Java™ Platform

Standard Ed. 6

java.util

Class 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.

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 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

Java™ Platform

Standard Ed. 6