obj` - the element to insert
      Throws:
        `UnsupportedOperationException` - if the add method is not supported by the iterator implementation of the underlying list

    - #### set

          public void set(E obj)

      Replaces the last element that was returned by `next()` or `previous()`.

      This feature is only supported if the underlying list's `List.listIterator()` method returns an implementation that supports it.

      Specified by:
        `set` in interface `ListIterator<E>`
      Parameters:
        `obj` - the element with which to replace the last element returned
      Throws:
        `UnsupportedOperationException` - if the set method is not supported by the iterator implementation of the underlying list

    - #### reset

          public void reset()

      Resets the iterator back to the start of the list.
      Specified by:
        `reset` in interface `ResettableIterator<E>`

    - #### size

          public int size()

      Gets the size of the list underlying the iterator.
      Returns:
        the current list size

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