java.util

## Class LinkedList<E>

- java.lang.Object
- - java.util.AbstractCollection<E>
  - - java.util.AbstractList<E>
    - - java.util.AbstractSequentialList<E>
      - - java.util.LinkedList<E>

- Type Parameters:
    `E` - the type of elements held in this collection
  All Implemented Interfaces:
    Serializable, Cloneable, Iterable<E>, Collection<E>, Deque<E>, List<E>, Queue<E>

      public class LinkedList<E>
      extends AbstractSequentialList<E>
      implements List<E>, Deque<E>, Cloneable, Serializable

  Linked list implementation of the `List` interface. Implements all optional list operations, and permits all elements (including `null`). In addition to implementing the `List` interface, the `LinkedList` class provides uniformly named methods to `get`, `remove` and `insert` an element at the beginning and end of the list. These operations allow linked lists to be used as a stack, queue, or double-ended queue.

  The class implements the `Deque` interface, providing first-in-first-out queue operations for `add`, `poll`, along with other stack and deque operations.

  All of the operations perform as could be expected for a doubly-linked list. Operations that index into the list will traverse the list from the beginning or the end, whichever is closer to the specified index.

  Note that this implementation is not synchronized. If multiple threads access a linked list concurrently, and at least one of the threads modifies the list structurally, it must be synchronized externally. (A structural modification is any operation that adds or deletes one or more elements; merely setting the value of an element is not a structural modification.) This is typically accomplished by synchronizing on some object that naturally encapsulates the list. If no such object exists, the list should be "wrapped" using the `Collections.synchronizedList` method. This is best done at creation time, to prevent accidental unsynchronized access to the list:

         List list = Collections.synchronizedList(new LinkedList(...));

  The iterators returned by this class's `iterator` and `listIterator` methods are fail-fast: if the list is structurally modified at any time after the iterator is created, in any way except through the Iterator's own `remove` or `add` methods, the iterator will throw a `ConcurrentModificationException`. Thus, in the face of concurrent modification, the iterator fails quickly and cleanly, rather than risking arbitrary, non-deterministic behavior at an undetermined time in the future.

  Note that the fail-fast behavior of an iterator cannot be guaranteed as it is, generally speaking, impossible to make any hard guarantees in the presence of unsynchronized concurrent modification. Fail-fast iterators throw `ConcurrentModificationException` on a best-effort basis. Therefore, it would be wrong to write a program that depended on this exception for its correctness: the fail-fast behavior of iterators should be used only to detect bugs.

  This class is a member of the Java Collections Framework.

  Since:
    1.2
  See Also:
    `List`, `ArrayList`, `Vector`, Serialized Form

- - ### Field Summary

    - ### Fields inherited from class java.util.AbstractList

      `modCount`

  - ### Constructor Summary

    Constructors
    Constructor and Description
    `LinkedList()` Constructs an empty list.
    `LinkedList(Collection<? extends E> c)` Constructs a list containing the elements of the specified collection, in the order they are returned by the collection's iterator.

  - ### Method Summary

    Methods
    Modifier and Type | Method and Description
    --- | ---
    `boolean` | `add(E e)` Appends the specified element to the end of this list.
    `void` | `add(int index, E element)` Inserts the specified element at the specified position in this list.
    `boolean` | `addAll(Collection<? extends E> c)` Appends all of the elements in the specified collection to the end of this list, in the order that they are returned by the specified collection's iterator.
    `boolean` | `addAll(int index, Collection<? extends E> c)` Inserts all of the elements in the specified collection into this list, starting at the specified position.
    `void` | `addFirst(E e)` Inserts the specified element at the beginning of this list.
    `void` | `addLast(E e)` Appends the specified element to the end of this list.
    `void` | `clear()` Removes all of the elements from this list.
    `Object` | `clone()` Returns a shallow copy of this `LinkedList`.
    `boolean` | `contains(Object o)` Returns `true` if this list contains the specified element.
    `Iterator<E>` | `descendingIterator()` Returns an iterator over the elements in this deque in reverse sequential order.
    `E` | `element()` Retrieves, but does not remove, the head (first element) of this list.
    `E` | `get(int index)` Returns the element at the specified position in this list.
    `E` | `getFirst()` Returns the first element in this list.
    `E` | `getLast()` Returns the last element in this list.
    `int` | `indexOf(Object o)` Returns the index of the first occurrence of the specified element in this list, or -1 if this list does not contain the element.
    `int` | `lastIndexOf(Object o)` Returns the index of the last occurrence of the specified element in this list, or -1 if this list does not contain the element.
    `ListIterator<E>` | `listIterator(int index)` Returns a list-iterator of the elements in this list (in proper sequence), starting at the specified position in the list.
    `boolean` | `offer(E e)` Adds the specified element as the tail (last element) of this list.
    `boolean` | `offerFirst(E e)` Inserts the specified element at the front of this list.
    `boolean` | `offerLast(E e)` Inserts the specified element at the end of this list.
    `E` | `peek()` Retrieves, but does not remove, the head (first element) of this list.
    `E` | `peekFirst()` Retrieves, but does not remove, the first element of this list, or returns `null` if this list is empty.
    `E` | `peekLast()` Retrieves, but does not remove, the last element of this list, or returns `null` if this list is empty.
    `E` | `poll()` Retrieves and removes the head (first element) of this list
    `E` | `pollFirst()` Retrieves and removes the first element of this list, or returns `null` if this list is empty.
    `E` | `pollLast()` Retrieves and removes the last element of this list, or returns `null` if this list is empty.
    `E` | `pop()` Pops an element from the stack represented by this list.
    `void` | `push(E e)` Pushes an element onto the stack represented by this list.
    `E` | `remove()` Retrieves and removes the head (first element) of this list.
    `E` | `remove(int index)` Removes the element at the specified position in this list.
    `boolean` | `remove(Object o)` Removes the first occurrence of the specified element from this list, if it is present.
    `E` | `removeFirst()` Removes and returns the first element from this list.
    `boolean` | `removeFirstOccurrence(Object o)` Removes the first occurrence of the specified element in this list (when traversing the list from head to tail).
    `E` | `removeLast()` Removes and returns the last element from this list.
    `boolean` | `removeLastOccurrence(Object o)` Removes the last occurrence of the specified element in this list (when traversing the list from head to tail).
    `E` | `set(int index, E element)` Replaces the element at the specified position in this list with the specified element.
    `int` | `size()` Returns the number of elements in this list.
    `Object[]` | `toArray()` Returns an array containing all of the elements in this list in proper sequence (from first to last element).
    `<T> T[]` | `toArray(T[] a)` Returns an array containing all of the elements in this list in proper sequence (from first to last element); the runtime type of the returned array is that of the specified array.

    - ### Methods inherited from class java.util.AbstractSequentialList

      `iterator`

    - ### Methods inherited from class java.util.AbstractList

      `equals, hashCode, listIterator, removeRange, subList`

    - ### Methods inherited from class java.util.AbstractCollection

      `containsAll, isEmpty, removeAll, retainAll, toString`

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

      `finalize, getClass, notify, notifyAll, wait, wait, wait`

    - ### Methods inherited from interface java.util.List

      `containsAll, equals, hashCode, isEmpty, iterator, listIterator, removeAll, retainAll, subList`

    - ### Methods inherited from interface java.util.Deque

      `iterator`

- - ### Constructor Detail

    - #### LinkedList

          public LinkedList()

      Constructs an empty list.

    - #### LinkedList

          public LinkedList(Collection<? extends E> c)

      Constructs a list containing the elements of the specified collection, in the order they are returned by the collection's iterator.
      Parameters:
        `c` - the collection whose elements are to be placed into this list
      Throws:
        `NullPointerException` - if the specified collection is null

  - ### Method Detail

    - #### getFirst

          public E getFirst()

      Returns the first element in this list.
      Specified by:
        `getFirst` in interface `Deque<E>`
      Returns:
        the first element in this list
      Throws:
        `NoSuchElementException` - if this list is empty

    - #### getLast

          public E getLast()

      Returns the last element in this list.
      Specified by:
        `getLast` in interface `Deque<E>`
      Returns:
        the last element in this list
      Throws:
        `NoSuchElementException` - if this list is empty

    - #### removeFirst

          public E removeFirst()

      Removes and returns the first element from this list.
      Specified by:
        `removeFirst` in interface `Deque<E>`
      Returns:
        the first element from this list
      Throws:
        `NoSuchElementException` - if this list is empty

    - #### removeLast

          public E removeLast()

      Removes and returns the last element from this list.
      Specified by:
        `removeLast` in interface `Deque<E>`
      Returns:
        the last element from this list
      Throws:
        `NoSuchElementException` - if this list is empty

    - #### addFirst

          public void addFirst(E e)

      Inserts the specified element at the beginning of this list.
      Specified by:
        `addFirst` in interface `Deque<E>`
      Parameters:
        `e` - the element to add

    - #### addLast

          public void addLast(E e)

      Appends the specified element to the end of this list.

      This method is equivalent to `add(E)`.

      Specified by:
        `addLast` in interface `Deque<E>`
      Parameters:
        `e` - the element to add

    - #### contains

          public boolean contains(Object o)

      Returns `true` if this list contains the specified element. More formally, returns `true` if and only if this list contains at least one element `e` such that `(o==null ? e==null : o.equals(e))`.
      Specified by:
        `contains` in interface `Collection<E>`
      Specified by:
        `contains` in interface `Deque<E>`
      Specified by:
        `contains` in interface `List<E>`
      Overrides:
        `contains` in class `AbstractCollection<E>`
      Parameters:
        `o` - element whose presence in this list is to be tested
      Returns:
        `true` if this list contains the specified element

    - #### size

          public int size()

      Returns the number of elements in this list.
      Specified by:
        `size` in interface `Collection<E>`
      Specified by:
        `size` in interface `Deque<E>`
      Specified by:
        `size` in interface `List<E>`
      Specified by:
        `size` in class `AbstractCollection<E>`
      Returns:
        the number of elements in this list

    - #### add

          public boolean add(E e)

      Appends the specified element to the end of this list.

      This method is equivalent to `addLast(E)`.

      Specified by:
        `add` in interface `Collection<E>`
      Specified by:
        `add` in interface `Deque<E>`
      Specified by:
        `add` in interface `List<E>`
      Specified by:
        `add` in interface `Queue<E>`
      Overrides:
        `add` in class `AbstractList<E>`
      Parameters:
        `e` - element to be appended to this list
      Returns:
        `true` (as specified by `Collection.add(E)`)

    - #### remove

          public boolean remove(Object o)

      Removes the first occurrence of the specified element from this list, if it is present. If this list does not contain the element, it is unchanged. More formally, removes the element with the lowest index `i` such that `(o==null ? get(i)==null : o.equals(get(i)))` (if such an element exists). Returns `true` if this list contained the specified element (or equivalently, if this list changed as a result of the call).
      Specified by:
        `remove` in interface `Collection<E>`
      Specified by:
        `remove` in interface `Deque<E>`
      Specified by:
        `remove` in interface `List<E>`
      Overrides:
        `remove` in class `AbstractCollection<E>`
      Parameters:
        `o` - element to be removed from this list, if present
      Returns:
        `true` if this list contained the specified element

    - #### addAll

          public boolean addAll(Collection<? extends E> c)

      Appends all of the elements in the specified collection to the end of this list, in the order that they are returned by the specified collection's iterator. The behavior of this operation is undefined if the specified collection is modified while the operation is in progress. (Note that this will occur if the specified collection is this list, and it's nonempty.)
      Specified by:
        `addAll` in interface `Collection<E>`
      Specified by:
        `addAll` in interface `List<E>`
      Overrides:
        `addAll` in class `AbstractCollection<E>`
      Parameters:
        `c` - collection containing elements to be added to this list
      Returns:
        `true` if this list changed as a result of the call
      Throws:
        `NullPointerException` - if the specified collection is null
      See Also:
        `AbstractCollection.add(Object)`

    - #### addAll

          public boolean addAll(int index,
                       Collection<? extends E> c)

      Inserts all of the elements in the specified collection into this list, starting at the specified position. Shifts the element currently at that position (if any) and any subsequent elements to the right (increases their indices). The new elements will appear in the list in the order that they are returned by the specified collection's iterator.
      Specified by:
        `addAll` in interface `List<E>`
      Overrides:
        `addAll` in class `AbstractSequentialList<E>`
      Parameters:
        `index` - index at which to insert the first element from the specified collection
        `c` - collection containing elements to be added to this list
      Returns:
        `true` if this list changed as a result of the call
      Throws:
        `IndexOutOfBoundsException` - if the index is out of range (`index < 0 || index > size()`)
        `NullPointerException` - if the specified collection is null

    - #### clear

          public void clear()

      Removes all of the elements from this list.
      Specified by:
        `clear` in interface `Collection<E>`
      Specified by:
        `clear` in interface `List<E>`
      Overrides:
        `clear` in class `AbstractList<E>`

    - #### get

          public E get(int index)

      Returns the element at the specified position in this list.
      Specified by:
        `get` in interface `List<E>`
      Overrides:
        `get` in class `AbstractSequentialList<E>`
      Parameters:
        `index` - index of the element to return
      Returns:
        the element at the specified position in this list
      Throws:
        `IndexOutOfBoundsException` - if the index is out of range (`index < 0 || index >= size()`)

    - #### set

          public E set(int index,
              E element)

      Replaces the element at the specified position in this list with the specified element.
      Specified by:
        `set` in interface `List<E>`
      Overrides:
        `set` in class `AbstractSequentialList<E>`
      Parameters:
        `index` - index of the element to replace
        `element` - element to be stored at the specified position
      Returns:
        the element previously at the specified position
      Throws:
        `IndexOutOfBoundsException` - if the index is out of range (`index < 0 || index >= size()`)

    - #### add

          public void add(int index,
                 E element)

      Inserts the specified element at the specified position in this list. Shifts the element currently at that position (if any) and any subsequent elements to the right (adds one to their indices).
      Specified by:
        `add` in interface `List<E>`
      Overrides:
        `add` in class `AbstractSequentialList<E>`
      Parameters:
        `index` - index at which the specified element is to be inserted
        `element` - element to be inserted
      Throws:
        `IndexOutOfBoundsException` - if the index is out of range (`index < 0 || index > size()`)

    - #### remove

          public E remove(int index)

      Removes the element at the specified position in this list. Shifts any subsequent elements to the left (subtracts one from their indices). Returns the element that was removed from the list.
      Specified by:
        `remove` in interface `List<E>`
      Overrides:
        `remove` in class `AbstractSequentialList<E>`
      Parameters:
        `index` - the index of the element to be removed
      Returns:
        the element previously at the specified position
      Throws:
        `IndexOutOfBoundsException` - if the index is out of range (`index < 0 || index >= size()`)

    - #### indexOf

          public int indexOf(Object o)

      Returns the index of the first occurrence of the specified element in this list, or -1 if this list does not contain the element. More formally, returns the lowest index `i` such that `(o==null ? get(i)==null : o.equals(get(i)))`, or -1 if there is no such index.
      Specified by:
        `indexOf` in interface `List<E>`
      Overrides:
        `indexOf` in class `AbstractList<E>`
      Parameters:
        `o` - element to search for
      Returns:
        the index of the first occurrence of the specified element in this list, or -1 if this list does not contain the element

    - #### lastIndexOf

          public int lastIndexOf(Object o)

      Returns the index of the last occurrence of the specified element in this list, or -1 if this list does not contain the element. More formally, returns the highest index `i` such that `(o==null ? get(i)==null : o.equals(get(i)))`, or -1 if there is no such index.
      Specified by:
        `lastIndexOf` in interface `List<E>`
      Overrides:
        `lastIndexOf` in class `AbstractList<E>`
      Parameters:
        `o` - element to search for
      Returns:
        the index of the last occurrence of the specified element in this list, or -1 if this list does not contain the element

    - #### peek

          public E peek()

      Retrieves, but does not remove, the head (first element) of this list.
      Specified by:
        `peek` in interface `Deque<E>`
      Specified by:
        `peek` in interface `Queue<E>`
      Returns:
        the head of this list, or `null` if this list is empty
      Since:
        1.5

    - #### element

          public E element()

      Retrieves, but does not remove, the head (first element) of this list.
      Specified by:
        `element` in interface `Deque<E>`
      Specified by:
        `element` in interface `Queue<E>`
      Returns:
        the head of this list
      Throws:
        `NoSuchElementException` - if this list is empty
      Since:
        1.5

    - #### poll

          public E poll()

      Retrieves and removes the head (first element) of this list
      Specified by:
        `poll` in interface `Deque<E>`
      Specified by:
        `poll` in interface `Queue<E>`
      Returns:
        the head of this list, or `null` if this list is empty
      Since:
        1.5

    - #### remove

          public E remove()

      Retrieves and removes the head (first element) of this list.
      Specified by:
        `remove` in interface `Deque<E>`
      Specified by:
        `remove` in interface `Queue<E>`
      Returns:
        the head of this list
      Throws:
        `NoSuchElementException` - if this list is empty
      Since:
        1.5

    - #### offer

          public boolean offer(E e)

      Adds the specified element as the tail (last element) of this list.
      Specified by:
        `offer` in interface `Deque<E>`
      Specified by:
        `offer` in interface `Queue<E>`
      Parameters:
        `e` - the element to add
      Returns:
        `true` (as specified by `Queue.offer(E)`)
      Since:
        1.5

    - #### offerFirst

          public boolean offerFirst(E e)

      Inserts the specified element at the front of this list.
      Specified by:
        `offerFirst` in interface `Deque<E>`
      Parameters:
        `e` - the element to insert
      Returns:
        `true` (as specified by `Deque.offerFirst(E)`)
      Since:
        1.6

    - #### offerLast

          public boolean offerLast(E e)

      Inserts the specified element at the end of this list.
      Specified by:
        `offerLast` in interface `Deque<E>`
      Parameters:
        `e` - the element to insert
      Returns:
        `true` (as specified by `Deque.offerLast(E)`)
      Since:
        1.6

    - #### peekFirst

          public E peekFirst()

      Retrieves, but does not remove, the first element of this list, or returns `null` if this list is empty.
      Specified by:
        `peekFirst` in interface `Deque<E>`
      Returns:
        the first element of this list, or `null` if this list is empty
      Since:
        1.6

    - #### peekLast

          public E peekLast()

      Retrieves, but does not remove, the last element of this list, or returns `null` if this list is empty.
      Specified by:
        `peekLast` in interface `Deque<E>`
      Returns:
        the last element of this list, or `null` if this list is empty
      Since:
        1.6

    - #### pollFirst

          public E pollFirst()

      Retrieves and removes the first element of this list, or returns `null` if this list is empty.
      Specified by:
        `pollFirst` in interface `Deque<E>`
      Returns:
        the first element of this list, or `null` if this list is empty
      Since:
        1.6

    - #### pollLast

          public E pollLast()

      Retrieves and removes the last element of this list, or returns `null` if this list is empty.
      Specified by:
        `pollLast` in interface `Deque<E>`
      Returns:
        the last element of this list, or `null` if this list is empty
      Since:
        1.6

    - #### push

          public void push(E e)

      Pushes an element onto the stack represented by this list. In other words, inserts the element at the front of this list.

      This method is equivalent to `addFirst(E)`.

      Specified by:
        `push` in interface `Deque<E>`
      Parameters:
        `e` - the element to push
      Since:
        1.6

    - #### pop

          public E pop()

      Pops an element from the stack represented by this list. In other words, removes and returns the first element of this list.

      This method is equivalent to `removeFirst()`.

      Specified by:
        `pop` in interface `Deque<E>`
      Returns:
        the element at the front of this list (which is the top of the stack represented by this list)
      Throws:
        `NoSuchElementException` - if this list is empty
      Since:
        1.6

    - #### removeFirstOccurrence

          public boolean removeFirstOccurrence(Object o)

      Removes the first occurrence of the specified element in this list (when traversing the list from head to tail). If the list does not contain the element, it is unchanged.
      Specified by:
        `removeFirstOccurrence` in interface `Deque<E>`
      Parameters:
        `o` - element to be removed from this list, if present
      Returns:
        `true` if the list contained the specified element
      Since:
        1.6

    - #### removeLastOccurrence

          public boolean removeLastOccurrence(Object o)

      Removes the last occurrence of the specified element in this list (when traversing the list from head to tail). If the list does not contain the element, it is unchanged.
      Specified by:
        `removeLastOccurrence` in interface `Deque<E>`
      Parameters:
        `o` - element to be removed from this list, if present
      Returns:
        `true` if the list contained the specified element
      Since:
        1.6

    - #### listIterator

          public ListIterator<E> listIterator(int index)

      Returns a list-iterator of the elements in this list (in proper sequence), starting at the specified position in the list. Obeys the general contract of `List.listIterator(int)`.

      The list-iterator is fail-fast: if the list is structurally modified at any time after the Iterator is created, in any way except through the list-iterator's own `remove` or `add` methods, the list-iterator will throw a `ConcurrentModificationException`. Thus, in the face of concurrent modification, the iterator fails quickly and cleanly, rather than risking arbitrary, non-deterministic behavior at an undetermined time in the future.

      Specified by:
        `listIterator` in interface `List<E>`
      Specified by:
        `listIterator` in class `AbstractSequentialList<E>`
      Parameters:
        `index` - index of the first element to be returned from the list-iterator (by a call to `next`)
      Returns:
        a ListIterator of the elements in this list (in proper sequence), starting at the specified position in the list
      Throws:
        `IndexOutOfBoundsException` - if the index is out of range (`index < 0 || index > size()`)
      See Also:
        `List.listIterator(int)`

    - #### descendingIterator

          public Iterator<E> descendingIterator()

      Description copied from interface: `Deque`
      Returns an iterator over the elements in this deque in reverse sequential order. The elements will be returned in order from last (tail) to first (head).
      Specified by:
        `descendingIterator` in interface `Deque<E>`
      Returns:
        an iterator over the elements in this deque in reverse sequence
      Since:
        1.6

    - #### clone

          public Object clone()

      Returns a shallow copy of this `LinkedList`. (The elements themselves are not cloned.)
      Overrides:
        `clone` in class `Object`
      Returns:
        a shallow copy of this `LinkedList` instance
      See Also:
        `Cloneable`

    - #### toArray

          public Object[] toArray()

      Returns an array containing all of the elements in this list in proper sequence (from first to last element).

      The returned array will be "safe" in that no references to it are maintained by this list. (In other words, this method must allocate a new array). The caller is thus free to modify the returned array.

      This method acts as bridge between array-based and collection-based APIs.

      Specified by:
        `toArray` in interface `Collection<E>`
      Specified by:
        `toArray` in interface `List<E>`
      Overrides:
        `toArray` in class `AbstractCollection<E>`
      Returns:
        an array containing all of the elements in this list in proper sequence
      See Also:
        `Arrays.asList(Object[])`

    - #### toArray

          public <T> T[] toArray(T[] a)

      Returns an array containing all of the elements in this list in proper sequence (from first to last element); the runtime type of the returned array is that of the specified array. If the list fits in the specified array, it is returned therein. Otherwise, a new array is allocated with the runtime type of the specified array and the size of this list.

      If the list fits in the specified array with room to spare (i.e., the array has more elements than the list), the element in the array immediately following the end of the list is set to `null`. (This is useful in determining the length of the list only if the caller knows that the list does not contain any null elements.)

      Like the `toArray()` method, this method acts as bridge between array-based and collection-based APIs. Further, this method allows precise control over the runtime type of the output array, and may, under certain circumstances, be used to save allocation costs.

      Suppose `x` is a list known to contain only strings. The following code can be used to dump the list into a newly allocated array of `String`:

               String[] y = x.toArray(new String[0]);

      Note that `toArray(new Object[0])` is identical in function to `toArray()`.
      Specified by:
        `toArray` in interface `Collection<E>`
      Specified by:
        `toArray` in interface `List<E>`
      Overrides:
        `toArray` in class `AbstractCollection<E>`
      Parameters:
        `a` - the array into which the elements of the list are to be stored, if it is big enough; otherwise, a new array of the same runtime type is allocated for this purpose.
      Returns:
        an array containing the elements of the list
      Throws:
        `ArrayStoreException` - if the runtime type of the specified array is not a supertype of the runtime type of every element in this list
        `NullPointerException` - if the specified array is null

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