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>, List<E>, Queue<E>

public class LinkedList<E>
extends AbstractSequentialList<E>
implements List<E>, Queue<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 (deque).

The class implements the Queue interface, providing first-in-first-out queue operations for add, poll, etc. Other stack and deque operations could be easily recast in terms of the standard list operations. They're included here primarily for convenience, though they may run slightly faster than the equivalent List 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 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 the 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, Collections.synchronizedList(List), Serialized Form

Field Summary

 

Fields inherited from class java.util.AbstractList
modCount

 

Constructor Summary
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
boolean	add(E o)           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 o)           Inserts the given element at the beginning of this list.
void	addLast(E o)           Appends the given 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.
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 in this list of the first occurrence of the specified element, or -1 if the List does not contain this element.
int	lastIndexOf(Object o)           Returns the index in this list of the last occurrence of the specified element, or -1 if the list does not contain this 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 o)           Adds the specified element as the tail (last element) of this list.
E	peek()           Retrieves, but does not remove, the head (first element) of this list.
E	poll()           Retrieves and removes the head (first element) of 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 in this list.
E	removeFirst()           Removes and returns the first element from this list.
E	removeLast()           Removes and returns the last element from this list.
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 the correct order.
<T> T[]	toArray(T[] a)           Returns an array containing all of the elements in this list in the correct order; 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

 

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.

getFirst

public E getFirst()

Returns the first element in this list.

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.

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.

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.

Returns:

the last element from this list.

Throws:

NoSuchElementException - if this list is empty.

addFirst

public void addFirst(E o)

Inserts the given element at the beginning of this list.

Parameters:

o - the element to be inserted at the beginning of this list.

addLast

public void addLast(E o)

Appends the given element to the end of this list. (Identical in function to the add method; included only for consistency.)

Parameters:

o - the element to be inserted at the end of this list.

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 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 List<E>

Specified by:

size in class AbstractCollection<E>

Returns:

the number of elements in this list.

add

public boolean add(E o)

Appends the specified element to the end of this list.

Specified by:

add in interface Collection<E>

Specified by:

add in interface List<E>

Overrides:

add in class AbstractList<E>

Parameters:

o - element to be appended to this list.

Returns:

true (as per the general contract of Collection.add).

remove

public boolean remove(Object o)

Removes the first occurrence of the specified element in this list. If the 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).

Specified by:

remove in interface Collection<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 the 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. (This implies that the behavior of this call is undefined if the specified Collection is this list, and this list is nonempty.)

Specified by:

addAll in interface Collection<E>

Specified by:

addAll in interface List<E>

Overrides:

addAll in class AbstractCollection<E>

Parameters:

c - the elements to be inserted into 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 first element from the specified collection.

c - elements to be inserted into this list.

Returns:

true if this list changed as a result of the call.

Throws:

IndexOutOfBoundsException - if the specified 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 element to return.

Returns:

the element at the specified position in this list.

Throws:

IndexOutOfBoundsException - if the specified 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 element to replace.

element - element to be stored at the specified position.

Returns:

the element previously at the specified position.

Throws:

IndexOutOfBoundsException - if the specified 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 specified 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 removed.

Returns:

the element previously at the specified position.

Throws:

IndexOutOfBoundsException - if the specified index is out of range (index < 0 || index >= size()).

indexOf

public int indexOf(Object o)

Returns the index in this list of the first occurrence of the specified element, or -1 if the List does not contain this 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 in this list of the first occurrence of the specified element, or -1 if the list does not contain this element.

lastIndexOf

public int lastIndexOf(Object o)

Returns the index in this list of the last occurrence of the specified element, or -1 if the list does not contain this 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 in this list of the last occurrence of the specified element, or -1 if the list does not contain this element.

peek

public E peek()

Retrieves, but does not remove, the head (first element) of this list.

Specified by:

peek in interface Queue<E>

Returns:

the head of this queue, or null if this queue 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 Queue<E>

Returns:

the head of this queue.

Throws:

NoSuchElementException - if this queue is empty.

Since:

1.5

poll

public E poll()

Retrieves and removes the head (first element) of this list.

Specified by:

poll in interface Queue<E>

Returns:

the head of this queue, or null if this queue is empty.

Since:

1.5

remove

public E remove()

Retrieves and removes the head (first element) of this list.

Specified by:

remove in interface Queue<E>

Returns:

the head of this queue.

Throws:

NoSuchElementException - if this queue is empty.

Since:

1.5

offer

public boolean offer(E o)

Adds the specified element as the tail (last element) of this list.

Specified by:

offer in interface Queue<E>

Parameters:

o - the element to add.

Returns:

true (as per the general contract of Queue.offer)

Since:

1.5

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 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 index is out of range (index < 0 || index > size()).

See Also:

List.listIterator(int)

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 the correct order.

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 the correct order.

See Also:

Arrays.asList(Object[])

toArray

public <T> T[] toArray(T[] a)

Returns an array containing all of the elements in this list in the correct order; 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 collection 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.

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 a is not a supertype of the runtime type of every element in this list.

NullPointerException - if the specified array is null.